Walls, windows and roofs for the Canadian climate: a summary of the current basis for selection and design

(1)

Publisher’s version / Version de l'éditeur:

Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n’arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca.

Questions? Contact the NRC Publications Archive team at

PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information.

https://publications-cnrc.canada.ca/fra/droits

L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site

LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB.

Special Technical Publication (National Research Council of Canada. Division of

Building Research), 1973-10

READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE. https://nrc-publications.canada.ca/eng/copyright

NRC Publications Archive Record / Notice des Archives des publications du CNRC :

https://nrc-publications.canada.ca/eng/view/object/?id=ae1707e2-c586-45f3-8bb3-e8f4b3a35ac4

https://publications-cnrc.canada.ca/fra/voir/objet/?id=ae1707e2-c586-45f3-8bb3-e8f4b3a35ac4

NRC Publications Archive

Archives des publications du CNRC

This publication could be one of several versions: author’s original, accepted manuscript or the publisher’s version. / La version de cette publication peut être l’une des suivantes : la version prépublication de l’auteur, la version acceptée du manuscrit ou la version de l’éditeur.

For the publisher’s version, please access the DOI link below./ Pour consulter la version de l’éditeur, utilisez le lien DOI ci-dessous.

https://doi.org/10.4224/40001180

Access and use of this website and the material on it are subject to the Terms and Conditions set forth at

Walls, windows and roofs for the Canadian climate: a summary of the

current basis for selection and design

(2)

PRICE $3.00

WALLS,

WINDOWS AND ROOFS

FOR THE

CANADIAN CLIMATE

BY

J. K. LATTA

A SLIMMARY OF THE CURRENT BASIS

FOR SELECTION AND DESIGN

SPECIAL TECHNICAL PUBLICATION NO.l

OF THE

DIVISION OF BUILDING RESEARCH

(3)

FOREWORD

The building designer of the post had to rely on experience or on experiment. When the reasons for the success of past designs were not known i t was not possible to identify the significance of departures from them, and thus the new design inevitably became an experiment. I t has been necessary, therefore, i n establishing a sound basis for rational design, to seek to understand why various arrangements did or d i d not perform. This has been one of the major tasks o f building research institutes i n various countries.

The enclosures o f buildings merit special attention i n every country. They must always be designed with due regard for the materials, the conditions, and the practices of the country, and so can seldom be studied adequately elsewhere. As studies progressed at DBR and at other building research institutes and the essential elements o f performance began to be appreciated, it became necessary to emphasize putting the results to work. The Canadian Building Digests and Building Science Seminars were useful i n communicating with designers and others, but they were not enough. Something more was needed by way of a thorough but simple treatise relating a l l the relevant bits o f information previously presented i n many individual publications The author,

o

research officer with the Division, accepted this challenge. He has followed closely the studies and the conclusions arising out o f the work of his colleagues i n the Division over the years, but the selection and presentation are his, as are the explanations and the examples which ore intended to promote the best possible understanding of building enclosure design and performance.

Ottawa October

1973

N

.B.

Hutcheon Director, DBR/NRC

(4)

PREFACE

E v e r since m a n c e a s e d t o be a cave d w e l l e r he h a s been a builder.

In f a c t , i t w a s h i s ability t o build that enabled h i m t o leave the cave.

Slowly, a s society evolved t o w a r d s the p r e s e n t d a y technological e r a , the

m e t h o d s and techniques u s e d in building evolved with g r e a t e r and g r e a t e r

r e f i n e m e n t of d e t a i l , and t h e slow t e m p o of construction in t h o s e d a y s

allowed t i m e f o r t h i s p r o c e s s of t r i a l and e r r o r .

When one a d m i r e s a n old building that h a s been standing f o r c e n t u r i e s ,

p e r h a p s , one i s in f a c t a d m i r i n g t h o s e f o r m s of construction that w e r e

t r i e d and found t o be s u c c e s s f u l . But what of the e r r o r s ? One cannot help

but think t h a t the e r r o r s that led t o t h e development of t h e flying b u t t r e s s e s

of the old c a t h e d r a l s , f o r example, m u s t have been v e r y c o s t l y indeed.

During t h e l a s t few d e c a d e s the demand f o r new buildings h a s been

s o g r e a t and a l l c o s t s have been r i s i n g s o f a s t that t h e r e h a s been l i t t l e

t i m e f o r slow and painstaking development of construction m e t h o d s . In an

endeavour t o speed t h e p r o c e s s of building and t o cut c o s t s , new m a t e r i a l s

and m e t h o d s of a s s e m b l y have been used without the benefit of l o n g - t e r m

p e r f o r m a n c e . The design and construction p r o c e s s h a s become m o r e

f r a g m e n t e d with m a n u f a c t u r e r s and s u b - t r a d e s designing and installing

a wide v a r i e t y of components and m a t e r i a l s . The buildings t h e m s e l v e s

have a l s o b e c o m e m o r e complicated, l a r g e l y a s a r e s u l t of i n c r e a s i n g

d e m a n d s f o r g r e a t e r c o n t r o l of i n t e r i o r conditions. Heating, ventilating

and e l e c t r i c a l s y s t e m s have a l s o become m o r e e l a b o r a t e , s o m e t i m e s a s a

d i r e c t r e s u l t of t h e design of the e x t e r i o r wall. T h i s multiplicity of t r a d e s

and the d e s i r e d speed of construction m a k e c o n t r o l of the whole p r o c e s s

m o r e difficult a n d , not infrequently, one t r a d e m a y be working a t odds

with a n o t h e r . T h e installation of one thing m a y m a k e t h a t of a subsequent

one m o r e difficult o r n e c e s s i t a t e the r e m o v a l o r d a m a g e of a previously

i n s t a l l e d i t e m with t h e r e s u l t that t h e building m a y not p e r f o r m a s intended.

During the l a s t 1 5 t o

20

y e a r s , t h e r e h a s been a growing s t o r e of

knowledge of the underlying physical phenomena that c o n t r o l t h e p e r f o r m a n c e

of buildings. T h i s i s s o m e t i m e s r e f e r r e d t o a s "building science".

T h e r e

i s , of c o u r s e , a scientific b a s i s f o r m o s t of t h i s knowledge but the p r o c e s s

of designing and constructing

a'

building i s s t i l l f a r f r o m being scientific

and might b e t t e r be d e s c r i b e d a s a n a r t i n that m a n y d e c i s i o n s m u s t be

m a d e on a subjective o r intuitive b a s i s . M u c h of t h i s information i s

s c a t t e r e d i n various publications and, a s f a r a s i s known, h a s n e v e r been

collected together and p r e s e n t e d in one self-contained package.

(5)

The purpose of t h i s book then i s t o p r e s e n t the existing b a s i s f o r the

design of e x t e r n a l walls and roofs in s o f a r a s they m u s t act t o withstand

the Canadian climate and contain the d e s i r e d inside conditions. A s know-

ledge of the subject expands,

it i s probable that some of the m a t e r i a l

p r e s e n t e d h e r e will have t o be modified, r e s t a t e d o r enlarged. In a book

s u c h a s t h i s , one cannot hope t o cover a l l a s p e c t s of wall and roof design

and m a n y subjects such a s s t r u c t u r a l strength, f i r e r e s i s t a n c e , acoustical

p r o p e r t i e s and a e s t h e t i c s a r e not covered. T h e r e i s scope f o r many

l a r g e r , m o r e detailed and a l l - e m b r a c i n g books on the limited subject m a t t e r

which has been covered a s well a s on the m a n y other subjects that have

been omitted completely.

It i s hoped that the book will be of use t o a r c h i t e c t u r a l students and

o t h e r s who need t o a s s i m i l a t e the b a s i c design principles a s rapidily a s

possible without being overwhelmed with technical o r scientific jargon,

With t h i s objective, the style of presentation h a s been kept relatively

informal. P o s s i b l y the book could be used a s the b a s i s of a c o u r s e of

instruction in schools of a r c h i t e c t u r e o r building technology. It i s a l s o

hoped that i t m a y find a place in practicing design offices where the

bustle of design and the need t o reconcile m a n y conflicting demands m a y

lead t o b a s i c principles being violated and where a reasonably concise

statement of some of these principles m a y be of u s e in straightening things

out again.

No c l a i m i s made that the principles and t h e o r i e s expounded in t h i s

book a r e a l l the r e s u l t of original work by the author. On the c o n t r a r y ,

they a r e l a r g e l y a consolidation and r e s t a t e m e n t of the work of many

o t h e r s . Naturally, the principal s o u r c e of information i s the various pub-

lications of the Division of Building R e s e a r c h and in p a r t i c u l a r the Canadian

Building Digest s e r i e s . Additional information has been drawn f r o m other

s o u r c e s outside of DBR. A l i s t of a l l a u t h o r s would be excessively long

and r e f e r e n c e should be m a d e t o the bibliographies a t the end of each

chapter. The u s e of a l l information i s gratefully acknowledged a s a r e the

m a n y pertinent comments and suggestions made by various people within

DBR who have reviewed p a r t s o r a l l of the text. Special thanks a r e due

t o M r . George K u e s t e r who p r e p a r e d m a n y of the s p e c i a l drawings and

the a r c h i t e c t u r a l d e t a i l s contained in the Appendix.

(6)

CONTENTS

CHAPTER I

--

THE PROBLEM

Why do we build buildings? The inside conditions

Human beings and a s s o c i a t e d activities Some non -human occupancies

The outside conditions The weather Other f a c t o r s CHAPTER LI

--

BUILDING MATERIALS

The n a t u r e of m a t e r i a l s The d e t e r i o r a t i o n of m a t e r i a l s Water T e m p e r a t u r e Ultraviolet radiation F r o s t Ice lensing C o r r o s i o n Rotting of wood

Radiation, oxygen and t e m p e r a t u r e Durability

CHAPTER ILI

- -

PHYSICS HEAT

T e m p e r a t u r e g r a d i e n t s

Conductivity and conductance

Calculation of t e m p e r a t u r e gradients Heat l o s s f r o m b a s e m e n t s

PSYCHROMETRY Vapour diffusion

Calculation of vapour flow AIR MOVEMENT

Wind p r e s s u r e s Stack effect

Ventilation p r e s s u r e s CHAPTER IV

--

THE CONTROL OF HEAT

Insulation Insulating a basement T h e r m a l b r i d g e s T h e r m a l breakage of windows Connections Control of s o l a r radiation Methods of control Ice d a m s

CHAPTER V

--

THE CONTROL O F WATER Water vapour

Movement of w a t e r vapour by a i r c u r r e n t s Movement of water vapour by diffusion

Movement of water vapour by a t e m p e r a t u r e gradient Liquid w a t e r

Capillarity M omenturn Gravity Wind p r e s s u r e

(7)

CHAPTER VI

- -

THE BASIC SOLUTION The e s s e n t i a l r e q u i r e m e n t s C o n t r o l of a i r flow Control of r a i n penetration C o n t r o l of h e a t flow Wall cladding Roofing C o n t r o l of vapour diffusion The b a s i c configuration C o m p a r i s o n of two w a l l d e s i g n s Roofs and roof t e r r a c e s

Rain-tight joints

(8)

CHAPTER I

- -

THE PROBLEM

WHY DO WE BUILD BUILDINGS?

All over the world new buildings a r e being built and old ones r e p a i r e d or demolished to m a k e way f o r others. In Canada during the late

1960's s o m e $7 to $8 billion, representing about 10 p e r c e n t of the g r o s s national product, w e r e spent on this activity each year. Thus i t i s not unreasonable to a s k why do people build buildings? Or, if one wishes t o reduce the s c a l e of the question, why does one live in a house?

In the v a s t m a j o r i t y of c a s e s one builds a building to p r o t e c t the occupants and contents f r o m inclement weather. It i s t r u e that in some c a s e s protection i s a l s o needed against animals o r i n s e c t s and against thieves or vandals but in the g r e a t m a j o r i t y of c a s e s the initial r e q u i r e - m e n t i s f o r protection f r o m the weather. This protection i s given by separating the inside f r o m the uncontrolled conditions outside s o that the conditions inside can be modified and controlled to s o m e extent. It i s the basic function of the walls, roof and f l o o r s of the building to effect this separation.

Since these walls, roof and f l o o r s f o r m an enclosure they can conveniently be r e f e r r e d to collectively a s the building enclosure. Any fenced a r e a could be called an enclosure but although it h a s some of the separation c h a r a c t e r - i s t i c s of a building,for n o r m a l purposes i t cannot reasonably be considered to be one. I t i s the technical design of a building enclosure in i t s function a s a s e p a r a t o r of inside and outside conditions that will be discussed in the following chapters. If this separation i s not required, then a building a s such i s not required, only a s t r u c t u r a l f r a m e w o r k to support the various components in t h e i r d e s i r e d locations.

A complete separation, however, i s not always n e c e s s a r y nor even desirable. A g r e e n - house with a n opaque roof would not work v e r y well; lumber s t o r e d f o r a i r seasoning r e q u i r e s only a roof to keep the r a i n off and provisions made to e n s u r e a flow of a i r around and through the pile to c a r r y away the excess moisture. The d e g r e e of separation will a l s o v a r y with the weather, the s e a s o n of the year and the time of day. Sunlight s t r e a m i n g in through a window c a n be intolerable a t t i m e s but on other occa- sions m a y be m o s t welcome. Thus the building m u s t not only be a n efficient s e p a r a t o r but m u s t in s o m e r e s p e c t s be selective in what i t excludes and when.

Keeping w a r m and d r y a r e probably the p r i m e objectives in constructing m o s t buildings. I t i s c l e a r l y impossible to achieve these objec-

tives if the wind i s allowed to blow f r e e l y through the building. Unless the wind i s controlled by the building enclosure i t i s not possible to control any of the internal conditions satisfactorily except f o r some p a r t i a l p r o t e c - tion f r o m the sun and r a i n but even with this p a r t i a l protection, snow may blow right through. Thus to fulfil i t s function the building enclosure m u s t control the flow of a i r . The importance of this f a c t can hardly be o v e r - s t r e s s e d f o r , a s we s h a l l s e e l a t e r , f a i l u r e to control the movement of a i r through, and within, the thickness of the building enclosure can lead to many s e r i o u s problems of building d e t e r i o r a - tion and of f a i l u r e to p e r f o r m i t s intended function satisfactorily. I t i s probably worth- while to d i g r e s s a t this point and to anticipate l a t e r discussions to point out that the a i r flows concerned need not be r i g h t through the building enclosure but can be within the thickness of the enclosure itself. F u r t h e r m o r e , the f o r c e s producing these c u r r e n t s of a i r a r e not n e c e s s a r i l y those associated with strong winds; convection c u r r e n t s produced by d i f f e r - ences in t e m p e r a t u r e can in many c a s e s be of g r e a t e r importance.

Even when the wind has been excluded f r o m the building i t will s t i l l b e n e c e s s a r y during m o s t of the year t o a d j u s t the i n t e r n a l t e m p e r - a t u r e by m e a n s of a heating o r cooling system. The s i z e of this s y s t e m will depend upon the balance between the r a t e a t which h e a t i s gen- e r a t e d within the building and gained f r o m e x t e r n a l s o u r c e s s u c h a s the sun, and the r a t e a t which i t l e a k s away. If the c o s t of, and the s p a c e occupied by, this system, with a l l i t s attendant piping and duct work, and if the c o s t of operating i t a r e of-no consequence, then one can m e r e l y provide a big enough s y s t e m and pump the h e a t around to maintain the d e s i r e d i n t e r n a l conditions. Even then control of these conditions would be v e r y difficult because of local cooling o r heating effects a t v a r i o u s locations. In p r a c t i c e i t i s highly unlikely that t h e s e c o s t ard s p a c e r e q u i r e m e n t s can be ignored and s o the building enclosure m u s t e x e r c i s e s o m e control over the flow of heat. P a r t of this control will be directed toward the c o n t r o l of h e a t gain f r o m the sun and radiant h e a t l o s s to a c l e a r night sky. Of these, heat gain f r o m the sun i s probably t h e m o r e i m p o r t - a n t f o r , although the p r i m a r y objective m a y be to keep warm, we do not want to be too w a r m .

In addition to the need to c o n t r o l the heat gain f r o m the whole s p e c t r u m of s o l a r radiation, specific p a r t s of the s p e c t r u m m a y have

(9)

t o b e controlled for other reasons. In p a r t i c - ular the effects of ultraviolet radiation on organic m a t e r i a l s which either f o r m a p a r t of the building o r a r e s t o r e d in i t should be considered.

The second m a i n objective in building a building i s to keep dry, and this entails a lot m o r e than shedding vertically falling r a i n off the roof. Rain driven a l m o s t horizontally by gale-force winds m u s t a l s o be excluded a s m u s t snow which can sometimes d r i f t in through quite s m a l l openings if t h e r e i s a c u r r e n t of a i r to c a r r y it.

Then t h e r e a r e m o i s t u r e problems which can originate inside the building because of the water vapour c a r r i e d in the a i r . This water vapour will condense on any sufficiently cool surface. The condensed water can then d i s - figure o r damage both the contents of the building and the f a b r i c of the building itself. The condensation will not always occur where i t can be s e e n but m a y be in the walls o r roof. In s o m e c a s e s i t will collect in l a r g e quantities a s f r o s t which will be r e l e a s e d a s m o i s t u r e a s the t e m p e r a t u r e r i s e s .

In s u m m a r y i t can be said that a building e n c l o s u r e i s composed of e x t e r n a l walls, a roof and those floors, o r p a r t s of floors, that a r e not totally enclosed within the building. The basic function of a building enclosure i s to keep the outside out and the inside in, except f o r those things which we wish t o enter o r leave the building. These m u s t be controlled s o that they enter or leave the building a t such points and in such quantity and m a n n e r that they neither change the inside conditions too much nor h a r m the building e n c l o s u r e in p a s s - ing through it.

A complete l i s t of a l l the r e q u i r e m e n t s of the building enclosure would be v e r y long but f o r our purposes in the discussion that follows they c a n be limited t o the following six:

1. Control of a i r flow 2. Control of heat flow

3. Control of sunlight and other f o r m s of radiant energy

4. Control over the entry of r a i n and snow 5. Control of water vapour

6.

Satisfactory performance during i t s s e r v i c e life.

T h e r e a r e many other r e q u i r e m e n t s such a s control of f i r e , control of noise, s t r u c t u r a l strength, aesthetic qualities, economy of construction, operation and maintenance a l l of which m u s t be taken into account in the over - a l l design, I t is, however, beyond the scope of this book to cover in detail a l l of these subjects and

s o in g e n e r a l they will be excluded f r o m the discussions that follow. This exclusion should not be taken a s a n indication that they a r e either not important or that t h e r e i s a conflict between the solution t o wall and roof design to m e e t the s i x basic r e q u i r e m e n t s and that required f o r the control of f i r e o r acoustics, o r any of the o t h e r s . This being s o i t i s preferable t o

explain the principles underlying good enclosure design without the added complications of these f u r t h e r constraints.

The sixth r e q u i r e m e n t

- -

satisfactory p e r f o r - mance

- -

i s on the other hand m o s t g e r m a n e to the theme. I t is, in fact, the c r u x of the m a t t e r , for many buildings a r e built a t p r e s e n t in a manner which f r o m t i m e t o t i m e c a u s e s s e r i o u s problems in the control of the i n t e r i o r conditions or which lead t o rapid d e t e r i o r a t i o n and e x c e s s i v e maintenance c o s t s . The m e a n s taken t o m e e t the f i r s t five r e q u i r e m e n t s of the building e n c l o s u r e m u s t be such a s will m e e t the sixth requirement.

In the past, changes in the design of the building e n c l o s u r e evolved slowly and largely on the b a s i s of t r i a l and e r r o r . Today, with new m a t e r i a l s and methods of construction available, the tempo of change h a s i n c r e a s e d dramatically. I t is no longer adequate t o wait f o r the passage of t i m e t o d e t e r m i n e the suit- ability of any design f o r in the meantime many m o r e buildings will have been built. I t i s n e c e s - s a r y to a s s e s s a t l e a s t in general t e r m s , the probability of the design being satisfactory, before work s t a r t s on the site, even if the ultimate t e s t m u s t s t i l l be t r i a l by use. How- ever, the dice can be loaded in the d e s i g n e r ' s favour i f c e r t a i n principles a r e adhered to throughout both the design and construction of the building.

THE INSIDE CONDITIONS

Having decided that the function of the building enclosure i s t o p r o t e c t the inside conditions f r o m the weather outside i t i s reasonable that the next s t e p should be to d e t e r m i n e what we want to protect. That is, what conditions a r e we going t o t o l e r a t e inside the building o r alternatively what conditions a r e we going to s t r i v e to c r e a t e and maintain?

Different occupancies can s e t widely differing conditions inside a building. A cold s t o r a g e _- - locker will r e q u i r e a t e m p e r a t u r e of l e s s than 0" F whereas a building f o r human occupancy will need inside t e m p e r a t u r e s of around 7 0 ° F . F o r s a t i s f a c t o r y u s e e v e r y effort m u s t be m a d e to maintain these conditions. On the other hand s o m e occupancies will c r e a t e c e r t a i n conditions that a r e not r e a l l y required but can

(10)

be accepted. P a p e r making, for example, the house by household activities a s shown in does not need high t e m p e r a t u r e s and humidities Table I. 1.

in the building but the n a t u r e of the p r o c e s s and the equipment c u r r e n t l y used makes them a l m o s t inevitable. In this c a s e the internal conditions do not need to b e controlled s o c l o s e - ly but can be allowed to fluctuate over a wide range.

A building enclosure will be required to handle the w o r s t conditions without d i s t r e s s and s o one m u s t balance the c o s t and effort r e q u i r e d to design and construct such a n en- c l o s u r e against the s i m i l a r c o s t and effort required t o control the internal conditions m o r e closely. With this in mind l e t us proceed by reviewing both the effects of some activities on the i n t e r n a l conditions and a l s o the conditions which should be maintained to enable o t h e r s to be c a r r i e d on satisfactorily. I t i s n e c e s s a r y to use the expression "should be maintained" f o r in a l l too many instances in the p a s t the l i m i t to the internal conditions h a s been s e t by the inability of the building enclosure to maintain the d e s i r e d conditions satisfactorily.

T h i s review i s not intended to be in any way comprehensive and one m u s t turn to other s o u r c e s for m o r e detailed information. I t i s however intended to give an o v e r - a l l p i c t u r e of the effects and r e q u i r e m e n t s of some types of occupancy s o that discussions of s a t i s f a c t o r y solutions to the problem of the building en- c l o s u r e can be s e t against a reasonably factual background.

Human Beings and Associated Activities People a r e warm-blooded a n i m a l s and m u s t maintain the inner organs of their bodies a t a relatively uniform t e m p e r a t u r e around 98.6" F. Food is used a s a fuel which i s converted into energy some of which may leave the body in the f o r m of external work. The balance i s available to maintain the body temperature. Since the body produces heat continuously i t m u s t a l s o l o s e 'heat continuously otherwise i t will overheat like a c a r with no water in its radia- tor. A basic r a t e of heat production during s l e e p i s about 250 B t u l h r . , the h e a t equivalent of about 75 watts. As the bodily activity in- c r e a s e s s o does the r a t e of h e a t production with approximate values of 400 B t u l h r (120 watts) when awake but sedentary, 650 B t u / h r (190 watts) f o r light work and 2400 B t u f h r (700 watts) f o r heavy work.

In addition t o the heat produced by p e r s o n s in a building t h e r e i s a l s o a n associated p r o - duction of moisture, which i s c a r r i e d in the a i r in the f o r m of water vapour. In a house with a family of four this has been estimated to be about 12 lb p e r day. Additional quantities of m o i s t u r e will be added to the a i r inside

These f i g u r e s show that approximately 15 to 20 lb (1 1 / 2 to 2 gallons) of m o i s t u r e p e r day may be introduced into a house with four occupants under n o r m a l living conditions and that this can r i s e to a s much a s 40 to 50 lb (4 o r 5 gallons) p e r day on washdays. A gas range in the kitchen will a l s o add m o i s t u r e to the a i r f r o m water vapour which i s one of the products of combustion when g a s i s burnt.

Heat and m o i s t u r e a r e not the only problems associated with human occupancy; people m u s t a l s o have a i r to breathe. F r e s h d r y outdoor a i r cmtains about 21 p e r c e n t 0 and 0. 03 p e r

2

cent C 0 2 on a volume b a s i s (the r e m a i n d e r being mainly nitrogen). Significant variations in these proportions can r e n d e r i t unfit f o r human use. F o r prolonged exposure a minimum concentration of 16 p e r cent 0 2 and a maximum concentration of 0.5 p e r cent C02 ( s o m e t i m e s extended to 1.5 p e r cent) a r e commonly accept-. ed standards.

A person, when seated, usually inhales about 18 c u f t of a i r p e r h r . The exhaled a i r contains about 16 p e r c e n t 0 2 and about 4 p e r cent COZ. Thus, if only 1 8 c u f t p e r h r of f r e s h a i r w e r e provided via a face m a s k f o r each p e r s o n in a continuously occupied s p a c e the concentrations of OZ and C 0 2 would a p - proach these levels. If the f r e s h a i r i s suppli- ed to the room and thoroughly mixed with the room a i r , each p e r s o n would ultimately inhale a mixture of half f r e s h and half room a i r and the C 0 2 level would approach 2 p e r cent. Exposure f o r even a s h o r t time to a GO2 level of even 2 p e r cent would r e s u l t in a t e m p o r a r y l o s s of vitality and ability. If, however, ten t i m e s this amount of f r e s h a i r w e r e provided to the room (180 c u f t p e r h r or 3 c u f t per minute), the ultimate CO level would be only 1110th of 4 p e r cent i. e. '6.4 p e r cent and the OZ deficiency would be only 0. 5 p e r cent, in- stead of 5 p e r cent. Approximately 3 cfm p e r p e r s o n may thus be r e g a r d e d a s the minimum r a t e of supply of outdoor a i r , or equivalent, that i s required to control, within accepted limits, the concentration of CO a r i s i n g f r o m r e s p i r a t i o n of people a t r e s t ; o h y one tenth of this i s r e q u i r e d to maintain the required levels of 02.. Consumption of 0 and production of COZ mc r e a s e with a c tivify, and ventilation r e q u i r e m e n t s i n c r e a s e correspondingly. F o r people who a r e standing, the values a r e about 50 p e r cent higher than for those seated.

People use light to s e e what they a r e doing. An illumination of 100 ft-candles provided by r e c e s s e d fluorescent fixture r e q u i r e s about 0.5 kw of e l e c t r i c power f o r each 100 sq f t of floor a r e a , m o s t of which sooner or l a t e r

(11)

TABLE I. 1 MOISTURE PRODUCED BY VARIOUS HOUSEHOLD ACTIVITIES FOR A FAMILY O F FOUR

Activity

Cooking (3 m e a l s p e r day) Dishwashing (3 m e a l s p e r day) Bathing

--

Shower

- -

Tub

Clothes washing ( p e r week) Clothes drying indoors o r with

unvented d r y e r ( p e r week) Floor mopping ( p e r 100 sq f t ) Occupants (family of four p e r day)

a p p e a r s a s heat within the building.

If t h e s e a r e s o m e of the effects of human beings on the conditions in the building what conditions should we s t r i v e t o maintain s o that people c a n live and work in them satisfactorily? These will v a r y with the n a t u r e of the activity being c a r r i e d on; with s e d e n t a r y occupations a n a i r t e m p e r a t u r e of 70 t o 75

"

F would be d e s i r a b l e whereas with light activity this r a n g e could be reduced by s o m e 5 OF. Humidity levels a r e not s o c r i t i c a l . F o r p e r s o n s a t r e s t o r doing light work they c a n v a r y between about 20 per c e n t and 70 p e r c e n t r e l a t i v e humidity provided that the a i r t e m p e r a t u r e i s l e s s than 7 8 ° F .

People t h e m s e l v e s a r e t h e r e f o r e reasonably adaptable but what of the activities they m a y be doing? The surgeon in the operating t h e a t r e m a y be happy with a considerable variation in conditions but i t i s e s s e n t i a l f o r considerations of safety that the r e l a t i v e humidity should not drop below 50 p e r cent. This humidity level i s n e c e s s a r y to prevent electrostatic c h a r g e s building up, with the danger of a s p a r k c r e a t i n g an explosion with flammable anesthetics. The e l e c t r o s t a t i c c h a r g e s v a r y with the r e l a t i v e humidity r i s i n g f r o m s o m e b a s e f i g u r e a t

z e r o R. H. to a maximum which i s v e r y often in the range of 20 to 30 p e r cent

R.

H. Above this value the s u r f a c e m o i s t u r e on t h e m a t e r i a l allows the c h a r g e t o leak away until i t d r o p s to z e r o a t high humidities. The values will of c o u r s e v a r y with different m a t e r i a l s .

F o r this s a m e r e a s o n of e l e c t r o s t a t i c c h a r g - ing, textile m i l l s a r e always r u n a t v e r y high humidity levels. At lower l e v e l s the m a t e r i a l , which i s produced on high speed machines,

M o i s t u r e Produced l b

together making i t difficult to handle. Relative humidities of 80 o r 90 p e r c e n t a r e often r e - quired to o v e r c o m e this.

An office building m a y be thought of a s occupied predominantly by people and f o r the bulk of the s p a c e in the building this i s t r u e . However many offices a r e now using machines of one s o r t o r another and s o m of these, p a r - t i c u l a r l y computers, will r e q u i r e s p e c i a l conditions t o enable t h e m to o p e r a t e m o s t s a t i s - factorily. Once again controlled humidities a t relatively high levels of 50 p e r c e n t o r m o r e m a y be required.

The range within which t h e r e l a t i v e humidity i s controlled c a n be just a s i m p o r t a n t i n s o m e c a s e s a s the a c t u a l value maintained. Many m a t e r i a l s s w e l l and s h r i n k with changes in their m o i s t u r e content which in t u r n depends upon the r e l a t i v e humidity of the a i r around them. If the r e l a t i v e humidity i s allowed to fluctuate over a c o n s i d e r a b l e range this continual swelling and shrinking c a n damage a r t i c l e s m a d e f r o m such m a t e r i a l s particularly if different components swell and s h r i n k by different amounts. Thus i t happens s o m e t i m e s that s o m e p i e c e s of wooden f u r n i t u r e loosen a t the joints o r v e n e e r s c r a c k o r peel off. A r t i - f a c t s in a r t g a l l e r i e s ahd m u s e u m s a l s o c a n

suffer damage f o r the s a m e reason. As mentioned e a r l i e r the paper-making p r o c e s s tends to c r e a t e high t e m p e r a t u r e s and humidities in the building even though they a r e not s t r i c t l y n e c e s s a r y f o r the p r o c e s s . During v i s i t s to s o m e 30 p a p e r m i l l s during 1960-61, t e m p e r a t u r e and humidity levels w e r e m e a s u r

-

ed i n the machine r o o m s a t v a r i o u s locations. The r e s u l t s of t h e s e m e a s u r e m e n t s a r e given in Table I. 2.

(12)

T A B L E 1.2. MACHINE ROOM CONDITIONS M P A P E R M I L L S

Location Temp. R. H.

"

F %

A v e r a g e

. . .

.

. . .

.Over - a l l

Under roof a t wet end Under roof a t d r y end By w a l l a t wet end By w a l l a t d r y end H i g h e s t

t e m p e r a t u r e s Under roof, d r y end of d r y e r s Under roof, o v e r d r y e r s L o w e s t

t e m p e r a t u r e s F l o o r level, opposite b r e a s t r o l l F l o o r level, opposite r e e l H i g h e s t R.H..

. . .

.

.Under roof, o v e r headbox

F l o o r level, a t wet end L o w e s t R. H..

.

. .

.

.In

_{penthouse, o v e r d r y e r s} In a t t i c , a b o v e c o m p l e t e c e i l i n g S o m e Non-human O c c u p a n c i e s A n i m a l s . Animals, o t h e r t h a n h u m a n s have d i f f e r e n t r a t e s of h e a t and m o i s t u r e p r o d u c t i o n and a l s o h a v e d i f f e r e n t r e q u i r e m e n t s f o r t h e conditions i n s i d e t h e building. D a i r y c o w s in s t a l l s in a b a r n will g i v e off s o m e 3500 B t u / h r / 1 0 0 0 l b body weight when the b a r n t e m p e r a t u r e i s a b o u t 4 0 ° F . At t h e s a m e t i m e t h e y will p u t 0.9 l b of w a t e r / h r / 1 0 0 0 l b body weight i n t o the a i r . T h e s e r a t e s of h e a t and m o i s t u r e p r o d u c t i o n c a n l e a d t o c o n s i d e r a b l e d i f f i c u l t i e s

i n s e l e c t i n g a s u i t a b l e ventilation r a t e in cold w e a t h e r . If s u f f i c i e n t a i r is p a s s e d t h r o u g h t h e building t o c a r r y the m o i s t u r e away the i n s i d e t e m p e r a t u r e will d r o p below t h e d e s i r e d l e v e l . T h e ventilation a i r can, of c o u r s e , b e h e a t e d t o o v e r c o m e t h i s b u t t h i s i s a n a d d e d e x p e n s e f o r t h e f a r m e r .

T h e conditions t h a t a r e c o n s i d e r e d d e s i r a b l e f o r v a r i o u s c l a s s e s of l i v e s t o c k a r e given in

T a b l e I. 3. A s would b e expected, with t h e e x c e p t i o n of one-week-old chicks, a n i m a l s c a n t o l e r a t e a m u c h g r e a t e r v a r i a t i o n i n t e m p e r a t u r e than c a n h u m a n s . S t o r a g e of F a r m P r o d u c e F r u i t s and v e g e t a b l e s h a v e t h e i r own o p t i m u m conditions f o r s t o r a g e . In g e n e r a l t h e t e m p e r - a t u r e should b e j u s t above t h e f r e e z i n g point of t h e i t e m being s t o r e d which is u s u a l l y two o r t h r e e d e g r e e s below t h e f r e e z i n g point of w a t e r . T h u s t h e t e m p e r a t u r e should b e a r o u n d 30" t o 32

"

F. T h e r e l a t i v e h u m i d i t i e s r e q u i r e d a r e a l w a y s v e r y high and a r e u s u a l l v i n the 85 p e r - c e n t t o 95 p e r c e n t r a n g e . In addition, a p p l e s c a n b e s t o r e d in s p e c i a l a t m o s p h e r e s i n which t h e a m o u n t s of c a r b o n dioxide and oxygen a r e c o n t r o l l e d . While i n s t o r a g e , f r u i t s and v e g e t a b l e s g i v e off h e a t which m u s t b e d i s s i p a t e d if t h e d e s i r e d s t o r a g e conditions a r e n o t t o b e exceeded. _- T h e a m o u n t of h e a t p r o d u c e d i n c r e a s e s with a n i n - c r e a s e in s t o r a g e t e m p e r a t u r e and m a y b e m o r e t h a n doubled when the t e m p e r a t u r e r i s e s f r o m 3 2 ° F t o 4 0 ° F with p o s s i b l y e v e n g r e a t e r i n - c r e a s e s when t h e t e m p e r a t u r e r i s e s t o 6 0 ° F . F o r m e a t s t o r a g e , t e m p e r a t u r e s of l e s s than 0 ° F a r e r e q u i r e d . T h i s p r o d u c e s t h e u n u s u a l s i t u a t i o n f o r a building in t h a t t h e i n s i d e i s c o l d e r than o u t s i d e f o r the m a j o r p o r t i o n of t h e y e a r . F u r t h e r i n f o r m a t i o n a b o u t t h e d e s i r a b l e conditions under which a n i m a l s should b e housed, p l a n t s g r o w n a n d c r o p s s t o r e d c a n b e obtained f r o m both t h e Handbook of F u n d a m e n t a l s and t h e Applications Volume of t h e A m e r i c a n S o c i e t y of Heating, R e f r i g e r a t i n g a n d A i r

-

Conditioning E n g i n e e r s and f r o m t h e C a n a d i a n C o d e f o r F a r m Buildings. F r o m t h i s b r i e f r e v i e w of t h e e f f e c t s o f , and d e s i r e d conditions f o r , v a r i o u s o c c u p a n c i e s i t c a n b e s e e n t h a t a wide r a n g e of t e m p e r a t u r e s and r e l a t i v e h u m i d i t i e s m a y e x i s t i n s i d e buildings. T e m p e r a t u r e s c a n v a r y f r o m below 0 ° F t o o v e r 1 0 0 ° F and the r e l a t i v e h u m i d i t y f r o m 2 0 p e r c e n t t o 95 p e r cent. Whether t h e s e conditions a r e s p e c i f i c a l l y c r e a t e d and m a i n - t a i n e d o r a r e the b y p r o d u c t of s o m e p a r t i c u l a r a c t i v i t y t h e building e n c l o s u r e m u s t b e con- s t r u c t e d e i t h e r t o a s s i s t in m a i n t a i n i n g t h e m

(13)

TABLE I. 3. RECOMMENDED TEMPERATURE AND HUMIDITY LIMITS FOR CLOSED ANIMAL PRODUCTION

BUILDINGS*

- - - -

-Inside Temp., OF. Inside Relative

C l a s s of Livestock Recommended Range

**

_Humidity,

₇₀

~ eommended Range c Dairy cattle C O W S 2 0 7 5 2 5 7 5 calves 5 0 8 0 2 5 7 5 calves-6wks. 0 8 0 (if d r a f t - f r e e ) Beef c a t t l e 0 8 0 2 5 7 5

Sheep and goats 0 80 5 0 7 5

Swine b r e e d e r s 4 5 7 0 5 0 7 5 finishers 6 0 70 5 0 7 5 piglets 7 0

9

0 5 0 7 5 Poultry chicks ( 1 s t week) 85

9

5 5 0 7 5 hens 2 0 8 5 5 0 75 turkeys 5 0 7 0 50 7 5 Rabbits 20 85 5 0 7 5 H o r s e s 2 0 85 2 5 7 5 Notes

*

Sainsbury, D. Animal Health and Housing. Bailliere, Tindall and Cassell, London 1967. **Lower temperatures may b e tolerated but usually r e s u l t s in increased feed consumption.

At temperatures below 3 2 ° F freezing of s e r v i c e s m u s t be prevented.

o r to tolerate them; but in either c a s e i t m u s t do s o without suffering any damage.

Finally the r e a d e r m u s t be cautioned that while a pound of a i r a t a high temperature contains m o r e heat than that pound a t a low t e m p e r - a t u r e i t does not n e c e s s a r i l y follow that a high relative humidity indicates m o r e m o i s t u r e in the a i r than a low one. For example, a i r a t 7 0 ° F and 30 p e r cent R. H. will contain m o r e m o i s t u r e than a i r a t 0 ° F and 100 p e r cent R.H.

This subject will however be explored m o r e thoroughly in the section dealing with psyc hro- m e t r y and will not be pursued f u r t h e r here.

THE OUTSIDE CONDITIONS

Having decided upon the conditions which m u s t either be tolerated or maintained inside a building the next step in the design sequence i s to find out what the outside conditions a r e likely to be. Unless the building i s in outer space, submerged below water o r buried underground these conditions will b e s e t by the weather conditions a t the site. All buildings which a r e not r a i s e d above ground on columns do in f a c t have p a r t of the building enclosure

on or below ground s o the weather i s not the only factor that m u s t be considered. Never- theless i t i s the predominant one and i t i s a s well to examine it with a reasonable degree of c a r e .

The weather has been defined a s "state of the atmosphere a t a definite time and place with r e s p e c t to heat o r cold, wetness o r d r y - ness, c a l m o r storm, c l e a r n e s s or cloudi- ness. I t According to this definition, the c l e a r atmospheric conditions which p e r m i t the sun to shine through a r e considered a s p a r t of the weather. but the sunshine itself i s not. The sun, however, cannot be ignored and i t s

effects on a building m u s t be taken into account. Thus the outside conditions will be s e t by the weather, the sun, and the s o i l conditions under the ground floor slab or outside the basement walls and floor.

The Weather

L e t us take the definition of the weather and look a t each of i t s c h a r a c t e r i s t i c s in turn. T e m p e r a t u r e

-

Air

The f i r s t of these i s "heat o r cold" which would normally be termed the t e m p e r a t u r e and since we a r e dealing with the atmosphere this m e a n s the a i r temperature. F o r design

(14)

16 I

I

10

20

30

40

50

60

70 TEMPERATURE,

O F

Fig

.

I. 1

Monthly average ground temperatures

measured in clay soil under natural

surface cover at Ottawa between

(15)

purposes, values m u s t be selected which will give the l i m i t s to a range of t e m p e r a t u r e within which the building will be expected to p e r f o r m satisfactorily. A limiting low value f o r winter conditions i s required and a corresponding high value f o r s u m m e r . Should the t e m p e r a t u r e v a r y outside this range some d i s t r e s s both to the occupants and to the f a b r i c of the building enclosure m a y occur.

If

the d e s i g n v a l u e h a s been well chosen this d i s t r e s s will be minor and of s h o r t duration.

The u s e of the a v e r a g e t e m p e r a t u r e during one or m o r e months in the winter would not be s a t i s f a c t o r y a s a winter design t e m p e r a t u r e since the outside t e m p e r a t u r e wouldbe below the a v e r a g e about half the time during the period considered. The lowest t e m p e r a t u r e ever r e c o r d e d i s a l s o unsatisfactory for i t i s usually too s e v e r e . In m o s t c a s e s t h e r e i s no need t o design a building s o that the inside t e m p e r a t u r e will never drop below the design value. The r e s u l t s a r e not catastrophic if a home or office or shop i s unc ornfortable f o r a few hours, or in e x t r e m e c a s e s , even f o r a day or two. With the heating s y s t e m working to i t s maximum capacity the inside t e m p e r a t u r e will drop quite slowly because of the r e l e a s e of heat s t o r e d in the f a b r i c and contents of the building.

This suggests basing the outside design temp- e r a t u r e on the a v e r a g e of the t e m p e r a t u r e s f o r the coldest day in each year, or on the tenth o r fifteenth coldest hour in a n average winter month. The choice depends to s o m e extent on the r e c o r d s that a r e available and on the techniques to be employed in the analysis.

In Canada the hourly t e m p e r a t u r e readings in January f o r ten y e a r s have been s o r t e d by machine f o r a number of stations and tables have been drawn up showing the number of hours a t each t e m p e r a t u r e f o r each station. F r o m the 7440 hourly t e m p e r a t u r e readings a "1 p e r c e n t design temperature" can be selected such that 1 p e r cent of the readings l i e a t or below this value. This m e a n s that on the a v e r a g e in January seven o r eight hours out of the total of 744 would have t e m p e r a t u r e s a t o r below the 1 p e r c e n t design value. T e m p e r a t u r e s selected in this way a g r e e reasonably well with the design t e m p e r a t u r e s a r r i v e d a t by experience in many localities in Canada and the United States. F o r dwellings, this value i s probably unnecessarily low and the corresponding 2 i per c e n t design t e m p e r a t u r e i s a m o r e reasonable value for general use. This means that in a n average January t h e r e would be 18 o r 19 hours with outdoor t e m p e r a t u r e s a t o r below the design t e m p e r a t u r e . If these hours a r e distributed over a few nights they will r e s u l t a t w o r s t in a few hours slightly below 7 0 ° F within the building, m o s t likely in the e a r l y morning.

The problem of keeping a building comfort- ably cool in s u m m e r is s i m i l a r but, a t l e a s t in Canada, i s l e s s c r i t i c a l . Outside a i r t e m p e r a - t u r e s r a r e l y reach 100°F. This i s only twenty five d e g r e e s above the a r b i t r a r y comfort t e m - p e r a t u r e of 7 5 ° F . Summer design t e m p e r a t u r e s c a n be obtained in exactly the s a m e way a s winter design t e m p e r a t u r e s . In Canada the hourly t e m p e r a t u r e s in July a r e generally used, a s July i s the w a r m e s t month in m o s t p a r t s of the country.

Temperature-Outeide Basements

Below g r a d e the situation i s modified con- siderably by the m a s s of s o i l which i s i n t e r - posed between the wall and the outside a i r . Heat flow f r o m the basement is s t i l l ultimately to the a i r but the t h e r m a l capacity of the soil, which i s much g r e a t e r than that of a wall above grade, modifies the effect of the a i r t e m p e r - a t u r e to a significant degree.

On a t e r r a i n devoid of s t r u c t u r e s the ground s u r f a c e t e m p e r a t u r e v a r i e s cyclically with the annual weather cycle. The variation i s a l m o s t

sinusoidal and i s reflected below g r a d e with a n amplitude that d e c r e a s e s with increasing depth until, a t about 3 0 t o 50 ft, the t e m p e r

-

a t u r e r e m a i n s essentially constant throughout the year a t a value called the m e a n annual ground t e m p e r a t u r e , Tm. If the s u r f a c e v a r i a - tion h a s a n amplitude, A, then the maximum and minimum s u r f a c e t e m p e r a t u r e s will be

Values of Tm and A will v a r y with geographic locations and s u r f a c e cover.

Because of the t h e r m a l diffusivity of the soil, variations in subsoil t e m p e r a t u r e lag m o r e and m o r e in time behind s u r f a c e t e m p e r a t u r e s a s depth i n c r e a s e s . At depths of 10 to 15 f t this time lag can generally be m e a s u r e d in months. F i g u r e I. 1 shows values that i l l u s t r a t e the t i m e lag involved. T h e r m a l diffusivity i s a m e a s u r e of the r a t e a t which a change in t e m p e r a t u r e will s p r e a d through a body. It i s proportional to conductivity and inversely proportional to the volumetric heat capacity.

The construction of a building with a heated basement establishes a new t e m p e r a t u r e r e g i m e in the surrounding s o i l which v a r i e s with the annual periodic fluctuation in ground surface t e m p e r a t u r e . The new t e m p e r a t u r e r e g i m e can be determined by combining t h r e e s e p a r a t e t e m p e r a t u r e effects. The f i r s t and basic t e m - p e r a t u r e component i s the mean annual ground temperature, Tm, a t the s i t e of building. The second t e m p e r a t u r e component i s that r e s u l t - ing f r o m placing in the s o i l a t t e m p e r a t u r e Tm a heated basement a t t e m p e r a t u r e T i . The

(16)

Fig.

1.2 Ground temperature variation

10 cm below the surface.

Lines of equal amplitude.

(17)

t h i r d t e m p e r a t u r e c o m p o n e n t i s the influence of t h e f l u c t u a t i o n s in g r o u n d s u r f a c e t e m p e r a t u r e d u e t o c l i m a t i c c o n d i t i o n s and f o r t h e p u r p o s e of c a l c u l a t i n g t h e m a x i m u m h e a t l o s s f r o m the basement, t h e condition of i n t e r e s t i s Tm

-

A. I n f o r m a t i o n about Tm i s n o t r e a d i l y a v a i l - a b l e f o r m a n y l o c a t i o n s i n Canada, but a s i t i s r e l a t e d t o t h e m e a n annual a i r t e m p e r a t u r e , Ta, which is a l w a y s s l i g h t l y c o l d e r than T m . a s u i t a b l e e x t e r n a l d e s i g n a i r t e m p e r a t u r e will be given by s u b t r a c t i n g t h e a m p l i t u d e of ground s u r f a c e t e m p e r a t u r e variation. A, f r o m Ta, i. e. Ta

-

A. T h i s s u b s t i t u t i o n will p r o b a b l y b e s a t i s f a c t o r y f o r p r a c t i c a l p u r p o s e s when one c o n s i d e r s t h e m a n y v a r i a b l e s involved, e. g. the p r e s e n c e o r a b s e n c e of snow cover. r a d i a n t h e a t e x c h a n g e a t the ground s u r f a c e , and t h e l i m i t e d knowledge of p r e c i s e v a l u e s of both Tm a n d A.

T a b l e I. 4 g i v e s v a l u e s of Tm. A, and Ta under n a t u r a l snow conditions f o r v a r i o u s l o c a t i o n s in Canada a n d c a n be u s e d in conjunc- t i o n with the m a p given in F i g u r e I. 2 t o e s t i - m a t e a d d i t i o n a l v a l u e s of A. T h i s m a p is p a r t of one p r e p a r e d by Jen-Hu Chang giving a n n u a l r a n g e i n ground t e m p e r a t u r e a t a d e p t h of 10 c m (4 in.). I t m a y be s e e n that, in Canada, v a l u e s of A l i e within a r a n g e of 1 0 ° F . Values of t h e m e a n a n n u a l a i r t e m p e r a t u r e , T a , c a n be obtained f r o m l o c a l m e t e o r o l o g i c a l r e c o r d s . Rain, Snow and Humidity

F r o m t i m e t o t i m e i n v a r i o u s c o u n t r i e s a t t e m p t s a r e m a d e t o develop s o m e m e a s u r e of the i n t e n s i t y of driving r a i n which h a s t o b e r e s i s t e d by t h e building e n c l o s u r e by combining t h e r a t e of r a i n f a l l with t h e wind s p e e d a t t h a t t i m e . Until s u c h t i m e a s r a i n p e n e t r a t i o n r e s i s t a n c e c a n b e a s s e s s e d with a g r e a t e r d e - g r e e of confidence than i t c a n a t p r e s e n t , however. t h e s e d r i v i n g r a i n i n d i c e s a r e of only l i m i t e d u s e . They c a n b e u s e d a s a b a s i s f o r s e l e c t i n g a component s u c h a s a window f o l - lowing a t e s t a s a n indication of t h e d e g r e e of c a r e t h a t m u s t b e t a k e n in t h e d e s i g n a n d d e - tailing of t h e building e n c l o s u r e . O n e c a n d e t e r m i n e t h e h e a t flow o r t e m p e r - a t u r e g r a d i e n t t h r o u g h the building e n c l o s u r e by c a l c u l a t i o n p r o v i d e d r e a s o n a b l y a c c u r a t e i n s i d e a n d o u t s i d e t e m p e r a t u r e s a r e a v a i l - able. T o a s s e s s the r e s i s t a n c e t o r a i n p e n e - t r a t i o n however, n o c o m p a r a b l e m e t h o d s of c a l c u l a t i o n e x i s t . As w i l l b e s e e n l a t e r , the m e t h o d s used t o c o n t r 01 r a i n p e n e t r a t i o n a r e m o r e o r l e s s the s a m e r e g a r d l e s s of the in- t e n s i t y of t h e r a i n f a l l . S o m e intuitive f e e l i n g n e e d s t o b e developed f o r t h e conditions a t t h e e x t r e m e s of the r a n g e of r a i n f a l l i n t e n s i t y but l i t t l e m o r e i s n e e d e d a s f a r a s t h e d e s i g n of t h e building e n c l o s u r e is c o n c e r n e d . M o s t buildings a r e , a t s o m e t i m e o r a n o t h e r , s u b - j e c t e d t o r e a s o n a b l y h e a v y r a i n f a l l s a c c o m p a n i e d by s t r o n g winds a n d t h e y m u s t e x c l u d e the r a i n a t t h e s e t i m e s j u s t a s w e l l a s o t h e r buildings l o c a t e d w h e r e t h e s e c o n d i t i o n s o c c u r m o r e f r e q u e n t l y .

If one could b e c e r t a i n that h e a v y r a i n s would n e v e r o c c u r a n d t h a t the building would n e v e r b e s u b j e c t e d t o f r e e z i n g c o n d i t i o n s a f t e r t h e envelope h a s s o a k e d up c o n s i d e r a b l e a m o u n t s of w a t e r t h e n one could u s e a s o m e w h a t d i f f e r - e n t d e s i g n . And f o r s o m e o c c u p a n c i e s s o m e d e g r e e of r a i n p e n e t r a t i o n i s a c c e p t a b l e . One a u t o m a t i c a l l y a c c e p t s t h i s philosophy when one builds a c a r p o r t r a t h e r t h a n a g a r a g e ; v e r t i c a l l y falling r a i n will b e excluded but wind-driven

r a i n will not. T h e m a j o r i t y of buildings in Canada, however, m u s t be d e s i g n e d with a v i e w t o excluding t h e r a i n c o m p l e t e l y .

On the o t h e r hand the i n t e n s i t y of r a i n f a l l is of g r e a t i m p o r t a n c e i n the d e s i g n of the d r a i n - a g e s y s t e m and a l s o , i n s o m e c a s e s , i n the s t r u c t u r a l d e s i g n b u t both of t h e s e m a t t e r s a r e o u t s i d e t h e s c o p e of t h i s book. T h e s u b j e c t of snow is i n m u c h the s a m e c a t e g o r y . Snow l o a d s a r e of g r e a t i m p o r t a n c e t o t h e s t r u c t u r a l d e s i g n e r and h e m u s t e x e r c i s e g r e a t c a r e when c o n s i d e r i n g t h e p r o b a b i l i t y of snow d r i f t i n g f r o m one p a r t of t h e s t r u c t u r e onto a n o t h e r . T h e d e s i g n e r of t h e building e n c l o s u r e m u s t a l s o c o n s i d e r this m a t t e r f o r i n m o s t c a s e s h e is t h e one who w i l l s a y what s h a p e t h e o v e r - a l l s t r u c t u r e will t a k e . He will d e t e r m i n e t h e r e l a t i v e s i z e and l e v e l s of the v a r i o u s c o m p o n e n t s on t h e building, what p a r a p e t s will b e built, the s i z e and l o c a t i o n of c a n o p i e s , b a l c o n i e s and s u n s h a d e s ; a l l of which c a n influence l o c a l snow l o a d s . He should a l s o g i v e thought t o t h e p o s s i b l e e f f e c t s of w e t snow which m a y cling a n d build up in s o m e l o c a t i o n s . When i t m e l t s i t m a y r e l e a s e c o n s i d e r a b l e q u a n t i t i e s of w a t e r t o w e t the building e n c l o s u r e with t h e d a n g e r of s u b s e q u e n t d a m a g e on f r e e z - ing. At t h e o t h e r e x t r e m e v e r y f i n e powder snow c a n b e blown i n t h r o u g h q u i t e s m a l l openings. C a s e s a r e on r e c o r d of s n o w being blown i n t o a t t i c s in t h i s way and, a f t e r m e l t i n g . d a m a g i n g t h e c e i l i n g below. A s with r a i n f a l l , t h e d e s i g n e r of t h e building e n c l o s u r e s e l d o m n e e d s a n y e x a c t f i g u r e s t o u s e in c a l c u l a t i o n s . He should, however, t r y t o develop s o m e s o r t of i n t u i t i v e feeling f o r t h e behaviour of snow and t h e way i t c a n d r i f t and a c c u m u l a t e .

Snowflakes of falling snow c o n s i s t of i c e c r y s t a l s with t h e i r well-known c o m p l e x p a t - t e r n s . Owing t o t h e i r l a r g e r a t i o of s u r f a c e a r e a t o weight t h e y f a l l t o the ground r e l a t i v e l y slowly. F r e s h l y f a l l e n snow is u s u a l l y v e r y l o o s e and fluffy, with a s p e c i f i c g r a v i t y of about 0.05 t o 0. 1 (1 120th t o 11 10th of w a t e r ) . I m m e d i a t e l y a f t e r landing, however, t h e snow c r y s t a l s s t a r t t o change: t h e thin, n e e d l e - l i k e

(18)

TABLE I. 4 VALUE O F Tm, A and Ta UNDER NATURAL SNOW CONDITIONS

P l a c e

Swift Current, Sask. Guelph, Ontario Ottawa, Ontario Toronto, Ontario

Ste. Anne d e l a Pocati'ere, Que. Fredericton, N. B.

Charlottetown, P. E. I. St. John's, Nfld. Saskatoon, Sask.

projections begin to sublime and the c r y s t a l s gradually become m o r e like s m a l l i r r e g u l a r l y shaped grains. This r e s u l t s in settlement of the snow and a f t e r a few days the specific gravity will usually have increased to about 0.2. This compaction i n c r e a s e s further with time and specific gravities of about 0.3 will often have been attained a f t e r about a month, even a t below-freezing t e m p e r a t u r e s . Longer periods of w a r m weather a s well a s r a i n falling into the snow (a possibility that m u s t be in- cluded in determining design loads) may in- c r e a s e this density even f u r t h e r .

As a simple r u l e f o r estimating loads f r o m snow depths the specific gravity can be considered to be about 0.2 to 0.3. In other words, each inch of snow r e p r e s e n t s a load of about 1 to 1 pounds p e r s q u a r e foot, depending mainly on the age of the snow.

The t h i r d f e a t u r e covered by the t e r m "wet- n e s s or dryness" i s the humidity of the a i r . Everyone blames the humidity for t h e i r d i s - comfort on hot muggy s u m m e r days. What i s this humidity that i s talked about? Suffice i t to say f o r now that the humidity of the a i r can be taken a s a m e a s u r e of the amount of water vapour in the a i r . Usually this i s not expressed in absolute t e r m s of s o many pounds of water vapour p e r pound of d r y a i r but a s a m e a s u r e

of the amount of water vapour in the a i r r e l a t i v e to the amount the a i r i s capable of c a r r y i n g a t that t e m p e r a t u r e . Thus we get the r e l a t i v e humidity expressed a s a percentage. F o r example, a i r a t 50 p e r cent

R.H.

h a s half of the amount of water vapour which i t i s capable of c a r r y i n g a t that t e m p e r a t u r e .

Thus when dealing with the design of the building enclosure and the associated problems of condensation i t i s v e r y largely meaningless to give the relative humidity a s a percentage

without giving the corresponding a i r t e m p e r a t u r e . The quantity of water vapour in the a i r can be expressed directly a s the weight of water vapour contained in unit weight of d r y a i r . This i s called the humidity r a t i o o r mixing r a t i o and i s usually given in g r a i n s of water vapour p e r pound of d r y a i r . L e s s d i r e c t methods of giving the quantity of water vapour in the a i r a r e to give the vapour p r e s s u r e or the dewpoint t e m p e r a t u r e . F o r any given m i x t u r e of a i r and water vapour in the a t m o s p h e r e the humidity ratio, vapour p r e s s u r e and dewpoint t e m p e r a t u r e will r e m a i n constant f o r a l l a i r t e m p e r a t u r e s above the dewpoint t e m p e r a t u r e . Below that t e m p e r a t u r e water vapour will condense out of the a i r and, naturally, the values will change. At Ottawa, for example, the

m e a n monthly r e l a t i v e humidity i s a t a maximum of 78 p e r c e n t in December, dropping slightly (to 75 p e r cent) in January, and to a minimum of 58 p e r cent in May, and rising to 66 per cent and 68 p e r cent in July and August respectively. The humidity ratio, however, i s a t a minimum of 8 grains/pound of dry a i r in January and F e b r u a r y rising to a maximum of 69 g r a i n s / l b in July. Thus we s e e that although the m e a n relative humidity m a y be higher in winter than in s u m m e r the quantity of water vapour in the a i r i s l e s s . This v e r y g e n e r a l presentation of relative humidity will be discussed in detail in the Section under "Psychometry" in Chapter

LII.

Wind

The third a s p e c t of the weather that m u s t be considered i s the s t a t e of

the

_{a t m o s p h e r e} with r e s p e c t t o c a l m or s t o r m ; or, in a word, wind.

Wind usually r e f e r s to the movement of a i r p a r a l l e l to the e a r t h ' s s u r f a c e and f o r building design purposes we a r e concerned only with winds in the lowest few hundred f e e t of the a t - mosphere. The roughness of the e a r t h ' s surface,

(19)