Construction Details for Air Tightness. Record DBR Semin./ Wksp.

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Construction Details for Air Tightness. Record DBR Semin./ Wksp.

NATIONAL RESEARCH COUNCIL OF CANADA

DIVISION OF BUILDING RESEARCH

CONSTRUCTION DETAILS FOR AIR TIGHTNESS

Record of the DBR Seminar/Workshop

October, November 1977 and January 1978

Proceedings No.

. 3

of the

Division

of Building Research

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TABLE OF CONTENTS

Page

Design Principles

G.O. Handegord

1

Application of Design Principles in Practice

J.C. Perreault*

7

Masonry Walls

K.N. Burn

13

Metal Stud Walls

Wall/Roof Junctions and Soffits

R.L. Quirouette

2

1

R.G. Turenne

2

5

The following people assisted in the preparation of material for the

seminar and in its presentation: D.C. Tibbetts, DBR Halifax; C.P. Hedlin

and H.W. Orr, DBR Saskatoon; W.H. Ball, DBR Vancouver; and R.G. Brand,

Carleton University, Ottawa.

1)IX-i I GN I'R I NC I 1'1,IiS

by

G . O .

Handegord

Although the importance of air tightness has been stressed for the

past 25 years in the technical literature dealing with the design of

building enclosures, we still construct buildings that provide their

owners with excessive energy costs, that make the occupants uncomfortable

because of drafts and the infiltration of contaminated air, and that

suffer from problems of condensation and the deterioration of their

components and materials, which require high costs for repair and

maintenance. Air tightness, accordingly, should be the primary objective

in

the design and construction of walls, windows, floors and roofs of

buildings.

The following reasons for the lack of achievement of air tightness

in practice may be cited: (1) a lack of appreciation by designers,

builders and materials manufacturers of its importance; (2) the

assumption that by simply specifying an air vapour barrier the problem

is solved;

(3)

a resignation to the belief that in the practical case

air tightness cannot be achieved; and (4) the traditional attitude

that some air leakage is necessary in order to provide adequate ventilation.

The notion that air leakagc is

necessary

for ventilation must bc

dispelled. Instead, air leakage through the building enclosure must be

regarded as undesirable, unnecessary and uneconomical, because air

leaking into a building cannot be treated or conditioned, nor can its

rate of supply or distribution throughout the building be controlled.

Similarly, air leaking out of a building not only constitutes a waste

of energy but results in condensation and deterioration of the building

fabric. Any openings in the building envelope and even those in floors

and partitions disrupt the intended operation of heating, ventilating and

air-conditioning systems, make it difficult to control noise, and increase

the danger of the spread of fire and smoke. In addition, a wall that

suffers from air leakage may also suffer from rain leakage if it is

exposed to wetting by rain.

In a Canadian Building Digest dealing with fundamental considerations

in the design of exterior walls for buildings,

l

Dr.

N. B. Hutcheon provided

a list of requirements, the first seven of which, termed the "barrier

requirements," related to the control of the flow of mass and energy

through the enclosure. All of these flows, except for those involving

light and solar or other radiation, are influenced by the size, location

and distribution of cracks and openings in the building enclosure.

It has been demonstrated that substantial amounts of heat can flow

through small cracks and openings. These also provide the primary means

for the transport of water vapour into the cold regions of walls, which

takes place mainly by the mass flow of air rather than by vapour diffusion

t h r o u g h m a t e r i a l s . Recent i n f o r m a t i o n c o n c e r n i n g t h e c o n t r o l o f n o i s e shows t h a t a r e l a t i v e l y s m a l l opening i n a w a l l can a l t e r c o n s i d e r a b l y i t s sound t r a n s m i s s i o n p r o p e r t i e s ; t h u s an opcning e q u a l t o one onc- h u n d r e d t h of one p e r c e n t o f t h e a r e a o f t h e s e p a r a t i n g element c a n r e d u c e i t s sound t r a n s m i s s i o n c l a s s by 10 dB. S i m i l a r l y , o p e n i n g s i n a w a l l a f f e c t i t s f i r e r e s i s t a n c e ; t h u s i n t h e "Standard Method o f Small-

s c a l e F i r e T e s t s of Walls and P a r t i t i o n s P e n e t r a t e d by Small P i p e s , " i s s u e d by U n d e r w r i t e r s 1 L a b o r a t o r i e s of ~ a n a d a , ~ one o f t h e f a i l u r e c r i t e r i a i s t h e development of a n opening I f t h r o u g h which h o t g a s e s i s s u e a t a s u b s t a n t i a l r a t e . " T h i s can o c c u r when t h e o p e n i n g s a r e no l a r g e r t h a n 0 . 1 s q i n . ( 0 . 6 crn2).

A i r l e a k a g e t h r o u g h o p e n i n g s r e s u l t s from a i r p r e s s u r e d i f f e r e n c e s c r e a t e d by wind f o r c e s , s t a c k e f f e c t , and t h e o p e r a t i o n o f f a n s . The p r e s s u r e c r e a t e d by wind depends on t h e geometry o f t h e b u i l d i n g , a s w e l l a s t h e w i n d ' s v e l o c i t y and d i r e c t i o n . I n most c a s e s , wind p r e s s u r e w i l l n o t exceed i t s s t a g n a t i o n p r e s s u r e , which i s t h e p r e s s u r e r e s u l t i n g from t h e c o n v e r s i o n o f t h e w i n d ' s k i n e t i c e n e r g y t o s t a t i c f o r c e . The g r e a t e s t v a r i a t i o n i n wind p r e s s u r e o c c u r s a t t h e e d g e s o f a b u i l d i n g , w h i l c n c g a t i v c p r e s s u r c o r s u c t i o n i s c r e a t e d by v o r t i c e s and t u r b u l e n c e on t h e leeward s i d e and o v e r r o o f s . The amount o f s u c t i o n developed i s

i l l f lucnccd by t h c p a r a p e t h c i g h t

,

and n c g n t i v c p r c s s u r c s s c v c r a l tirncs :IS

I ; l r r s c ' a s t h c s t a g n a t i o n p r c s s u r e may o c c u r . The p r e s s u r e d i f f e r e n c e a c r o s s t h e e n c l o s u r e due t o s t a c k e f f e c t r e s u l t s from t h e d i f f e r e n c e i n t e m p e r a t u r e o f t h e a i r i n s i d e and o u t s i d e t h e b u i l d i n g . The e x t e r i o r b a r o m e t r i c p r e s s u r e v a r i e s w i t h h e i g h t a s d o e s t h e p r e s s u r e i n t h e b u i l d i n g , b u t t h e v a r i a t i o n w i t h h e i g h t i s l e s s i n s i d e b e c a u s e t h e a i r i s l e s s d e n s e . When t h e r e i s l i t t l e r e s t r a i n t t o v e r t i c a l a i r f l o w i n t h e b u i l d i n g , a n inward movement of a i r w i l l t a k e p l a c e t h r o u g h t h e o p e n i n g s i n t h e

lower h a l f of t h e b u i l d i n g w h i l e an outward movement o c c u r s t h r o u g h o p e n i n g s i n t h e u p p e r h a l f of t h e e n c l o s u r e . Where t h e o p e n i n g s i n t h e e n c l o s u r e a r e u n i f o r m l y d i s t r i b u t e d , t h e maximum p r e s s u r e d i f f e r e n c e s a c r o s s t h e w a l l s w i l l o c c u r a t t h e bottom and t h e t o p , and t h e r e w i l l be no p r c s s u r e d i f f e r e n c e a c r o s s t h c o p e n i n g s a t some p o i n t n e a r m i d - h c i g h t whcrc t h e i n s i d c and o u t s i d e p r e s s u r e s a r e t h e same, i . e . , t h e n e u t r a l p r c s s u r e p l a n e . I f t h e b u i l d i n g i s p r e s s u r i z e d by means o f f a n s o r b l o w e r s , t h e i n c r e a s e d i n t e r i o r p r c s s u r e w i l l r e s u l t i n t h e l o w c r i n g o f t h e n c u t r a l p r e s s u r e p l a n e . An e x c e s s o f e x f i l t r a t i o n o v e r i n f i l t r a t i o n w i l l t a k e p l a c e t h r o u g h t h e w a l l s ; a c c o r d i n g l y , t h e t o t a l a i r exchange w i l l b e i n e x c e s s o f t h a t e x p e r i e n c e d when no f a n s a r e u s e d . T h i s a p p r o a c h i s sometimes u s e d t o e l i m i n a t e a i r i n f i l t r a t i o n i n t h e lower f l o o r s , w i t h t h e o b j e c t o f improving t h e c o m f o r t o f t h e o c c u p a n t s o r p r e v e n t i n g t h e e n t r y o f c o n t a m i n a t e d a i r . Although t h e s e o b j e c t i v e s may be a c h i e v e d , such p r e s s u r i z a t i o n w i l l n o t o n l y r e s u l t i n a n a d d i t i o n a l e n e r g y l o s s , b u t by i n c r e a s i n g t h e q u a n t i t y o f a i r moving outward t h r o u g h t h e b u i l d i n g e n v e l o p e , i t i n c r e a s e s t h e r i s k o f c o n d e n s a t i o n and d e t e r i o r a t i o n .

I f a n e x h a u s t system i s employed, t h e d e c r e a s e d p r e s s u r e w i t h i n t h e b u i l d i n g r e s u l t s i n t h e r a i s i n g o f t h e n e u t r a l p r e s s u r e p l a n e . A c c o r d i n g l y , a i r movement w i l l be inward o v e r most o f t h e w a l l a r e a , w i t h e x f i l t r a t i o n t h r o u g h t h e e x h a u s t f a n and t h r o u g h t h o s e p o r t i o n s o f t h e b u i l d i n g t h a t a r e a t a h i g h e r p r e s s u r e t h a n o u t s i d e . T h i s w i l l a l s o i n c r e a s e t h e a i r exchange r a t e and e n e r g y l o s s , w i t h p o s s i b l e d i s c o m f o r t due t o d r a f t s on lower f l o o r s .

The e f f e c t o f wind i s t o c r e a t e d i f f e r e n t c o n d i t i o n s on t h e windward and leeward s i d e s o f

a

b u i l d i n g . On t h e windward s i d e , t h e o u t s i d e p r e s s u r e i n c r e a s e s w i t h r e s p e c t t o t h a t i n s i d e and t h e n e u t r a l p r e s s u r e p l a n e i s r a i s e d ; a c c o r d i n g l y , a i r i n f i l t r a t i o n w i l l o c c u r o v e r more o f t h e windward f a c e , w h i l e e x f i l t r a t i o n o c c u r s o n l y t h r o u g h t h e a r e a where t h e i n t e r i o r p r e s s u r e e x c e e d s t h a t o u t s i d e . I n c o n t r a s t , on t h e leeward s i d e , s u c t i o n i s produced, t h e n e u t r a l p r e s s u r e p l a n e i s lowered, and e x f i l t r a t i o n o c c u r s o v e r most o f t h e leeward f a c e o f t h e b u i l d i n g .

Thc maximum p r c s s u r c r e s u l t i n g from s t a c k c f f c c t dcpcnds on t h c

b u i l d i n g h e i g h t and t h e t e m p e r a t u r e d i f f e r e n c e between t h e i n s i d e and o u t s i d e . For a 2 0 - s t o r e y b u i l d i n g m a i n t a i n e d a t 70°F (21°C) i n s i d e , s u r r o u n d e d by a i r a t -20°F (-2g0C), a p r e s s u r e d i f f e r e n c e e q u i v a l e n t t o t h a t produced by a wind o f 25 mph (40 km/h) i s g e n e r a t e d a s a r e s u l t o f s t a c k a c t i o n . S i n c e a v e r a g e wind v e l o c i t i e s a r e u s u a l l y i n t h e o r d e r o f 1 0

-

15 mph (16

-

24 km/h) it c a n be r e a d i l y a p p r e c i a t e d t h a t s t a c k e f f e c t u s u a l l y o v e r r i d e s wind p r e s s u r e f o r h i g h b u i l d i n g s u n d e r w i n t e r c o n d i t i o n s .

Because t h e p r e s s u r e d i f f e r e n c e s c r e a t e d by mechanical systems a r e r e q u i r e d f o r a i r d i s t r i b u t i o n w i t h i n t h e b u i l d i n g , and b e c a u s e n o t h i n g can b e done t o change t h e p r e s s u r e d i f f e r e n c e s r e s u l t i n g from wind and s t a c k e f f e c t , t h e o n l y s o l u t i o n t o minimizing t h e i r e f f e c t s i s t o r e d u c e t h e s i z e and number o f o p e n i n g s t h r o u g h t h e e n c l o s u r e . The o b j e c t i v e , t h e r e f o r e , s h o u l d be t o p r o v i d e a s a i r t i g h t a n e n c l o s u r e as p o s s i b l e . The r e q u i r e m e n t s f o r f r e s h a i r and f o r e x h a u s t must be met by a s e p a r a t e v e n t i l a t i o n system c a p a b l e o f a d j u s t m e n t and c o n t r o l t o meet t h e s p e c i f i c n e e d s o f t h e occupancy.

I t h a s been g e n e r a l l y assumed t h a t l e a k a g e t h r o u g h t h e b u i l d i n g e n c l o s u r e o c c u r s p r i m a r i l y a t t h e d o o r s and windows where r e c o g n i z a b l e j o i n t s o c c u r . The Handbook o f Fundamentals o f t h e American S o c i e t y o f H e a t i n g , R e f r i g e r a t i n g and A i r - C o n d i t i o n i n g ~ n ~ i n e e r s ~ p r o v i d e s inform- a t i o n on l e a k a g e t h r o u g h d o o r s and windows a s w e l l a s t h a t o c c u r r i n g between t h e i r f r a m e s and t h e w a l l i n which t h e y a r e i n s t a l l e d . Values o f

a i r

l e a k a g e a r e a l s o p r o v i d e d f o r masonry c o n s t r u c t i o n and wood-frame walls. A masonry wall, f o r example, w i l l a l l o w s u b s t a n t i a l a i r l e a k a g e b e c a u s e o f t h e numerous small f i s s u r e s and o p e n i n g s between t h e u n i t s and m o r t a r ; by s i m p l y p l a s t e r i n g s u c h a wall, however, i t s a i r t i g h t n e s s i s improved, i n some c a s e s by a f a c t o r o f 100.

Studies undertaken on houses in Ottawa have demonstrated that the

air leakage associated with windows and doors constitutes about one fifth

of the total, while the leakage through the cciling and outsidc walls

ranges from

8

to

70

per cent of the total. Studies on large buildings

have shown actual air leakage rates far in excess of the values listed

by the National Association of Architectural Metal Manufacturers for

curtain walls or windows. It is important that all possible leakage

paths be identified in order that appropriate drawings, details and

specifications can be developed.

Leakage openings can exist in hollow frame walls of wood or metal,

either in the framing components themselves or created as the holes and

openings cut for services. If cladding materials are not drawn tight to

metal studs, leakage paths are formed. In wood-frame walls, subsequent

shrinkage df the framing members can result in gaps between these members

and the interior or exterior finishes applied to them, allowing air to

flow between the wallspaces. Polyethylene film and other membrane barriers

have been employed in wood-frame construction to provide a seal over these

openings.

Although the plastcring of masonry walls provides a substantial

degree of air tightness, it is often omitted in such places as the wall

area above the level of a dropped ceiling or that behind a radiator unit.

In some cases, the furred out vertical shafts provide channels for the

movement of air from lower levels into the space above a dropped ceiling.

It should not be assumed that the application of gypsum board or

other sheet material to a masonry wall will provide the same degree of

air tightness as plaster. Because of the uneven surface of the masonry,

intimate contact will not be achieved between it and the surface of the

sheet material unless a full bed of adhesive is used. If not, the open

space between the masonry and the sheets provides a path for air leakage.

In addition to the openings in a building enclosure caused by

expansion and shrinkage of materials, are those resulting from deflection

caused by structural forces on the building and its components. Cracks and

openings so formed allow air leakage to take place, with the danger of

resulting condensation and deterioration.

An important consequence of air leakage is the waste of energy.

Extremely leaky buildings, under pressure differences due to stack

effect alone, suffcr losses due to air leakage amounting to as much

as 60

per cent of the total energy required for heating in winter. If

such buildings had been constructed to meet the air tightness require-

ments of organizations such as the National Association of Architectural

Metal Manufacturers, the heating load due to infiltration could have

been reduced to

6

per cent of the total.

There is merit in improving the air tightness of floor systems

and partition walls, particularly in high-rise buildings, because this

reduces the pressure difference acting across the exterior walls.

Improved air tightness also assists in the control of ventilation and

air distribution, including the control of smoke movement in the case

of fire. In addition, it enables a more equitable apportioning of

energy charges to be made between the individual units in apartment

buildings

.

Improving the air tightness of walls, windows, floors and roofs

must be regarded as an essential step toward energy conservation in

building. Air leakage should never be relied upon for ventilation but

should be recognized as an impediment to the proper operation and

control of a ventilating system. Such a system, having a specific

means of air supply and exhaust, should be required in all buildings

to provide an adjustable fresh air supply and used-air exhaust, and

should incorporate energy recovery devices. Under such conditions

it will be easier to evaluate the actual air leakage characteristics

of the completed building, easier to monitor the energy needs

associated with ventilation, and easier to predict the performance

of the building under the anticipated climatic conditions.

References

1

tlutchcon, N.B. Requirements for exterior walls. National Research

Council of Canada, Division of Building Research, CBD

48,

1963.

2

Underwriters' Laboratories of Canada. Standard method of small

scale fire tests of walls and partitions penetrated by small

pipes. ULC-S115-1977.

3

_{American Society of Heating, Refrigerating and Air-conditioning}

Engineers. Handbook of Fundamentals. Issued every four years.

APPLICATION OF DESIGN PRINCIPLES IN PRACTICE

by J.C. Perreault

Webster defines thc word "detail" as

a

noun meaning a small or

unimportant part, but

a

construction detail

is

not unimportant, rathcr

it is of extreme importance. It should not be small, on the contrary,

it should be big, easy to read and easy to understand.

A good or bad detail will often make the difference between a

good or bad installation. Building designers should bear in mind that

those who actually build the buildings usually have no design background.

They should not be forced to guess the designer's intention, or to play

the role of designer, but should only be expected to build carefully, as

detailed. That is why a detail must be precise, easy to understand and

practicable. There should be no guess-work; if a problem arises in

understanding the function or purpose of any item or specification or

if a detail does not appear to be good practice, it should be queried.

If a detail appears to be impossible to build, no attempt should be made

to build it.

Attention to detail in design and construction is of prime impor-

tance if air tightness is to be achieved. In this regard one single

thing will make a detail a success or a failure. This is the presence

of holes. Just as Joseph Fouch6, Minister of Police under Bonaparte,

used to say, "If you are faced with a seemingly unsolvable mystery,

cherchez la femme" ["Look for the woman"), so too in the investigation

of building problems an appropriate saying would be

...

"Cherchez le trout'

(I1Look for the holef1). In practically all cases, it will work.

Many examples of building problems directly resulting from openings

or holes can be documented. In a certain hotel, for example, water

appeared at the ceiling line of many bathroom walls, badly staining them.

As usual, the first call went to the roofer, but because the problem

occurred in mid-winter it could not be blamed on a roof leak. Because

there were no windows in the bathrooms the window installer could not

be blamed either. The leakage was eventually traced (by putting dye in

the toilet water) to holes in the soil pipes. The pipes had inadvertently

been placed in the wall directly behind the location chosen for the

installation of the towel racks. In drilling holes in the walls to

install the racks, the workman in many instances also drilled holes into

the soil pipes, providng the paths for leakage.

Another example of "cherchez le trou" involved a shopping centre

that had an outside pedestrian area beneath a large roof overhang (1:igurc I*).

Water dripping out of openings, intended originally to scrve as air vents

for the soffit space, did not concern the management except that the

water froze on the sidewalk, forming patches of ice that were hazardous

to the customers. The first proposed solution to the problem was simply

t o i n s t a l l h e a t i n g c a b l e s i n t h e s i d e w a l k t o p r e v e n t t h e w a t e r from f r e e z i n g , b u t it was d e c i d e d t o t r y t o f i n d o u t what was c a u s i n g t h e d r i p s .

The overhang was formed by e x t e n d i n g t h e c o r r u g a t e d s t e e l r o o f deck o v e r a c o n c r e t e b l o c k w a l l . The s o f f i t s p a c e was e n c l o s e d by t h e r o o f deck on t o p , by t h e b l o c k w a l l t h a t s e p a r a t e d t h e s o f f i t s p a c e from t h e b u i l d i n g , by p r e c a s t c o n c r e t e p a n e l s on t h e o t h e r t h r e e s i d e s , and on t h e bottom by s t u c c o a p p l i e d t o mesh, which was suspended by w i r e s t h a t p a s s e d t h r o u g h h o l e s i n t h e f l u t e s o f t h e c o r r u g a t c d d e c k .

I n t h e s o f f i t s p a c e i t was found t h a t f r o s t , i n some p l a c e s t o a d e p t h o f 1 1 / 2 i n . (38 mm), had formed on t h e c o n c r e t e p a n e l s , t h e s t e e l t r u s s members and t h e s u s p e n s i o n w i r e s . Large masses o f i c e were a l s o p r e s e n t , a p p a r e n t l y formed from w a t e r m e l t e d from t h e f r o s t , which had r u n down t h e p a n e l s u r f a c e and had f r o z e n i n t o i c e n e a r t h e a i r v e n t s . When t h e t e m p e r a t u r e i n t h e s o f f i t s p a c e became warm enough, t h e i c e m e l t e d and t h e w a t e r - d r i p p i n g problem began.

A t f i r s t it was t h o u g h t t h a t t h e h o l e s t h a t c a u s e d t h e problem, by a l l o w i n g warm m o i s t a i r t o move from t h e b u i l d i n g t o t h e s o f f i t s p a c e where t h e w a t e r v a p o u r condensed on c o l d s u r f a c e s , were t h o s e p r o v i d e d by t h e f l u t e s o f t h e r o o f d e c k , which formed o p e n i n g s between t h e t o p o f t h e b l o c k w a l l and t h e u n d e r s i d e o f t h e d e c k . Although t h e drawing of t h e d e t a i l o f t h i s a r e a c a l l e d f o r t h e s e s p a c e s t o be f i l l e d , t h e m a t e r i a l s p e c i f i e d , g l a s s wool i n s u l a t i o n , d i d n o t p r o v i d e a n a i r s e a l , b u t s e r v e d m e r e l y t o f i l t e r t h e a i r p a s s i n g t h r o u g h . I t was r e p l a c e d by p o l y u r e t h a n e

foam, and c a u l k i n g was a p p l i e d t o any o p e n i n g t h a t c o u l d b e f o u n d , b u t even a f t e r t h i s had been done i c e c o n t i n u e d t o form i n t h e s o f f i t s p a c e , and t h e w a t e r - d r i p p i n g problem c o n t i n u e d .

Another p a t h f o r a i r l e a k a g e was e v e n t u a l l y d i s c o v e r e d , t h i s b e i n g t h e s p a c e i n t h e u p p e r f l u t e s o f t h e d e c k . A i r l e a k a g e by way o f t h e s e f l u t e s was p o s s i b l e b e c a u s e o f h o l e s d r i l l e d i n t h e f l u t e s t o t a k e t h e w i r e s o f t h e suspended s o f f i t ( F i g u r e 1 ) . Water formed by t h e conden- s a t i o n o f w a t e r vapour c o l l e c t e d i n t h e f l u t e s , and when i t s l e v e l r e a c h e d t h a t o f t h e h o l e s i t flowed o u t and down t h e w i r e s , many o f which were c o a t e d w i t h i c e . The a i r t i g h t n e s s o f a b u i l d i n g c a n be p r e d i c t e d from a s t u d y of t h e d e t a i l e d d r a w i n g s , which u s u a l l y r e v e a l o p e n i n g s i n t h e e n c l o s u r e , p a r t i c u l a r l y when c o r r u g a t e d r o o f d e c k s a r e u s e d , whose s p a c e s a r e d i f f i c u l t t o s e a l . I n f a c t on many d r a w i n g s t h e i r s e a l i n g i s n o t even s p e c i f i e d . The d e t a i l e d d r a w i n g s a l s o o f t e n r e v e a l a r e a s where i t i s i m p o s s i b l e t o i n s t a l l an a i r o r vapour b a r r i e r ; i t i s a p p r o p r i a t e t h a t d e s i g n e r s u s e a d o t t e d l i n e t o r e p r e s e n t a vapour b a r r i e r b e c a u s e i n p r a c t i c e it i s u s u a l l y i n c o m p l e t e o r f u l l o f h o l e s .

When t h i n g s go wrong i n a b u i l d i n g , a good p l a c e t o s t a r t t h e "cherchez l e t r o u " t e c h n i q u e i s a t t h e window h e a d s , where many p i e c e s o f m e t a l u s u a l l y p e n e t r a t e t h e w a l l , making i t d i f f i c u l t , i f n o t

i s tlic wall ;111ovc a suspended c c i l i n g , fro111 which tlic v;lpour h3rric.r and i n s u l a t i o n a r e f r e q u e n t l y o m i t t e d , t h u s a l l o w i n g a i r l e a k a g e t o t a k e p l a c e t h r o u g h t h i s p a r t o f t h e w a l l .

I n s u l a t i o n and vapour b a r r i e r s a r e t h e two most m i s u n d e r s t o o d i t e m s i n c o n s t r u c t i o n t o d a y . Many b u i l d e r s and d e s i g n e r s a p p a r e n t l y do n o t know e x a c t l y what t h e s e two t h i n g s a r e supposed t o do. Sometimes d e s i g n e r s g e t c a r r i e d away and p r o v i d e two vapour b a r r i e r s on t h e same e l e m e n t , b u t t h e y n e v e r seem t o be a b l e t o c o n n e c t t h e vapour b a r r i e r of one element w i t h t h a t o f a n o t h e r e l e m e n t .

I n t h e c h o i c e and a p p l i c a t i o n o f m a t e r i a l f o r b u i l d i n g , c a r e must be t a k e n t o e n s u r e t h a t one s p e c i f i e d m a t e r i a l o r i n s t a l l a t i o n does n o t d e s t r o y o r i n t e r f e r e w i t h o t h e r s p e c i f i e d i t e m s . Sometimes two m a t e r i a l s , seemingly good i n t h e m s e l v e s , when combined t o g e t h e r can harm t h e o t h e r . I n a c e r t a i n b u i l d i n g , f o r example, f o i l - c o v e r e d i n s u l a t i o n was s p e c i f i e d t o be g l u e d t o t h e back o f p r e c a s t w a l l

c l a d d i n g , a d i f f i c u l t i n s t a l l a t i o n j o b because o f a l l t h e p r e c a s t a n c h o r s , sway b r a c e s and f u r r i n g c h a n n e l s t h a t t h e i n s u l a t i o n had t o be c u t around and s e a l e d t o . But i n a d d i t i o n , t h e s p e c i f i c a t i o n c a l l e d f o r wet c o n c r e t e t o be poured a g a i n s t t h e aluminum f o i l , t o c o v e r p r e c a s t a n c h o r s and

p r o v i d e f i r e s e p a r a t i o n between f l o o r s . The p r o d u c t d a t a s h e e t f o r t h e i n s u l a t i o n , however, c l e a r l y i n d i c a t e d t h a t : (1) t h e m a t e r i a l backing t h e f o i l c o n t a i n s w a t e r s o l u b l e s a l t s , and i f i t becomes wet t h e s e s a l t s have a c o r r o s i v e a c t i o n on t h e f o i l ; (2) t h e f o i l , when p l a c e d i n c o n t a c t w i t h wet p l a s t e r o r m o r t a r , w i l l c o r r o d e . I n s p i t e o f t h e s e w a r n i n g s , however, b o t h s p e c i f i c a t i o n and drawing f o r t h e f o i l ' s a p p l i c a t i o n i n t h i s i n s t a n c e c a l l e d f o r wet cement t o b e poured a g a i n s t i t .

An i n t e r e s t i n g c a s e o f problems a r i s i n g from a i r l e a k a g e i n v o l v e d one o f t h e f i r s t h i g h - r i s e b u i l d i n g s b u i l t on t h e r a i n - s c r e e n p r i n c i p l e i n A l b e r t a . An a c c o u n t o f t h i s b u i l d i n g and i t s performance, p r e p a r e d by G . O . Handegord f o r a DBR B u i l d i n g S c i e n c e Seminar,? r e f e r r e d t o t h e c o n s t r u c t i o n and t h e problem a s f o l l o w s : "The w a l l c o n s i s t s of a n i n n e r wythe of 8 - i n . (200 mm) t h i c k common b r i c k t o g e t h e r w i t h c a s t - i n - p l a c e r e i n f o r c e d c o n c r e t e s p a n d r e l beams. Expanded p o l y s t y r e n e i n s u l a t i o n , 1 i n . ( 2 5 mm) t h i c k , was i n s t a l l e d on t h e e x t e r i o r w i t h s p o t a p p l i e d a s p h a l t a d h e s i v e . I ' "The o u t e r r a i n s c r e e n i s a 4 - i n . (100 mm) t h i c k p r e c a s t con- c r e t e p a n e l 18 i n . (460 mm) l o n g and a p p r o x i m a t e l y 5 f t (1.5 m) wide suspended t o c r e a t e a n a i r s p a c e 1 i n . (25 mm) wide between i t s i n n e r f a c e and t h e o u t e r f a c e of t h e i n s u l a - t i o n . The s p a c e i s open t o t h e o u t s i d e a t t h e bottom above

? DBR Audio-Visual P r e s e n t a t i o n s No. 12A and 12B, Feburary 1972. P r e s e n t e d a t t h e B u i l d i n g S c i e n c e Seminar f f W a l l s , Windows, and Roofs," October 1971.

the window head and connected to the outside at the top by

means of a special aluminum sill. The spandrel beams provide

the air vapour barrier above the suspended ceiling and a

polyethylene film acts in this capacity over furring on the

inside of the masonry wall. The furring provides a means

for securing the metal induction units and supporting thc

vapour

barrier."

"In the early spring of the building's first year of occupancy,

the outside temperature rose to a sudden and unusual high of

40°F ( 5 ° C )

under sunny conditions.

A

series of icicles formed

along the bottom edge of the precast panels on every floor and

on every orientation except the south."

!'It was eventually concluded that the primary source of

moisture causing the icicles was that which had found its

way into the inner wythe during the period of winter con-

struction. The problem has not recurred to our knowledge,

thus tentatively confirming the analysis. The situation

prompted additional comments by those concerned as an example

of the effectiveness of the system in allowing the escape

of moisture which might ordinarily be trapped inside the

wall to cause serious problems."

"Further observations by the designers indicated that some

of the board insulation had become detached from the inner

wythe as evidenced at the opening above window heads. The

problem was remedied by the installation of small wedges

between the precast panel and insulation. It is a credit

to the designers that these difficulties have been reported

by them in the literature for the benefit of others and the

improvement of design technology."

The problem, however, did not stop; icicle formations, not as large

as the first, showed up from time to time and the problem has required the

expenditure of $50,000 in investigation and remedial work. The owner found

it more and more costly to maintain heating and cooling operation and

something had to be done to reduce air leakage and revamp the insulation.

In applying the "cherchez le trou" technique to this building, a

source of air leakage was found where the anchors for the precast panels

were located above and below windows, and at the corners of the building.

The openings around the anchors were not sealed, permitting air leakage

to take place, with the resultant condensation and icicle formation.

As is evident by the variety of buildings that are completed daily

in Canada, there is no lack of creative genius and ingenuity on the part

of designers and builders within the construction industry, but there

seems to be a lack of understanding of the design process by builders.

Much would be gained

by

the buildcrs and owners if thcy tricd to unclcrstand

the design process. The architect's lot is not an easy one, even if one

discounts the frustrations attendant on being an adjudicator, financial

adviser, decorating consultant, and guardian of culture and aesthetics.

His job is not made easier by the proliferation of new and inadequately

understood methods and materials, and a quickened pace of construction.

The successful performance of the building depends on his ability to

choose the right material or system.

Just as builders must try to understand the designer's job, so must

the designer realize that putting up

a

modern structure is also a form-

idable task. The construction superintendent spends a third of his time

pushing papers and answering the phone. If he has to spend half of his

remaining time figuring out how to build a particular detail and once it

is done, he has to spend the other half of his time around a conference

table trying to explain why he did it that way or why it doesn't work

then he will not be able to do what he is good at doing, which is putting

up the building.

The problems resulting from the faulty design and construction of

buildings, and the need for joint action by designers and builders to

overcome them, have been well summarized in the following words prepared

by M.C. Baker for a DBR Building Science Seminarf which are worth

repeating as a summary of this account of the application of design

principles in practice:

"In this age of unprecedented technological expertise, it

should be possible to predict performance. The construction

industry, howcvcr, appcars to bc plagued by an evcr increasing

amount of extremely poor performance, including buildings

that leak, stream with condensation, overheat in summer or

whose appearance is marred by streaking, discoloration, and

material break-down on the exterior. It is obviously

necessary to get back to scientific principles in the

design and construction of enclosures for building. All

details must be examined to ensure the continuity of air and

vapour barriers, and the construction must be carried out in

a manner that achieves this continuity. There is no doubt

that the present unsatisfactory performance of many buildings,

and building elements, can be overcome by a concerted team

effort on the part of designers and builders."

DBR Audio-Visual Presentation No. 11, February 1972. Presented at

the Building Science Seminar "Walls, Windows, and Roofs," October

1971.

MASONRY WALLS

by K . N . Burn

In t h e f o l l o w i n g d i s c u s s i o n of masonry w a l l s i n r e l a t i o n t o t h e a i r t i g h t n e s s o f b u i l d i n g s , p a n e l w a l l s c o n s t r u c t e d o f c o n c r e t e b l o c k s w i l l mainly be c o n s i d e r e d . The primary f u n c t i o n o f such a w a l l i s t o c l o s e t h e s p a c e between t h e f l o o r s of a b u i l d i n g i n such a way t h a t t h e d e s i r e d and c o n t r o l l e d environment on t h e i n s i d e i s i s o l a t e d from t h e n a t u r a l environment on t h e o u t s i d e by means o f a b a r r i e r t h a t w i l l p r e v e n t t h e movement o f a i r and whatever i t c o n t a i n s . T h i s can o n l y be a c h i e v e d i f t h e p a n e l w a l l i s c o n s t r u c t e d of some m a t e r i a l t h a t i s n o t permeable t o a i r and i f i t i s f i t t e d i n t o t h e s p a c e between t h e f l o o r s w i t h o u t l e a v i n g c r a c k s o r h o l e s around t h e p e r i m e t e r .

Most common b u i l d i n g m a t e r i a l s , i n c l u d i n g c a s t - i n - p l a c e and p r e - c a s t c o n c r e t e , gypsum board, plywood, s h e e t metal and g l a s s a r e

r e l a t i v e l y impermeable t o a i r . When j o i n t s between components made o f t h e s e m a t e r i a l s a r e a d e q u a t e l y s e a l e d an e f f e c t i v e a i r s t o p o r a i r

b a r r i e r c a n be formed. I n c o n t r a s t , however, masonry w a l l s , c o n s t r u c t e d of c o n c r e t e b l o c k s h e l d t o g e t h e r w i t h m o r t a r , a r e q u i t e permeable t o a i r f o r a number o f r e a s o n s . Mortar s h r i n k s about 10 t i m e s a s much a s con- c r e t e block and t h e d i f f e r e n t i a l movement produces numerous f i n e t e n s i o n c r a c k s i n t h e m o r t a r . I n a d d i t i o n , when a block i s l a i d i n m o r t a r t h e r e may be a r e a s where bond does n o t develop between t h e bottom s u r f a c e o f

t h e block and t h e t o p s u r f a c e o f t h e m o r t a r . Another c a u s e o f t h e p e r m e a b i l i t y o f block w a l l s i s t h e i n c o m p l e t e f i l l i n g o f m o r t a r j o i n t s , p a r t i c u l a r l y t h e v e r t i c a l o n e s .

S h r i n k a g e o f c o n c r e t e b l o c k s a l s o p l a y s a p a r t i n t h e problem o f t h e p e r m e a b i l i t y o f such w a l l s . Concrete b l o c k s made w i t h dense a g g r e - g a t e may s h r i n k a s much a s 0.04 p e r c e n t a f t e r b e i n g p l a c e d , w h i l e l i g h t - weight b l o c k s g e n e r a l l y s h r i n k t w i c e a s much. Autoclaved b l o c k s s h r i n k

l e s s t h a n t h o s e c u r e d a t low p r e s s u r e , b u t even s o , t h e i r s h r i n k a g e i s n o t g e n e r a l l y l e s s t h a n 50 p e r c e n t of t h a t o f b l o c k s n o t a u t o c l a v e d . T h i s amount of s h r i n k a g e means t h a t f o r a 1 0 - f t (3 m) h i g h p a n e l w a l l t h e v e r t i c a l s h r i n k a g e amounts t o between 1/20 and 1/10 i n . ( 1 . 3

-

2.5 mm). H o r i z o n t a l s h r i n k a g e a l s o o c c u r s b u t i s u s u a l l y somewhat l e s s t h a n t h e v c r t i c a l becausc o f t h e r e s t r a i n t causcd by f r i c t i o n a l resistance t o movement a l o n g t h e b a s e of t h e w a l l under i t s own dead weight.

I t can bc s e c n , t h e r e f o r e , t h a t p a n e l w a l l s o f c o n c r e t c blocks placed w i t h i n a r i g i d s t r u c t u r a l frame p r o v i d i n g 20 f t (6 m ) bays and 1 0 f t ( 3 m) s p a c i n g between f l o o r s , could produce, because of s h r i n k a g e , a c o n t i n u o u s opening between each p a n e l and t h e frame, a t both s i d e s and a c r o s s t h e t o p , o f 1/20 i n . ( 1 . 3 mm) o r more. The q u a n t i t y of a i r

f l o w i n g through t h i s c r a c k , a t 1/10 i n . (2.5 mm) w a t e r p r e s s u r e d i f f e r e n c e , *

*

The p r e s s u r e d i f f e r e n c e o f 0.1 i n . ( 2 . 5 mm) o f w a t e r was s e l e c t e d f o r t h e s e examples because i t i s f r e q u e n t l y encountered i n a l l t y p e s o f b u i l d i n g s . P r e s s u r e d i f f e r e n c e s i n h i g h - r i s e b u i l d i n g s may be s e v e r a l t i m e s g r e a t e r and t h e a i r l e a k a g e volumes would i n c r e a s e a c c o r d i n g l y .

would be 120 cfm ( 3 m3/min) o r 7000 c u f t (190 m 3 ) p e r h o u r . T h i s amount o f a i r l e a k a g e when m u l t i p l i e d by t h e number o f p e r i m e t e r b a y s and f l o o r s i n a b u i l d i n g may r e p r e s e n t a v e r y l a r g e w a s t e o f e n e r g y . I n a d d i t i o n t o p e r i m e t e r a i r l e a k a g e t h e r e i s l e a k a g e o c c u r r i n g t h r o u g h t h e p a n e l i t s e l f . In b r i c k w a l l s , under a p r e s s u r e d i f f e r e n c e o f 1/10 i n . ( 2 . 5 mm) of w a t e r , t h e a i r l e a k a g e may amount t o 800 cu f t (22 m3) p e r h o u r , f o r an a r e a o f 100 s q f t (9 m2). Because o f t h e fewer m o r t a r j o i n t s i n a g i v e n a r e a o f c o n c r e t e b l o c k w a l l t h a n i n one o f b r i c k , i t can bc e s t i m a t e d t h a t t h e a i r l e a k a g e r a t e w i l l be betwecn 250 and 400 cu f t ( 6 . 7

-

1 0 . 8 m3) p e r h o u r , f o r an a r e a o f 100 s q f t ($1

.

I : o r t u n a t c l y , howcvcr, a masonry w a l l can be t r e a t e d t o perform a s a s u i t a b l e a i r b a r r i e r by a p p l y i n g t o i t a l a y e r o f m o r t a r , p l a s t e r o r some o t h e r m a t e r i a l such a s h e a v i l y t e x t u r e d p a i n t o r m a s t i c which w i l l s e a l t h e m u l t i t u d e o f s m a l l o p e n i n g s i n t h e u n i t s and t h e m o r t a r

j o i n t s . Tn t h i s way a i r l e a k a g e c a n be r e d u c e d t o a s l i t t l e a s one-hundredth o f t h a t t h r o u g h t h e untreated w a l l ; t h u s a i r l e a k a g e t h r o u g h 100 sq f t (9 m2) o f c o n c r e t e b l o c k w a l l u n d e r a p r e s s u r e

d i f f e r e n c e o f 1/10 i n . ( 2 . 5 rnm) w a t e r may b e reduced t o a v a l u e between 5 and 20 cu f t ( 0 . 1

-

0.6 m3) p e r hour.

In t h i s d i s c u s s i o n o f t h e e f f e c t on a i r t i g h t n e s s o f t h e s h r i n k a g e of c o n c r e t e p a n e l w a l l s , i t h a s been assumed t h a t t h e b u i l d i n g s t r u c t u r e i s r i g i d , which i n p r a c t i c e i s n o t t h e c a s e . S t e e l and r e i n f o r c e d con- c r e t e e x p e r i e n c e d i m e n s i o n a l changes w i t h changes o f t e m p e r a t u r e when l o a d i s a p p l i e d t o them, and, i n t h e c a s e o f c o n c r e t e , when s u b j e c t e d t o change i n m o i s t u r e c o n t e n t . I n o r d e r t o d e t e r m i n e r e a l i s t i c v a l u e s f o r t h e s i z e s o f t h e c r a c k s and h o l e s t h a t w i l l d e v e l o p a t t h e j o i n t between t h e s t r u c t u r a l frame and an i n f i l l p a n e l , it i s t h e r e f o r e n e c e s s a r y t o know how much and i n what d i r e c t i o n t h e s t r u c t u r e w i l l d e f l e c t u n d e r i t s s e r v i c e c o n d i t i o n s .

Under s u s t a i n e d compressive s t r e s s e s , c o n c r e t e s u f f e r s c o n s i d e r a b l e d e f o r m a t i o n c a l l e d " c r e e p , " which h a s been o b s e r v e d t o c o n t i n u e f o r

p e r i o d s o f up t o t e n y e a r s , a l t h o u g h t h e most s i g n i f i c a n t p a r t o f i t

generally o c c u r s i n from one t o two y e a r s . Columns s h o r t e n and beams

and f l o o r s c o n t i n u e t o d e f l e c t u n t i l t h e d e f o r m a t i o n s e v e n t u a l l y become 2 t o 5 t i m e s t h e i n i t i a l o r e l a s t i c d e f l e c t i o n s , b u t i n most c a s e s t h e r a t i o of c r e e p t o e l a s t i c d e f l e c t i o n f o r s t r u c t u r a l c o n c r e t e i s 2 1 / 2 t o 3. T h i s means t h a t i f t h e d e s i g n o f a s p a n d r e l beam c a l l e d f o r an e l a s t i c d e f l e c t i o n o f 11480 o f t h e s p a n , f o r a column s p a c i n g o f 20 f t (6 m) (which would b e 0 . 5 i n . ( 1 3

mm))

t h e c r e e p d e f l e c t i o n c o u l d be a f u r t h e r 1 . 5 i n . (38 mm). ( I f t h e d e s i g n d e f l e c t i o n were l i m i t e d t o 1/1000 of t h e s p a n t h e c r e e p d e f l e c t i o n c o u l d s t i l l be a s much a s 3/4 i n . (19 nun).) I n a d d i t i o n , a column t h a t d e f l e c t s 1/20 i n . ( 1 . 3 mmj e l a s t i c a l l y may show c r e e p s h r i n k a g e o f a f u r t h e r 1/10 i n . (2.5 mm) w i t h i n t h e f i r s t two y e a r s a f t e r c o n s t r u c t i o n .

Steel, unlike concrete, does not develop creep strains. It is a

more nearly elastic material in the range of stresses normally imposed

on it in building construction. Therefore, its deformations are propor-

tional to the load and are more predictable than for concrete.

For the development of the details used in this and the following

discussions, the general approach was adopted that the insulation be

placed outward of the structure and its panel walls to take advantage

of the increased dimensional stability that results when the temperature

variations they experience are minimized. The insulation, placed

immediately to the cold side of the air barrier, is protected by

a

rainscreen on the outside, which shields it from the weathering cffects

of sun, wind and rain.

A design approach to the foundation wall/ground floor junction is

illustrated in Figure

2.

The outer face of the concrete block panel

wall, placed flush with the foundation wall, supports the air barrier

that is formed by the application of

a

suitably textured mastic. Besides

sealing the cracks in the block faces and mortar joints, the mastic

may also serve as an adhesive for fixing semi-rigid glass-fibre insulation

to the wall. This configuration places the wall and foundation insulation

in the same plane, making it easy to form an unbroken cover and thus

avoid a thermal bridge at the edge of the floor slab.

It

also allows

the cutting of service chases in the inner face of the block without

interrupting the air barrier.

The insulation over the bascment wall may be foam plastic or

a

rigid glass-fibre insulation; the latter would serve the added purpose

of providing adcc~uatc

drainage through thc soil to tllc 1)crimctcr

ti

lc

thus avoiding thc cost of hauling in granular material for backfill.

If foam plastic insulation is used, it needs to be protcctcd against thc

effects of ultraviolet radiation and both types of insulation would need

protection against mechanical damage at this level, which may be

provided by an asbestos-concrete board, or by parging.

The brick rainscreen is supported by a normal-size shelf angle from

a wedge insert in the edge of the floor slab, and a spacer fabricated

from a hollow rectangular structural section. Adjustment in three

directions of the position of the shelf-angle is permitted by a

horizontal slot in the shelf angle, a vertical slot in the wedge insert

and a selection of sizes of sections for the spacer. This type of

support was chosen in preference to an angle with longer legs fastened

directly to the edge of the slab mainly because the heavier section

required would be much more difficult for the masons to handle. Where

steel reinforcement of the masonry wall is required to resist earth-

quake loads the block and brick wythcs must be reinforced and tied

together and to the structure.

Windows usually account for large heat losses, because of air

leakage through both the window sash and frame and through the space

between the frame and thc opcning in the wall into which it is fitted.

Good window design takes care of the first; the second is a construction

problem that arises from the common practice of fixing windows by

m e c h a n i c a l f a s t e n e r s . T h i s g e n e r a l l y l e a v e s a gap o f a t l e a s t 1 / 2 i n . (12 mm) wide a r o u n d t h e frame a n d , even though t h e f i n i s h materials on t h c i n s i d c may Ilidc i t , t h c gap r c m a i n s and a i r f i n d s i t s way t h r o u g h . A d e s i g n a p p r o a c h t o window o p e n i n g s i s shown i n F i g u r e 3 , where i n o r d e r t o make t h e a i r b a r r i e r c o n t i n u o u s , i t i s t u r n e d back o v e r a wood f r a m e b u i l t i n t o t h e w a l l o p e n i n g a s i n d i c a t e d (broken l i n e ) . I t may a l s o b e t u r n e d back d i r e c t l y o v e r t h e u p p e r f a c e o f t h e t o p c o u r s e o f c o n c r e t e b l o c k , w h i c h f o r t h i s p u r p o s e s h o u l d be s o l i d .

Wood may s h r i n k a s much a s one p e r c e n t a c r o s s t h e g r a i n , and i t may warp, c a u s i n g t h e j o i n t between i t and t h e c o n c r e t e b l o c k t o open up. A c c o r d i n g l y , i f m a s t i c were u s e d i t i s p o s s i b l e t h a t a b r e a k i n t h e a i r b a r r i e r would o c c u r a t t h e i n t e r f a c e . To p r e v e n t a n a i r l e a k from d e v e l o p i n g h e r e , a f l e x i b l e membrane s u c h a s b u t y l r u b b e r may be a d h c r c d t o t h e m a s t i c c o a t on t h e o u t c r f a c e o f t h c w a l l and t u r n e d o v e r t h c t o p o f t h e wood f r a m e , w i t h t h e c o n t i n u i t y o f t h e a i r b a r r i e r

n ~ a i n t a i n e d by t h e a p p l i c a t i o n o f a f l e x i b l e s e a l a n t t o t h e o p c n i n g between t h e membrane and t h e window f r a m e . The f r a m e i t s e l f , t h e i n n e r g l a z i n g s e a l and t h e i n n e r pane o f g l a s s o f t h e s e a l e d g l a z i n g u n i t t h e n form a c o n t i n u a t i o n o f t h e a i r b a r r i e r .

F i g u r e 4 p r o v i d e s d e t a i l s o f a window jamb, which a r e e s s e n t i a l l y t h c same a s t o t h o s e o f t h e s i l l . The a i r s t o p i s t a k e n t o t h e i n s i d e o f t h e window frame s o t h a t i t c a n be r e p a i r e d more e a s i l y t h a n i f i t were on t h e o u t e r s u r f a c e , e s p e c i a l l y where s e a l e d g l a z i n g u n i t s a r e u s e d . I n a d d i t i o n , t h e e d g e s o f c o n c r e t e b l o c k s a r e o f t e n r o u g h l y c a s t o r damaged d u r i n g h a n d l i n g , making i t d i f f i c u l t t o a c h i e v e a good s e a l a t t h e o u t e r c o r n e r .

The p r o p e r f u n c t i o n i n g o f a window o f t h i s t y p e i n c o l d w e a t h e r

depends upon t h e i n n e r p a r t o f t h e window f r a m e ( t h e aluminum box s e c t i o n ) b e i n g m a i n t a i n e d a t a t e m p e r a t u r e above t h e dew p o i n t , o t h e r w i s e conden- s a t i o n w i l l o c c u r . I n a d d i t i o n , b e c a u s e t h e t e n s i l e s t r e s s e s a t t h e e d g e s of t h e i n n e r pane o f g l a s s o f a s e a l e d g l a z i n g u n i t i n c r e a s e a s t h e edge t e m p e r a t u r e d e c r e a s e s , t h e p o s s i b i l i t y o f f r a c t u r e i s h c i g h t e n e d . I t i s i m p o r t a n t t h e r e f o r e t h a t c o l d a i r b e p r e v e n t e d from c i r c u l a t i n g f r c c l y i n t h c s p a c e s bctwccn t h c wedgcs. T h i s i s accomplisllcd by i n s t a l l i n g a n o u t e r c a u l k i n g o f c o m p r e s s i b l c foam p l a s t i c r o p e which i s s c c u r e d betwecn t h e c o n c r e t e b l o c k , t h c window frame and a m e t a l f i n i s h p l a t e a t t h e jambs and h e a d , a n d a t t h e m e t a l s i l l a s shown i n

F i g u r e 3.

An a p p r o a c h t o t h e d e s i g n o f t h e window head and f l o o r / w a l l j u n c t i o n i s g i v e n i n F i g u r e 5. The a r r a n g e m e n t shown a l l o w s t h e i n s u l a t i o n

b l a n k e t t o c o n t i n u e p a s t t h e edge o f t h e s l a b , broken o n l y by t h e s h e l f a n g l e s u p p o r t s , s p a c e d a t i n t e r v a l s o f 5 o r 6 f t ( 1 . 5

-

1 . 8 m), t h u s a v o i d i n g t h e m a s s i v e t h e r m a l b r i d g e t h a t o c c u r s when t h e edge o f t h e f l o o r s l a b i n t e r r u p t s t h e i n s u l a t i o n b l a n k e t .

The j o i n t b e n e a t h t h e edge o f t h e f l o o r s l a b a t t h e t o p o f t h e w a l l w i l l e x p e r i e n c e a r e v e r s a l o f movements. With d r y i n g s h r i n k a g e o f t h e p a n e l - w a l l , t h e j o i n t w i l l f i r s t o f a l l open f a i r l y u n i f o r m l y , f o l l o w e d by a c l o s i n g which w i l l be g r e a t e s t a t mid-span a s t h e p a n e l w a l l above i s b e i n g c o n s t r u c t e d . A f u r t h e r c l o s i n g , a g a i n g r e a t e s t a t mid-span, w i l l o c c u r a s c r e e p - s h o r t e n i n g o f t h e columns and c r e e p d e f l e c t i o n s o f t h e e d g e s o f t h e s l a b c o n t i n u e . Because t h e combined e l a s t i c and c r e e p d e f l e c t i o n s o f t h e edge o f t h e s l a b w i l l n o r m a l l y be g r e a t e r t h a n t h a t due t o d r y i n g s h r i n k a g e o f t h e wall, t h e t o t a l e f f e c t o f t h e s e movements w i l l be t o c l o s e t h e j o i n t . C o n s e q u e n t l y , u n l e s s s u f f i c i e n t s p a c e i s p r o v i d e d , t h e s l a b w i l l e v e n t u a l l y r e s t on t h e t o p o f t h e w a l l and b e g i n t o t r a n s f e r l o a d s t o i t , which c o u l d l e a d t o b u c k l i n g and c o l l a p s c . I n a h i g h - r i s e b u i l d i n g w i t h i d c n t i c a l f l o o r l a y o u t s above t h c f i r s t f l o o r o r two, t h e f l o o r s and beams a r e u s u a l l y o f t h e same s t r u c - t u r a l d e s i g n because t h e y c a r r y s i m i l a r l o a d s . Under t h e s e c o n d i t i o n s t h e e d g e s o f t h e s l a b s d e f l e c t a b o u t t h e same amount w i t h t h e r e s u l t t h a t t h e v e r t i c a l d i s t a n c e between f l o o r s i s l i k e l y t o be f a i r l y uniform

a c r o s s t h e s p a n . Only t h e c r e e p s h o r t e n i n g o f t h e columns w i l l be s i g n i f i c a n t l y d i f f e r e n t from s t o r e y t o s t o r e y because t h e a c t u a l amount v a r i e s w i t h t h e l e v e l o f s t r e s s and w i t h t h e r a t i o o f t h e a r e a o f s t e e l r e i n f o r c i n g t o t h a t o f t h e c o n c r e t e . N e v e r t h e l e s s t h e r e w i l l be some v a r i a t i o n s i n t h e movements o f t h e s l a b s and i n f i l l w a l l s , t h e r e f o r e t h e m a t e r i a l s u s e d t o b l o c k t h e s e p o t e n t i a l a i r - l e a k a g e p a t h s must be s u f f i c i e n t l y f l e x i b l e t o accommodate t h e opening and c l o s i n g o f t h e j o i n t s . I n a d d i t i o n , t h e d i f f e r e n t i a l movements between t h e edge o f t h e s l a b and t o p o f t h e p a n e l w a l l have i m p l i c a t i o n s f o r t h e d e t a i l s o f t h e b r i c k v e n e e r . S u f f i c i e n t s p a c e , f i l l e d w i t h a s u i t a b l y f l e x i b l c s e a l a n t t o e x c l u d e t h e r a i n , must be p r o v i d e d b e n e a t h t h e s t r u c t u r a l l y s u p p o r t e d s h e l f - a n g l e t o a v o i d t r a n s f e r r i n g compression s t r e s s e s from t h e s t r u c t u r e t o t h e b r i c k c l a d d i n g .

Design d e t a i l s i n t h e h o r i z o n t a l s e c t i o n o f a column and window jamb a r e sllown i n F i g u r e 0. _{The w a l l i n s u l a t i o n i s brought a s c l o s e} t o t h e window frame a s t h e c o n s t r u c t i o n w i l l a l l o w , b u t f i n i s h i n g t h e window s u r r o u n d w i t h b r i c k means t h a t t h e w a l l i n s u l a t i o n s t o p s some 5 o r 6 i n . (128

-

153 mm) away from t h e jamb. The r e s u l t a n t t h e r m a l b r i d g i n g means t h a t t h e window jambs w i l l be c o l d e r t h a n t h e head and

s i l l . A t t h e columns, t h e o u t e r s u r f a c e o f t h e w a l l i s i n t h e same p l a n e a s t h e column f a c e , s i m p l i f y i n g t h e a p p l i c a t i o n o f t h e m a s t i c and i n s u l - a t i o n b l a n k e t . The two j o i n t s between t h e p a n e l w a l l s and t h e column may be l e f t w i t h o u t m o r t a r , b u t a t t h e o u t e r f a c e t h e s p a c e s h o u l d be b r i d g e d by a s e a l a n t bead o f t h e p r o p e r geometry t o p e r m i t e x t e n s i o n of t h e j o i n t s a s t h e w a l l s h r i n k s , w i t h o u t e x c e s s i v e s t r a i n i n t h e s e a l a n t m a t e r i a l .

At each floor, and at the roof line as detailed in Figure

7,

the joint between the top of the panel wall and the underside of the concrete slab will present the same problem. The air stops must be flexible, durable and sufficiently tight that they cannot be dislodged by the highest pressure difference to which they will be subjected.

The provision of a continuous air boundary in a steel frame building with concrete block panel walls can be very difficult if the designer attcmpts to placc thcm in the same plane in the building envelope. However, a relatively simple alternative is suggested in Figure 8; that of extending the floor slabs beyond the outer surfaces of the structural steel by an amount sufficiently wide to allow the laying of uninterrupted concrete block walls. The outside surface can be treated to form the air barrier and the insulation installed in the same manner as was used for the concrete structure.

If the floor slab is not extended as shown in Figure 8, support of the shelf angles for brick veneer may require complicated appendages to the structure protruding through the panel walls and promoting air leakage. When necessary, shelf angles can be supported as they are at the edge of the slab of a reinforced concrete structure, but an easier solution for a building of only two or three storeys is to support brick cladding at foundation level and carry it up to the roof line.

Details of a horizontal section through a steel column and window jamb, essentially the same as that for the concrete structure except that

the column is now completely inside the panel wall, are given in Figure 9.

Here the vertical joint becomes a shrinkage control joint in the block wall.

It is sometimes desirable that steel columns be recessed into the walls, for example, in a school gymnasium. A solution for such a situation, which follows the approach of applying both air-barrier and insulation to continuous plane surfaces, is presented in Figure 10. The outer surface of the block wall is bridged by a strip of sheet steel attached to the flanges of the colurnn before thc masonry work is

started. Shrinkage of the wall will cause cracks to open between the wall and the flange faces, but these potential leakage paths are stopped with back-up caulking and sealant as indicated. At the base of the column care is needed to ensure that infiltration and circulation of cold air does not occur, because this would nullify the purpose of the insulation and lead to condensation on the interior surfaces of the column.

Resistance to lateral movement is provided by clips attached to the bottom flange of the spandrel beam, shown in Figure 11. They make it impossible to use caulking and a sealant bead as a corltinuous air stop at this joint. Continuity of the plane exterior surface is maintained by placing gypsum board or plywood sheets to cover the spandrel beam but flush with the outer face of the concrete block wall. Differential movements will tnkc place between the wall and the roof strl~cture, tlicrcforc 3 f 1cxiOlc scal, S L I C ~ I as a butyl-rubbcr mc~nl)~a~rc, sl~oulll I)c uscd to span the joint at the top of the wall.

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