The principles and dilemmas of designing durable house envelopes for the North

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The principles and dilemmas of designing durable house envelopes for

the North

Latta, J. K.

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ISSN 0701-5216

TRE RINCIPLES AND DILEMMAS OF DESIGNING

85-

n"

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by J&. Latta

Division of Building Research, National Research Council of Canada

TABLE OF CONTENTS

Introduction

The Functioo of the B u i l d i n g Envelope The Durability of Materials

Psychrometrg

-

Haw Does Condensation Occur in the Envelope? Dif fusf on

Air Movement

A i r Pressures an the Building Envelope Wind

Temperature

VentilatLan Systems

Vapour Barriers, A f r Barriers and Air-Vapour Barrlters Characteristics Required of an Air Barrier

Locatim of the A i r Barrier The W i n d Barrier

THE PRINC.IfLES AND DIL-S OF

DESIGNING

DURABLE HOUSE ENVELOPES FOR TEE NORTH b-Y

J.R. Latta

Starting a prime cause

are discussed

from the premise that condensation In the building envelope is of its d e t e r i o r a t i o n , the mechanisms that cause condensation and control measures explained. The conflicts that arise between some of these measures, the probability of achieving them under realistic construction conditions, and the p o s s i b l e need for f a i l - s a f e provisions should complete success not be achiived, are described.

INTRODUCTION

The keg t o durable buildings fn the North is the control of water in a l l its phases; s o l i d , liquid and gaseous, with particular emphasis on

control of water in the gaseous phase. This is effected by controlling heat flows, and a i r pressures and flaws. It is through an understanding of these phenomena and of some concepts of design derived from them t h a t durable

buflding envelopes can be created.

Obviously structural damage due t o overloading can be a problem but, except for damage: caused b y . f o u n d a t i o n failure, the reasons f u r a component breaking or being displaced due to overloading are usually clear. In mast c a s e s , the magnitude of the loads that mast be resisted can be estimated i n

advance and adequate strength b u i l t i n t o the building either by engineering d e s i g n or by conventional practice. Damage due t o frost heave or the

melting of permafrost i s related to heat flows and can be avoided by controlling such f l o w s . This will not be considered further here.

On

effects of water the causes are not always clear and it may be difficult to

select s u i t a b l e remedies. In many instances precise design procedures are not available and conventional construction practice may not be

satisfactory. In such cases, the designer =st work with basic p r i n c i p l e s of physics. Be may not be able to prove that one design i t 3 supexfor to

another but if such principles are followed they w i l l probably lead to t h e construct3on of a durable building. This is not exactly t h e sort of

philosophy that one would expound t o an astronaut before launching h i m i n t o

Epsce in a space capsule, but in more rrmndane affairs it may have to

suffice. If the bnildlng is needed it mst be hilt with whatever knowledge and technology are available at the t i m e . Failures due to deterioration can be annoying and inconvenient as well as c o s t l y to repair, but they seldom lead to injury or loss of Iffe.

THE FUNCTION OF

THE

In the vast majority of cases otre builds a h f l d i n g to protect the occupants m d contents from inclement weather. In some cases protection is a l s o needed against animals or insects and against thieves

or

most cases the i n i t i a l requfrewnt is for protection from the weather. This protection i s given by separating t h in s i d e from the uncontrolled, and in most respects uncontrollable, conditions outside so that the conditions i n s i d e cm k modified and cmntrolled to so= extent. The basic function of

the walls, roofs and floors of the building is to effect t h i s separation m d

they can c o l l e c t i v e l y b~ referred t o 9s the building envelope.

It

control the f l w

of

over-stressed, for failure t o control the movement of air through, and withh, the thickness of the b u i l d f n g envelope can lead t o serf- b u i l d i n g deterioration, as w e l l as faflure of the envelope t o perform i t s intended functim s a t i s f a c t o r i l y

.

differences in temperature can, in many cases, be of greater importance because they act steadily in me dfrection as long as a temperature

difference exists. Such temperaturn differences are not only between the

inside and outside of the b i l d i n g but a l s o exist within the thickness of the envelope. Thas the air flows need not be rf ght through the building envelope ht can be within its thickness.

THE DURABILITY W MATERIAIS

It: is natural for a designer or builder wha has had a bad experience with a particular mteri a1 t o d e c i d e never to use it again. That, after a l l , is called learning by experience. The trouble is that experiences often appear t o contradict ~ c other; a material h that f a l l e d an one buildiug perfonas quite satisfactorily c#l another. Thus, as experience grows, It becomes clear that the material failed because sf t h e particular manner in which it was used; o m m d i f ies the blanket cande-mnatian axul d e c i d e s that one won't use that material in that nranner

wain*

withstand.

Several factors, either slngly OK in combinatf on, affect the behaviour

of building materials; the three principal ones are w a t e r , emperarure and ultra-violet radiation. Of these three, water is probably the mst

important. There are other factors, h t in the absence

of

c h a n g ~ that must be allowed for if damage is to be avoided, Finishing materials m y ?E d i s f i g u r e d by water staining and, in extreme cases,

The moisture content of many materials is affected by the r e l a t i v e humidity t o which they are exposed, and mould growth can occur at high

relative humidities without the material being wetted by l i q u i d water. Thus water in i t s vapour phase can be r e s p o n s i b l e f o r physical deterioration. However the conditions appropriate f o r such deterioration can be changed quickly by a change in temperature or a small increase in a i r movement, The more serious cases of deterioration occur when the vapour condenses and soaks the materials. It i s much more d i f f i c u l t , and takes a considerable period of t i m e , t o dry o u t the materials once they have been wetted; by that

t i m e irreversable damage may have taken p l a c e .

&ring t h e long periods of cold weather in the North, water may

accumulate w i t h i n the fabric of the building envelope as snow, hoar frost or

ice. Probably little damage w i l l occur, b o t h because the materials w i l l not be wetted appreciably and because the low temperatures will inhibit wood rot and corrosion. As t h e outside temperature moderates, the accumulated snow

and i c e melt, leading to severe wetting of the materials at a time when warmer temperatures a l l o w corrosion and rot to proceed. In southern Canada some periods of r e l a t i v e l y mild weather during the winter usually release

any accumulated water and a l l o w the materials to dry o u t . The southern

s u m m e r s a l s o are r e l a t i v e l y long and warm and, in most cases, g i v e ample opportunity for drying. In the North, on the other hand, the summers may be

so short and cool that water accumulated during the previous winter cannot dry out and is carried over into the following winter; t h i s causes a

progressive b u i l d u p of water in the building envelope.

If one can control t h e water, one will go a long way towards making a durable building. But s i n c e one cannot r e l y upon s u i t a b l e periods of drying

to reduce the concentration of water in t h e building envelope, the amount of water that enters must be reduced to a minimum. Any additional measures to

e n a b l e water in t h e envelope to escape t o the o u t d o o r s constitute, i n

effect, a safety valve. Such safety valves, or fail-safe mechanisms, should be p r o v i d e d , but i f no water gets t o the problem location there will be none to escape. A skydiver has two parachutes; one that he intends to use and

one that he hopes he never has to; but i f the need should a r i s e , it is v i t a l . So it is w i t h designing a durable b u i l d i n g envelope; design i t so

t h a t water cannot g e t in, but if it should get in, provide a way for it to get out.

One can reduce the q u a n t i t y of water that enters the envelope by reducing the supply of water a t i t s source. T h i s has only l i m i t e d a p p l i c a b i l i t y in most northern houses, but it should not be ignored. O u t s i d e the building, water e x i s t s as rain and snow, which are not controllable. In the North fine wind-driven snow is probably the m o r e

troublesome. Inside the b u i l d i n g , w a t e r comes from many sources such as cooking, washing, people, wet materials and the unvented products of

combustion of gas-burning a p p l i a n c e s . Water from such sources becomes water vapour carried by the air. To control such sources may require some

modification of the living habits of t h e occupants. This is not something t h a t the designer can control and it would be unwise to r e l y upon it f o r a

permanent s o l u t i o n . On the other hand, provision c o u l d be made t o vent

moisture-producing appliances to the outdoors. P r o v i s i o n can also be made

to increase the rate o f v e n t i l a t i o n by mechanical means when the l e v e l of

The principal method of controlling the accumulation of water in t h e envelope is to reduce the rate at .which It is transferred t o any location where it may cause a problem. Much of t h f s water is deposited as

condensation and aa i f one wants t o make a durable b u i l d i n g , it is fmportant that the phenomenom of condensation be understood.

PSYCHROMETRY

-

Pspchrornetry is the branch of physics concerned with the physical and thermodynamic properties of mixtures of air and water wapour.

Water vapour I s one of several gaseous con~tituents of alr; the other principal ones are nitrogen, oxygen and carbon dioxide. Each exert6 its own partial pressure in proportion t o the amount of gas present; the sum of the

pressures makes up the total or barometric pressure of the air.

The maximum amount of water that can e x i s t i n the gaseous s t a t e

ivapour) In a given quantity of air is l i d t e d by t h e temperature. Thus, if any air-vapour mixture is cooled, a temperature will be reached at which it will be saturated, and if cooling is continued below this point, water w i l l condense. If the temperature at whfch the air becomes saturated ( t h e dew point) is above the freezing point, the vapour will condense t o a liquid; if

it is below freezing, the vapour w i l l condense as ice in the farm of hoar frost* For practical purposes,

in

between the mass of water vapour actually present per unit volume of a i r and the mass per unit volume it can contain when saturated a t the same

temperature may be taken as the r e l a t i v e humidity of the a i r .

Zr

quantities of vapour in t h e air, the relative h u d d i t y is a l s o given by the r a t i o between the actual vapour pressure and the saturation vapour pressure

a t the same temperature. Thus, if the temperature and r e l a t i v e humidity are k n m , the dew point temperature can be determined. First t h e actual vapour preasure of the air-vapour mixture is obtained from the product of the

relative humidity (expressed i n decimal form) and the saturation vapour

pressure for the given temperature obtained from psychrometric tables. Then the dew p o i n t is located i n the tables as t h e temperature that has a

saturation vapdur pressure equal to t h i s actual vapour pressure.

A

in

graphical representation of all p o s s i b l e conditions w i t h i n the temperature and humidity range for which the chart is constructed. One design of such a chart i s shown in Figure 1- The vertical scale represents the absolute

moisture content, defined as the number of kilogram of mo&sture per kilogram of dry air. The horizontal scale is the air temperature, scaled

from -20°C to +5S°C. The saturation curve

ClOOE

of the amount of moisture it could p o t e n t i a l l y hold (Figure 3 ) . This chart is often used t o calculate the dew point temperature of the i n s i d e air; from t h i s one can calculate the required thermal resistance of an assembly (such as a window) t o prevent condensation on its surface.

For example, suppose that the air i n a room is at 23°C and 50% HI; what would h the dew point temperature of the air? First, p l o t the intersection

of the inside air temperature with the measured relative humidity

(Figure 4). Second, move horizontally to the l e f t to intersect the 100% RH curve. T h i r d , project t k intersection downward until it intersects the temperature axis once mre, at about 12% Thfs is the dew point

temperature of the inside air* Therefore, if condensation appears on the surface of window glass, the temperature of the g l a s s r m s t be below U°C.

Conversely, w i t h an outside temperature of -$O°C a d a 25 km/hr wind and with a mom temperature of 21mC, the i n s i d e surface temperature of the c e n t r e of a double glazed windm w i l l be about 0%; then the rnaxtmum room relative humidtty kfore condensation occurs on the window is about 25%

( F i g u ~ 5 ) . Since the glass temperature w i l l

M

a surface temperature of about +;F°C, with lower values at the b o t t a . Thus the only practical way t o prevent condensation on windows in the North is to

b l w warm dry air over them, i-e. to reduce the relative humidity l o c a l l y . Since there is no "free lunch", this solution involves the extra cost of an

j -

-.---

+ - - 1 9 ? ~

- - -

HOW D O B COMDENSATIW OCCUR TM TJ33 EWELOPE?

For many years the response to the problem of water condensing in the buLlding euvelope has been to specify that a vapour barrier be installed on

the warm side. Unfortunately j u s t what mechanisms far vapour movement were

t o be controlled by the vapour barrier were not made clear, bst any s t a n d a r d for a vapour barrier material defined it on the basis of its a b i l i t y to resist the diffusion of vapour. No mention was made of the need t o a n t r o l air movement and

as

It is urn recognized that vapour d f f f u s i m is a relatively weak mechanism for moving water vapour into or out of the building envelope as

compared t o the movement of vapour by a current of air. Nevertheless Z t

will be d i s c u s s e d f i r s t in order to c l a r i f y the function af a vapour barrier before proceeding to a discussion of the m o r e important subject of a i r

leakage. D i f f u s i o n

When

p o i n t s , there will be a corresponding difference in vapour pressure. This w l l l cause a f l a * of water vapour from the point of higher concentration to

difference exists between two s i d e s of a material, the water vapour will diffuse through the material at a rate determined by the vapour pressure difference, the l e n g t h of the f l o w path, and the permeability t o water vapour of the particular material.

Vapour f l o w by d i f f u s i o n through a building envelope because of a

df fference in vapour pressure is very similar t o heat flow because of a

temperature difference, but with one major d i s t i n c t 5 a n . T h e maximum possible vapour pressure at any location is set by the saturation vapuur pressure at the location. Thus if the vapour pressure gradient through the envelope, given by consf deration of the inside and o u t s i d e vapour pressures and the vapour flow resistance of the varfow materials, c a l l s for a vapour pressure at any point that is above the saturation vapour pressure at that point, then condensation w i l l take p l a c e . This w i l l reduce the vapour concentration at all locations u n t i l the vapour pressure is reduced to the saturation vapour pressure. To prevent t h i s s i t u a t i o n from a r l s l n g

materials with high resistance t o vapour flow Cine. vapour barriers) are added near the high vapour pressure s i d e to depress the vapour pressure gradient to b e l o w the saturation vapour pressure gradient. A t the same

t i m e , materials w i t h low resistance t o vapour flow (e.g, breather membranes) are used on the low vapour pressure s i d e .

If the envelope is built: following these p r i n c i p l e s , to ensure that condensation as a result of vapour diffusion does not take place even under

the most adverse conditions, there w i l l always

t~

diffusion capacity to dry out any water that may accurnmlate from any other

cause. T h i s would rule out the use of som materials as sheathing d e s p i t e t h e i r othervise desirable properties. The situation i s n o t , however, one to

which r u l s of thumb should be a p p l i e d without thought. Insulating

materials with vapour permeabilities that would preclude them from being used as sheathing w i l l also raise the temperature of the i n s i d e face of the sheathfng. This will raf se the saturation vapour pressure at that location, which will help t o keep the saturation vapour pressure above t h e vapour

presstlre d i c t a t e d by c o n s i d e r a t i o n s of continuity of vapaur f l o w . Such

materials could s t i l l be used provided that a highly impermeable vapour barrier is used on the high vapour pressure side.

Vapour d i f f u s i o n is not a very powerful mechanism for moving water

vapour into the envelope but, because of the long periods of cold i n the North, it should not be neglected.

A i r Movement

In many respects a l l the variolls constituents of air act together a s though only one gas were present. The temperature of the q g e n will not be different f rum that of the nitrogen, and when a current of air moves the

carbon d i o x i d e , it w i l l also mve the water vapour. W s u n i f o r m i t y of temperature and movement among the various constituents was Implicit in the discussion of psychromet ry and condensation; where the air went, t h e water vapour went too; when one was cooled, so was the other.

Because of the small amount of water vapmr that cold outdoor air i s c a p a b l e of holding (see Figure 2), it frequently has a high relative

humidity, 802 RH or more. When it 5s warmd up to the building temperature, its RH drops to less than 10% and it w i l l be considered very dry although

nothing has actually changed except i t s temperature (A to B, Figure 6 ) . Water w i l l evaporate f r a m many sources within the building, including the occupants and their various activities, and from damp materials within the building, and w i l l raise the moisture content of the air ( 8 to C, Figure 6).

Noa, since the air in the b u i l d i n g does not remain stagnant but m s t be changed to provide needed ventilation, this humidified

&r

a higher misture content than it can carry at the outdom air temperature, t k r e is great potentfa1 for condensation to take place. The t r i & in msltag a durable btlilding is t o ensure that t h i s condensation does not take place where it w i l l be harmful,

If the b u i l d i n g envelope is completely sealed, then all of the a i r c k n g e w i l l take place through specially provided openings, probably via a mchanical v e n t i l a t i o n system. If the outlet duct from such

a

tetmlnates on the outer surface of the envelope, 611 is heavily insulated t o

i t P extremity, the outgoing air may remain above the dew point and

candensation w i l l not take place i n the duct i t s e l f . In such a case, the condensation w i l l Ize in the form of fog in the outdoor air, whfch may

collect as hoar frost around the ventilation outlet. The effect of d n d pressures on the inlet and exhaust d u c t s may rake it undesirable to

terminate the exhaust duct on the surface af the building envelope, but this question w i l l not be considered further here.

In practice, buildings are seldom completely airtight and so= leakage

takes place through the envelope. U n t i l r e l a t i v e l y recently, t h i s leakage

wrrs r e l i e d upon to provide the ventflation air and also the air required by cmbustion appliances, such as furnaces and stoves. Only in the last f e w gears have attempts been made to reduce the alr leakage of b u i l d i n g s as an energy economy masere; this has been advocated for years as the Bey t o

W l b i n g envelope durability. The i n s t a l l a t f u n of exhaust fans t o improve air quality in bathrooms and kitchens by exhausting contaminants from these lbcatfons, and the ducting of clothes d r y e r exhausts to the outdoors, a l l t a c i t l y assume that openings are available to allow air t o enter. With tighter building envelopes, t k major available openfng may k the chimney that is intended to exhaust the products of combustion from a furnace or a fireplace. Such chimneys may then b backdrafted, e s p e c i a l l y 'at s t a r t v p or l w b r n i n g rates, and the products of combustion &awn into the b i l d i n g . However this subject is also outside the scow of the present dtscussion of

M l d i n g durabilty.

What must be recognized is that there is a balance between the

qusntit ies of air entering and leaving the building.

M r

I s cold and hss little moisture in it, whereas air leaving d l 1 be warm and w i l l , in rmst instances, be considerably more mist. S h w l d this warm moist air le& out through the b u i l d i n g envelope, it w i l l p a s s through regions that are progressively colder. At some point it w i l l come fn contact with a cmponent in the envelape that fs b e l o w its dew point. When this happens,

s u e of the water vapour in the aft w i l l condense (C t o D to E, Figure 6 ) . H m much w i l l condense w i l l vary with the circumstances. If the leakage

pacrh is short and d i r e c t , the air m y flow rapidly to the out doors without

mu& v a p m condensing; j u s t as it do- in art exhaust duct from a fan. On

the other hand, if the air follows a tortuous path and flows over the inner

it to be cooled to the temperature of the sheathing a d large quantities of vapour d l 1 condense.

The foregoing discussi~n a s s u w that the air f l a r r is right through the building envelope from inside to outside, and t h i s i s probably the mjor

type aE air flar that causes condensation. Bowever, a convective afr flow that moves warm humid air from inside the h i l d i n g , cools it by contact with the colder portions of the envelope, and then leads it b a a fnto the

occupied spaces, can also l e a d to condensation. _{This is} a point to be considered when choosing a location fur the air barrier.

AIR PRESSURES ON TAE B U I L D I E ENVELOPE

For air to move f ran o w locatiun t o another, there must be a force t o

move it and a passage d o n g which it can move. Elimfnate e i t h e r one a i d the air stays where it is. The forces that may move air into and through the envelope are the differences in air pressure, which can b~ produced in a

variety of w a y s . Wind

Wind blowing on the b u l l d i n g w i l l produce a complicated d i s t r i h t l a n of. pressures and suctions. Fortunately, with regard to the mvement of

moisture into the building envelope, it is not necessary t o d e s c r i b them in d e t a i l . It is o n l y necessary to recognise two or three basic p r i n c i p l e s in order to get a general p i c t u r e of the wind pressure d i s t r i b u t i o n . The wind wfll be deflected over, under and round the b i l d i n g , and wherever there is

a change in directim ar in the speed of flow, there will be a change in air

pressure. If the change in direction is convex towards the h i l d i n g there

w i l l be en increase in pressure; if it is concave towards the building, there w i l l be a reduction in pressure. Thls will,

in

of

energy. With an elevated building the reduced passage under the building can IE seen clearly, but there are similar invisible reductions over and round the b u i l d i n g . So= distance away frm the building the air can be considered as flowing on undisturbed, and t h i s position of undisturbed flaw

can be taken as the outer, invisible w a l l of the reduced air f lar passage

(Figure 8). (It I s this speeding q of the air flow through the invisible pass-a@ over the top of an airplane w i n g that creates the necessary reduced pressure an the wing t o keep the plane fa the air.) D e t a i l s of wind load coefffcients m structures are given in the Supplement t o the Matliorral

Building b d e of Canada, together with procedures for the ~ l c u l a t i a n of the

maximum wind-induced loads on bulldings

.

W i n d pressures [ p o s i t i v e or negative) osl the outside Q$ a building w i l l be transmitted to the i n s i d e through any major openings i n the h i l d i n g envelope, Thus where a door, for example, is open on the leeward side of a

b u i l d i n g , the force t o be withstood by a wall on the windward side w f l l k the sum

of

Temperature

Differences in a i r pressure are a l s o caused by differences in

temperature. When considering the building as a whole, t h i s is generally referred t o as stack e f f e c t , although there w f l l also be pressure

d i f f e r e n c e s between the room and the envelope and within the envelope itself. Stack effect or convective air flows are not so dramatic as wind forces but they ney be much more damaging to a b u i l d i n g since they act

s t e a d i l y in one direction over long periods of time. This is p a r t i c u l a r l y

important In the North because of the long p e r i o d s of cold.

When air is heated i t expands, so each mbic foot of heated air is l e s s dense than the same volume of unheated air. ff two columns of air of q u a 1 height, one warm and the other cold, are p l a c e d s i d e by side, the denser t o l d a i r w i l l exert a greater pressure at the b t t o m than w i l l the Ughter warm air. I t will undercut the warm air, causing i t t o rise; this p r o d u c e s

the familiar draft: up a chimney, with w l d air entering at the fireplace, being heated and leaving at the chimney p o t . During cold weather a similar action occurs i n buildings, although the insf de-to-outsf de air temperature difference i s much less than in a chlmney: air flows in a t the bottom, is warmed, rises and flows out at the t o p .

S i n c e a i r f l o w s from high to law pressure areas, the pressure outside must be higher than that inside at the bottom and lower than that i n s i d e at the top. Thus, the pressure difference through the building envelope

changes from positive to negattve at some point in the height of t h e building and there must be a l e v e l at which this pressure difference is zero. A plane through t h e points on the b u i l d i n g perimeter of zero pressure d i f f e r e n c e is called tk neutral pressure plane.

If

t h e top and bottom of equal size, they impose an equal resistance t o f l o w and the pressure differences through them are theref ore of equal magnitude. F o r continuity of air flow, the pressures at t o p and bottom rmst be equal i n magnitude but o p p o s i t e i n s i g n and the point of zero pressure difference, and thus the neutral plane, will be at mid-height.

With real b u i l d i n g s the opcnlngs in the envelope will seldom b e

uniformly distributed from t o p t o bottom, k t in all cases the i n f law m s t

equal the outflow. If t h e openings at the bottom, for example, are larger than those at the t o p , a smaller pressure drop would be required through

them r e l a t i v e to the top openings to gfve the same flow. The neutral pressure plane would b lowered. Similarly if t h e openings at the top are larger than those at the bottom, the neutral pressure plane would be

raised

.

Ventilation Systems

A ventilation system must create pressure differences, at sonre points, between the inside of the h f l d i i l g and the mtside. Air must be drawn in somewhere and an equal voluue must be exhausted somewhere else. With a

natural v e n t i l a t i o n system, the wind and temperature induced forces (stack

e f f e c t ) are used to create the necessary air f l o w ; w i t h a mechanical

v e n t i l a t i o n system, fans are used t o do t h i s . Depending upon the d e s i g n of the system and the way it is operated, a mechanical ventilation system can create either a p o s i t i v e or a negative pressure on a building. If there is an excess of exhaust over supply, then the extra air required w i l l ba drawn

i n through some of the apenings i n the building envelope; the neutral plane will be raised. T h i s w i l l prevent moisture-laden a i r from i n f i l t r a t i n g i n t o

the building e n v e l o p e , which is good from t h e potnt of view of building durability, but it creates t h e p o e s i b i l i r y of chimneys being backdrafted, as

mentioned before, and

i r

condmsation and bufldiag deterioration. Since it is t h i s possibility of r a p i d d e t e r i o r a t i o n that one i s trying to avoid, this too I s an undesirable

sys tern.

In view of the possible problems, both with a suction on the b u i l d i n g and with a pressure, it is desirable t o design a ventilation system with balanced supply and exhaust. This is not an easy matter, since t h e

performance of t h e system may vary w i t h wind and temperature effects on the intake and exhaust ducts. There w i l l also be intermittant e f f e c t s of

special purpose f a n s , such as clothes dryer exhausts., and of chimney draft that w i l l b difficult to integrate into an overall ventilation system. The actions of rhc occupants I n opening doors, windows or vents and in changing the s e t t i n g s of dampers will also a f f e c t the operation of the system. In

any case, because of stack e f f e c t , t h e ventilation system can balance

pressures through the envelope at one level only.

Consrdering a l l of these factors, it is difficult to determine i n

advance t h e pressure d i f f e r e n c e s across the building e n v e l o p e or even t o be

sure of t h e d i r e c t i o n in which they w i l l act. fn general, however, during most of t h e period of c o l d weather, when condensatfon may accumulate in the b u i l d i n g envelope, there will probably be an air pressure inwards at t h e bottom a£ t h e b u i l d f n g and outwards at the top,

Since it w i l l be more or less inevitable t h a t , for long periods of time, there will be an air pressure t.ending t o move air outward6 from the building at some locations, and since this air d l 1 probably have more water vapour in it than it can carry a t the outside temperatures, the only way t o

stop condensation w i t h i n the building envelope is to eliminate a l l of the holes through which it could l e a k into the envelope, In other words, to be

durable the building envelope must be m d e a i r t i g h t ; this is a formidable task.

Formfdable or not it must be tackled. It is not a case of energy conservation, nor of t h e comfort of the occupants, but of the durability of the buildfng i t s e l f . Fortunately energy conservation and comfort are

a d d i t i o n a l benefits derived from an air-tight building.

VAPOUR BARRIERS, A I R BARRIERS AND AIR-VAPOUR BARRIERS

For many years the term "vapour barrier" has been used t o d e s c r i b e a

specific element that has a high resistance to the d i f f u s i o n of water vapour under the action of a vapour pressure difference. More recently, the

comparable term "air barrier" has been used to describe an element that s t o p s the movement of a i r under the action

of

element that: both s t o p s the mvement of air and has t h e characteristics of a vapour barrier. Unfortunately, since polyethylene film has been used

extensively as a vapour barrier, a l l three term tend t o conjure up visions

of i t s use in a l l cases. This is certainly m e p o s s i b i l i t y , h t it i s not the only one.

To reject the use of the term air barrser and just to t a l k about air-tightness has some drawbacks, since it is necessary to define the position wAthin the thickness of the b u i l d i n g envelope a t which one wishes

t o s t o p the passage of air. One muld talk about the plane

of

t o s i g n i f y the location, except that it need not be a l l in one plane, even within one component, and in f a c t has t o be continuous on all faces of the building.

I r

afr-vapour b a r r i e r , provided it is clearly understood that they do not imply

the exclusive use of thin f l e x i b l e membranes. Many materials can be used, ranging from c o a t s of p a i n t to solid concrete slabs, provided that they fulf fll the required f u n c t i o n s .

There is also the question as to what actually constitutes the a i r barrier. Is it the air-impermeable i t e m i t s e l f , even

ff

The former definition wfll b e used i n this d i s s e r t a t i o n , since i t

a l l o w s for greater flexibility in design. Materials can be selected f o r

their i n d i v i d u a l propertfes and a b i l i t y t o perform s p e c i f i c functions. They

can then be combined with other componants of the envelope that may k required for other purposes, without: those other componants being d e f i n e d as forming part of the air barrier itself. For example, wood studs are

required in the w a l l of a house to form part of the structure that s u p p o r t s

floors and r o o f s . They do not have t o be defined as forming part of the air

barrier, d e s p i t e the f a c t that they are one essential part of the support for a polyethylene film that provides the air-tight layer in the wall.

It IS essential that the loads that w i l l act on the air barrier be

recognized and s u i t a b l e support: be provided for all its components. The d r barrier and its structural support must act together as a system,

N o t only m u s t the strength of any particular material be considered,

but a l s o that of i t s attachments to its supports or the structure of the

b u i l d i n g , or t o other components of the air barrier. For example, a sheet of polyethylene m y be strong enough to withstand the air pressures when

backed by wood studs at 600 mn spacing, but it may rip off the top and

bottom plates and at points of overlap if it is j u s t s t a p l e d into place. It may a l s o r i p off the s t a p l e s when the wind direction changes and it is b e i n g blown away from the studs rather than back against them. It should always be remembered that strong suctions w i l l be acting on a building as well as positive pressures. J o i n t s between components rmst be j u s t as strong as the components themselves and m u s t r e t a i n that strength for the l i f e of the

b u i l d i n g . Thus caulking may need a backup suppart or to be clamped between rigid mmpwents. An adhesive t a p e may dry out and lose its adhesion after

a periob of t i m e ,

Characteristics Required of an Air Barrier

What then are t l ~ functioas of the air barrier? The most obvious

requirement, h t at the same time the m e that is most d i f f i c u l t t o f u l f i l l , is that it should b complete. A l l the various item and materfals that go t o make up the air barrier mst

k

it can be seen and repaired. However this presupposes that the occupants of the but lding recognize that the inside f i n i s h is the air barrier and realize its importance. Thfs ie unlikely t o IE tk case with =st occupants and while the inner surf ace is easy to r e p a i r , it is a l s o easy to damage.

The second essential function of the air barrier is that it must be strong enough to withstand the -ximum air pressures from the combined effects of wind, stadc action a d ventilation. The prim= function of any building is t o s t o p the d u d from blowing through it and the air barrier I s

the component in the envelope that has t o rEo t h i s . T h i s requirement a p p l i e s equally to all components of the air barrier: the glass in the window, a

sheet of plastic, tape over a cradc or caulking in a joist.

The strength requirement of t k air barrier also relates to the third requirement, that it be durable. The structure of the b u i l d i n g is expected to last far long periods, usually the useful l i f e of the buflding, without needing repairs. The air barrier is the princfple defence against

de terioraticm of the building envelope because of undesirable concentrations of water, and so is a l m o s t as important as the structrrre. This it should be required to have an equal durability, Clearly it will not b durable if it

does aot have s u f f i c i e n t strength, in a l l i t s components, t o withstand the loads imposed on it.

A fourth requirement is that the air barrier k adequately stiff or r i g i d ; a requirement that relates to three factors. F i r s t there is the previous requirement: for durability; an air barrfer that is constantly flexing back and forth m y become brittle and crack or work loose from its fastenings. Second, a flexible air barrier may d l s l o d e insulation that should be in contact with it and so create spaces fa which room or mtside aft may circulate. It may compress a mineral wool i n s u l a t i m behind it thus, temporarily at l e a s t , reducing i t s thermal resistance. Third,

pressure equali z a t f on must be maintained in a space behind the wetted facade as a means of controlling rain penetration. Pressure equallkation w f l l occur under steady w i d loads even though the air barrier ma9 be completely

flexible. With fluctuatlng wind pressures, hawever, air will be constantly pumping in and out of t h i s supposedly pressure-equalized space, and water may be forced inward. Just how mch stiffness is adequate has not been

determined as yet, but clearly an unsupported f l e x i b l e membrane is not stiff enough.

LOCATION OF THE AIR BARRIER

S3mply to say that the building envelope must stop the exchange of air between the outdoors and the inside of the b u i l d i n g is not enough. Problems can still arise if either room or outdoor a i r flaws i n t o and back out of the envelope without any through leakage at a l l ,

Because of the temperature differences between the room and t h e outer portions of the wall, convective air flow may take place. Warm moist room alr c o u l d enter a wall at a high level, be cooled by contact with the colder portions of the wall and return to the room at a lower l e v e l (Figure 9a).

If this air is cooled t o below its dew point temperature, some moisture will condense within the wall. While there is some evidence that t h i s does

occur, the s e v e r i t y of the problem has not been e s t a b l i s h e d . With the

extended periods of cold in the North, i t could be serious. Nor is i t known

by how much the effectiveness of the insulation is reduced, although clearly

there must be some r e d u c t i o n ,

Similarly cold outdoor air can enter a wall at a low level, be warmed

by contact with the warmer p o r t i o n s and return to the outdoors at a higher level (Figure 9h). In this case no condensation can take plaee in the

moving air stream s i n c e it I s b e i n g warmed, not cooled. In being warmed, however, it will cool the inside panel and if t h i s is cooled t o below the

dew p o i n t temperature of the room air, condensation will occur on the room sTde of the i n t e r i o r finish. The f o r c e s producing t h i s convective air flow from the outside are i d e n t i c a l w i t h those producing a convective flow from the inside, since both are produced by the same temperature difference. On the other hand, much larger a i r flows can take place within t h e insulated

cavities of the envelope because of wind a c t i o n . Condensation and even ice formation on the warm s i d e have resulted, which indicates a great reduction in the effectiveness of the i n s u l a t i o n . Thus stopping a i r from leaking Znto or out of the building is not enough to ensure a durable b u i l d i n g ,

Convective and wind-induced flows through and around the insulation must a l s o be stopped. These requirements, which are additional to the basic one of stopping through air flow, create a dilemma as to the correct l o c a t i o n of t h e air barrier in t h e t h i c k n e s s of the b u i l d i n g envelope.

Just as condensation in the envelope as a result of through air leakage (exfiltration) must be prevented by stopptng the air leak so must

condensation as a result of a convective flow be prevented by stopping the convective flow. T h i s can be achfeved in either of two ways. The holes on the w a r m side of the insulation through which the room a i r enters and leaves the envelope can be closed off by what can be called a "convection barrier" (Figure 9a); or the s p a c e s in the envelope on the cold side of the

i n s u l a t i o n that provide a passage for the convective flow can be e l i m i n a t e d , This can be achieved by installing a i r impermeable insulation in full

contact with the sheathing. This is not a matter of sealing the joints between the boards or panels of i n s u l a t i o n ; t o do that is to attempt to make

a convection barrier out of the insulation.

To s t o p wind-induced air flow through or around the insulation requires

t h a t e i t h e r the i n s u l a t i o n be covered with an a i r i m p e r m e a b l e l a y e r on the outside or that the insulated cavity have no e x i t through which the air c o u l d l e a v e . The f i r s t s o l u t i o n is an obvious one and s i n c e t h e function of

alternatively a "weather barrier

",

also k e e p rain and snow out of the building envelope. The second solutlon is akin to a box that is sealed on a l l faces except for an opening on the windward face. S i n c e t h e r e is no opening through which the wind c o u l d l e a v e

the box on the leeward s i d e , i t w i l l not enter on the windward. A small amount of a i r will e n t e r t h e box to pressurize it t o t h e w i n d pressure b u t thereafter there will be no air f l o w .

Thus to stop air movement through or i n t o and out of t h e b u i l d i n g

envelope, the need f o r three different s o r t s of barrier has been identified ( F i g u r e 10):

-

-

-

barrier, and since the f i r s t f u n c t i o n of the building e n v e l o p e is to s t o p the uncontrolled flow of a i r into and out of the b u i l d i n g , the combined p a i r

of barriers can be called t h e prime a i r barrier. S i n c e it does n o t matter where the air barrier is l o c a t e d , as far as stopping t h r o u g h a i r leakage

only is concerned, i t s location w i l l be determined by consideration of the

factors that may cause condensation, including the likelihood of a c h i e v i n g continuity under realistic construction conditions on the site.

When choosing a l o c a t i o n for the air barrier, the designer should consider three interrelated factors:

- the probability of being successful in preventing condensation;

-

-

To prevent condensation, room air m u s t not e n t e r the envelope; thus both through leaks and convection currents must be stopped. This would

argue for combining the convection and the infiltration/exfiltration

barriers to form the a i r barrier, which would thus have to be located near the warm side of the envelope. Unfortunately w i t h conventional

consrruction, it is d i f f i c u l t to ensure that a barrier in this locarion is complete and air-tight round the various components, structural and

otherwise, that may penetrate 5t. These difficulties can be overcome with suitable details, but construction becomes more d i f f i c u l t and costly and there are many opportunities for mistakes. These problems can be reduced by using less conventional designs, such as d o u b l e s t u d w a l l construction for houses, that bring the structure inside the major portion of t h e e n v e l o p e , and by l o c a t i n g t h e a i r barrier outside of the structure, with adequate I n s u l a t i o n outsfde of i t t o keep it above the dew p o i n t temperature of the room. There may s t i l l be some problems with attachment of the outer