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The need for improved airtightness in buildings
Handegord, G. O.
Ser
THI
B92
THE NEED FOR IFIPROVED AIRTTGilTNES5 I N
R U I C D I N G S ~
by
G.O. Wandegord
Infiltration and exfiltration cf a i r through t h e building envelope
h a s come to be accepted as an inerinsic feature o f a building; i n
residential c o n s t r u c t i o n some r e g a r d I t as necessary f o r t h e propeT
v e n t i l a t i o n and o p e r a t i o n of t h e house. In c o n t r a s t to these attitudes, it i s suggested t h a t a i r leakage wastes h e a t and is d e t r i m e n t a l t o the performance of a building.
A i r leaking i n t o a building cannot be treated or conditioned nor
can
its r a t e of supplyo r
distribution to t h e occupied space be controlled. Attempts to do so by intentional pressurization of t h ebuilding results
in
g r e a t e r waste of energy and promotes increasedcondensation and thus deterioration o f t h e building f a b r i c . A i r
leakage through a b u 5 l d i n g envelope, and between building compartments, can a l s o disrupt the intended o p e r a t i o n of h e a t i n g , ventilating and air-conditioning systems and places limitations on t h e control of n o i s e ,
f i r e 2nd smoke.
A i r Leakage in Buildings
A i r leakage occurs as a result of a i r p r e s s u r e differences created by f a n s , wind, and stack e f f e c t across unintentional openings in t h e building envelope [ I ) . L i t t l e can be done to change the pressure
d i f f e r e n c e s resulting from n a t u r a l climatic f a c t o r s and pressure
'k
T h i s r e p o r t was presented at t h e Engineering Foundation Conference on Ventilation vs Energy Conservation in Buildings, H e n n i k e r , NH, J u l y 1 9 7 7 ,
and will be included i n t h e proceedings of t h a t conference, to be
d i f f e r e n c e s created by mechanical systems are required 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 . Thc most d i r e c t approach is to
reduce t h e number o f openings through any s t r u c t u r a l b a r r i e r s by the
dcvelopmcnr and application of appropriate building dedails and
construction practices. The o b j e c t i v e should be to p r o v i d e as airtight
an enclosure as i s possible and to p r o v i d e the f r e s h air and exhaust ~ e q u i s e m e n t s of the occupancy by a separate ventilation system or systems capable of adjustment, control, or a d a p t a t i o n to meet t h e
s p e c i f i c needs of the occupants. Such an arrangement is a l s o necessary
f o r hea-t: recovery and it may be a d v i s a b l e to arrange i n l e t and exhaust
systems in close proximity t o each other in order to simplify heat exchange.
The d i r e c t energy c o s t s f o r h e a t i n g , cooling, humidifying, o r
dehumidifying t h e air leaking into a building have l o n g been recognized. Many o f t h e methods for e s t i m a t i n g t h e air leakage rate
are
related tod e s i g n calculations. These estimates, however, are based on extreme
conditions for purposes of e n s u r i n g an adequate maximum capacity of the room heating
o r
c o o l i n g unit. To obtain a more accurate p r e d i c t i o n , the methods f o r estimating the seasonal air leakage energy load mustt a k e i n t o account t h e influence of l o c a t i o n , s i z e and d i s t r i b u t i o n of
t h e a i r leakage openings, the v a r i a t i o n s
in
wind-induced pressures inspace and time and the simultaneous pressure differences created b y
s t a c k effect and air h a n d l i n g systems.
Laboratory t e s t s on prefabricated structural elements can be made
and f i e l d measurements employed t o determine the a c t u a l leakage characteristics of buildings and structural elements in
-
place. The necessarily complex calculations can be made by u s i n g computer models. The fact remains, however, t h a t t h e o n l y way t h i s energy component canbe lessened is by reducing t h e s i z e o r number of leakage openings. It is i m p o r t a n t , t h e r e f o r e , t o identify where t h e s e openings occur and
what changes-in details, materials and construction practices can be
Air Leakage P a t h s and Openings
There h a s been a general tendency t o assume that leakage t h r o u g h the building enclosure o c c u r s p r i m a r i l y at doors and windows where v i s i b l e j o i n t s occur. The ASHRAE Handbook of Fundamentals provides
i n f o r m a t i o n on a i r leakage through frame and masonry walls a?rd takes i n t o a c c o u n t cracks such as thasc between t h e frame of a window or door
and t h e wall construction in which it i s installed.
S t u d i e s undertaken on 6 houses
i n
Ottawa, Canada, demonstrated t h a tt h e leakage openings contributed by windows and doors constituted only
a b o u t o n e - f i f t h o f t h e t o t a l ; leakage through t h e ceiling and o u t s i d e walls ranged up to 70% o f the t o t a l ( 2 3 . Studies on larger buildings have shown t h e a c t u a l overall leakage r a t e s t o he far above t h a t
s p e c i f i e d by an i n d u s t r y standard f o r c u r t a i n walls ( 3 3 ,
The i n v e s t i g a t i o n of a i r leakage paths in actual buildings has
been o f special concern in Canada ever since air leakage was recognized
a s
the
most serious c o n t r i b u t o r t o difficultiesin
buildings caused by c o l d we at he^ condensation. The provisionof
an a i r b a r r i e r or an a i r and vapaur b a r r i e r combined has been stressed b o t h from a standpoint of condensation prevention and as a f e a t u r e of the d e s i g n o f building wallsto control r a i n penetration. The r o l e of srack e f f e c t h a s also been
emphasized s i n c e , in contrasc to wind, it provides a continuous f o r c e d u r i n g s h e c o l d weather to move m o i s t air outward through t h e upper
portions of a building.
In wood-frame residential buildings condensation in roof spaces is
thc most common problcrn as t h c m a i n paths f o r a i r lcakagc arc t h r o u g h partition walls into t h e a t t i c s p a c c ( 4 ) . These p a r t i t i o n s effectively constitute f l u e s f o r upward a i r movement under the action of stack
e f f e c t in w i n t e r . Construction p r a c t i c e s have resulted in many openings
between the interior of such partitions and the a t t i c space. Shrinkage
cracks between upper p l a t e and wallboard and holes drilled t h r o u g h p l a t e s to carry electrical wiring into the attic space f o r m the major
leakage paths. Gaps around plumbing stacks can be i d e n t i f i e d more readily and can be closed o r sealed o f f .
Measures t o Reduce A i r Leakage
Measures to reduce air leakage t h r o u g h ceilings in wood-frame
h o u s e s have been i n t r o d u c e d by authorities in Canada (5). Central
Mortgage and Housing Corporation now r e q u i r e s that vapour b a r r i e r s t r i p s
be p l a c e d over partitions and subsequently attached to the main vapour
b a r s i e r to
provide
a continuous membrane. Holes around electrical w i r i n g , plumbing and chimneys must be adequately sealed. Electricalwiring in roof spaces is being discouraged and, as an alternative,
f u r r e d spaces inward of rhc vapour barrier f o r wiring in both walls and ceilings are being promoted.
Air leakage t h r o u g h hollow concrete masonry walls is o f t e n much g r e a t e r Than normally assumed. Upward leakage i n t h e c a v i t y
is
notalways adequately blocked and parallel, random air flow passages occur
between gypsum wallboard f i n i s h e s and t h e block face (4). Because o f this, sheet f i n i s h material cannot be regarded as an equivalent a i r seal t o site-applied p l a s t e r or heavy textured p a i n t s and mastics.
In commorcial buildings the application of r i g i d i n s u l a t i o n and
built-up r o o f i n g above t h e r o o f deck results i n fewer openings from the i n t e r i o r of t h e space i n t o t h e roof/insulation system. A c r i t i c a l
location far leakage a n d condensation i n such buildings is associated
with the junction o f wall and r o o f . T h i s may be f u r t h k r a g g r a v a t e d when
dropped ceilings are provided f o r service spaces and t h e exterior wall
abovc t h c c e i l i n g i s l e f t unplastered or is inadequately t r e a t e d with
respect to airtightness. In some instances vertical service s h a f t s
p r o v i d e passages f o r unrestricted a i r flow to this critical l o c a t i o n , or into exterior walls ( 6 ) .
Air leakage through e x t e r i o r w a l l s can also occur where t h e s t r u c t u r a l system o r servfces penezrate The a i r b a r r i e r ,
o r
wherej o i n t s between dissimilar m a t e r i a l s o r components o c c u r . There often i s a g r e a t d i f f e r e n c e between t h e construction details as developed
on
a designer's drawing b o a r d and t h e result that occursin
the f i e l d . Masonry cannot be installed t i g h t l y to s t r u c t u r a l steel columns and beams.In principle, it i s p r e f e r a b l e to have the structural frame of a building inward and s e p a r a t e from t h e e x t e r i o r wall system. The wall
can t h e n i n c o r p o r a t e a more continuous s t r u c t u r a l a i r b a r r i e r and be protected from t h e f l u c t u a t i n g conditions of t h e w e a t h e r b y i n s u l a t i o n a p p l i e d t o t h e o u t s i d e . Exterior c l a d d i n g is best a p p l i e d following
t h e open r a i n screen principle to control r a i n p e n e t r a t i o n ( 7 ) .
C o n s i d e r a t i o n should a l s o be given to allowing access t o t h e interior wythe and air b a r r i e r f o r maintenance of the air s e a l at j o i n t s .
The d e t e r i o r a t i o n of extcrior structural elements of a building and damage t o the i n t e r i o r t h r o u g h a i r leakage and condensation have a n important bearing on the operaring and maintenance c o s t s and hence an
the long-term energy economics. I t i s hoped that consideration of these matters m i g h t change c u r r e n t "first c o s t " attitudes t o t h e more realistic l i f e cycle c o s t approach.
T h e r e is a l s o some m e r i t i n c o n s i d e r i n g t h e improvement in
airtightness of the i n t e r n a l floor systems, particularly in high-rise
b u i l d i n g s . ? ' h i s w i l l tend to redistribute t h e t o t a l pressure d i f f e r e n c e duc to s t a c k e f f e c t sa that t h e pressure d i f f e r e n c e a c t i n g across t h e extcrior wall on each f l o o r is r e d u c e d . Such an approach also has
mcrir; i n improving control of ventilation and a i r handling and
r e d u c i n g t h e air circulation between occupancies on d i f f e r e n t f l o o r s . It s h o u l d a l s o make it much e a s i e r to c o n t r o l 'smoke movement
in
buildings i n t h e case o f a f i r e s i t u a t i o n and enable a more e q u i t a b l e apportioningof
c n e r g y charges for space h e a t i n g between individual unitsin
apartment buildings.Conclusions
Improving t h e airtightness
of
walls, windows, f l o o r s and r o o f sis
an e s s e n t i a l
s t e p
toward energy conservation in buildings. Air leakage shouldnever
be r e l i e d uponf o r
v e n t i l a t i o no r
air supply and exhaust b u t should b e recognized as an impediment to the proper o p e r a t i o n andc o n t r o l
of
v e n t i l a t i o n systems. Specific ventilation systems should berequired in all buildings l o provide an adjustable or controllable r a t e
of f r e s h air supply and exhaust with consideration given to t h e
inclusion o f energy recovery methods.
E f f o r t s s h o u l d be made t o detrclop construction d e t a i l s ,
arrangements of materials and construction methods that will result
in
walls, windows, f l o o r s and roofs b e i n g more airtight with provisionmade for intentional and controllable v e n t i l a t i o n openings through t h e
components where necessary. If this is achieved, n o t only will
optimum o p e r a t i o n of t h e building be more readily attained, but it will
be easier t o evaluate the a c t u a l air Peakage characteristics of t h e completed
building,
easier t o monitor t h e energy a s s o c i a t e d withventilation, and easier to predict performance more accurately under
a n t i c i p a t e d climatic conditions.
References
(13 r i l l s o n , A. G . A ~ T leakage in b u i l d i n g s . Nat
.
Res. C o u n c i l Canada,Uiv. Bldg. Res., Canadian Building Digest 23, 1961.
( 2 ) Tamura, G.T. Measurements of air l e a k a g e characteristics of house cnclosurcs. AS!3RAE Trans., Vol. 81, Part 1, 1975. p . 202-211.
{ l < e p r i n t e d as NRCC 149501
.
[ I ] Tamtlra, G.T. and Shaw, C.Y. S t u d i e s on e x t e r i o r wall a i r tightness :lnd a i r infiltration of tall buildings. ASHRAE T r a n s . , V o l , 8 2 , 1'3rt 1, 1976, p. 122-134. [Reprinted as NRCC 15732).
(4) Tanura, G .T. Kuester, G . H . , and tlandegord, G - 0 . Condensat ion problems i n f l a t wood- frame i-oofs. Second I n t e r n a t . S!mposium on lrloisture Problems in Buildings, R o t t e r d m , Sept. 1974. Paper No. 2 . 6 . 2 . ( R e p r i n t e d as NRCC 14589)
.
(5) Measures t o reduce the incidence of damage from undue condensation.
C e n t r a l Mortgage and Housing Corporation, Builder's Bulletin
30. 228, Ottawa, July 1973.
( 6 ) Wilson, A . G . and Garden,
G.K.
Moisture accumulatian i n walls dueto air leakage. RILEEl/CTB Symposium, "Ploisture Problems in Suildings," l l e l s i n k i , 1965, Preprint 2 - 9 . [Reprinted as NRCC 91313,
7 ) Garden, G . K . Rain p e n e t r a t i o n and i t s control. Nat. Res. Council
