Humidity in Canadian buildings

Publisher’s version / Version de l'éditeur:

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

Questions? Contact the NRC Publications Archive team at

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

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

L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB.

Building Note (National Research Council of Canada. Division of Building Research), 1956-05-01

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

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

https://nrc-publications.canada.ca/eng/view/object/?id=cbb8ccd4-40ca-42f1-875d-8ec3d65c3b7e https://publications-cnrc.canada.ca/fra/voir/objet/?id=cbb8ccd4-40ca-42f1-875d-8ec3d65c3b7e

NRC Publications Archive

Archives des publications du CNRC

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

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

Humidity in Canadian buildings Hutcheon, N. B.

CANAI Ser TH1 B92 no. l g c . 2

BIJXI31FL

IW

CkNf-DIAR BUILDIN'ES

by

N.B. Hutcheon

( A talk

sf-ven

at the Canadian Chapters Confe-

r e n c e ,

ASHAE, T o r o n t o , February

14,

1955.

)

7 . J > ,.-

-h * :,+ ,.% . C

-

r

m e materials o f which a house

is

made m a g

normzlly c o n t a i n 2500 pounds o f w a t e r , T h e a i r contained

in the house a t any one time nil1 amount t o abollt

750

pounds

of

which about

5

pounds w i l l be w a t e r vaTour, The maintenance of t9is relatively s m a l l w a t e r content of the a i r at t h e proper

l e v e l c a n b e v e r y i m ~ o r t a n t since

it

may determine not only the

l e v e l of moisture c o n t e n t of" furnishings, and of m u c h of t h e

house structure

itself,

b u t a l s o r a t e s o f e v ~ l o r a t i o n or of moisture u p t a k e w h i c h m a y affect y e o p l e as well a s m a t e r i a l s ,

Excessive mofsture l e v e l s mag l e a d to r a p i d d e t e r i o r a t i o n or other faulty s e ~ v i c e f r o m many m a t e r i a l s .

Tke human

body i s a d a p t a b l e t o changing moisture conditions over a fairly wide range without serious ill effects

on

health, while under the same conditions many

materials e x h i b i t undesirable characteristics, The regula-

tion

of m o i s t w e l e v e l s in the air will often be more readily

j u s t i f i e d

on

t h e b a s i s of the response

o f

materials than

of

people,

In

some cases the absolute level of w a t e r c o n t e n t

is of concern while in o t h e r s

it

is the r e l a t i v e l e v e l , usually expressed as r e l a t i v e

humidity, which

becomes

of

h p o r t a n c e .

In

view of these complications, a b r i e f r e v i e w

of the p r o p e r t i e s of w a t e r vapour

in

air i s

in

o r d e r , before proceeding to f u r t h e r d i s c u s s i o n of humidity,

P r o p e r t i e s

of

Water

-

Vapour

in

Air

Water

vapour

in

a i r

is

a gas w " n i c h occupies

a l l the

space, along w i t h the air present,

I n

many ways the water vapour c a n act independently of the a i r , since in g e n e r a l i t s properties do not depend on the presence of the a i r ,

I t

exerts i t s o x m vapour pressure and c a n move about through

air

in a space,

or

move through materials under differences in i t s own

vanour pressure, independently o f

the

a i r ,

However,

when the air is moved suddenly

o r

is h e a t e d o r c o o l e d , the w a t e r

vapour

present is s i m i l a r l y a f f e c t e d , s o t h a t

it

i~ u s u a l l y necessary

to consider it a s a p a r t of an air-vapour r n l x t u r e .

Some of t h e characteristics

of

water v a p o u r ,

and

particularly t h e changes in properties of a m i x t u r e

of

a i r and water v a p o w can best be presented by a sinple p l o t of

water vapour c o n t e n t of t5c ~ i r a ~ h i n s t t e n p e r a t u r e , a s

shown

in

F i g u r e

1.

On

t h e

v e r t i c a l

s c a l e

is

shown

t h e

v a n o u r

c o n t e n t

a s a p ~ r c e n t a 5 e

bg

w e i z h t

of

t % e r d i e i ~ h t

of

dr:: a l r p r e s e n t . The

l i r n i t f n : ? cp

a n t i t i e s r.rhich c a n be h e l d st any

c l v e n t e m p e r a - t u r e z r e

shown by

t h e heavy

line nlarked

'saturation".

A t

7 5 ' ~ ~

f o r example, t h e ueter vapour c o n t e n t

is

1.9

p e r cent. A t

49'

( p o i n t

D )

t h e w a t e r vapour required f o r saturztian i s 0.7 ner cent r d h i l e

at

2 0 ~ ( ~ o i n t

F)

o n l y 0.2

per cent i s required.

If

at n temperature o f

7 r ° F

t h e r e i s less

than

1,9

p e r cent o f water vapour p r e s e n t , the air-vapour

m i x t u r e i s s e i d t o be g a r t i a l l y saturated, T!le d e g r e e of ssturation c a n be expressed

in

t a m s o f r e l a t i v e humiditg.

Cn

t h e v e r t i c a l

line

r e p r e s e n t i n g

7 5 O ~

a number of points can be l o c a t e d which correspond to the percentages of w a t e r

vapour p r e s e n t at v a r f o u s values of r e l a t i v e humidity. This c a n be extended t o o t h e r t e m p e r a t u r e s t o permit the construction of t h e c u r v e d l i n e s on t h e c h a r t s%ewing r e l a t i v e

humidity.

Un h s a t l n z or cooling

an

air-vapour

mixture

without

c h a n g t n g t h e w a t e r c o n t e n t , the percentage o f water p r e -

sent will obviously remain c o n s t a n t , and t h e s e processes

c a n be represented b y h o r i z o n t a l l i n e s on t h e c h a r t . Three processes t y p i c a l of cnndftfons which arise at times in

b u i l d i n g s a r e shown, e n d w i l l bs discussed.

D a m ~ n e s s in Basements

T'ne c o n d i t i o n s in a damp basement

on

a h u d d d a y

In

sm-ner might be represented by p a i n t A, c o r r e s p o n f i n g

to

100

p e r cent relative humidity a t

75'.

At

t h i s condition

m o l d s

will f o ~ m ,

w o o d will s w e l l and clothing and o t h e r ~ r t i c l e s w i l l be ('amp.

If,

however,

heat 5 s added

t o

tke n i x

t o

r a i s e i t s temper2ture t o y o 0 , t h e r e s u l t i n g

" n r o c e s s t '

can be r e p r e s e n t e d Sy t h e lLne A I3

on

t h e c h e r t . The final

condition, represented bg point B, shows t h a t t h e r e l z t i v e

h u m i d i t y w i l l - b e o n l y 62 per cent, Clothes w i l l now dry, a n d wood

will

s h r i n k t o a mare n o m a l condition. A d d i n g h e a t is

t h u s one w a g of' curing dampness

In

b a s e m e n t s a l t h o u g h

in

summer t; he c u r e m a y be l e s s acceptable

t h a n

the

a i l m e n t .

Point

C

represents a coAm-3n c o n d i t i o n in modern C a n a d i a n homes

i n

winter: 40 p e r c e n t r e l a t i v e h u m i d i t y at

75'.

The window

s u r f a c e s ,

however, may be only at

4q0,

T h i s

w

;.ndow s u r f a c e , as shown by t h e process l i n e C

I),

The

tenperzture

o f

1 1 9 ~

at N!IIC~ Kqis air-vapour m i x t u r e ,

if

cooled, becomes s a t u r a t e d , is k n o w n a s

i-t;s

d e w - ~ o i n t

A.

:

r

e

If

the window

surf

sce is colder then the dew-

p o i n t of

tth

eair v a p o w

rnixtme in contzlct

w i t h

it,

condensatian r , r i l l o c c u r ,

?or

t?le e x a i ~ ? l e represented by

the

process C

D E,

the a i r in contact rdth the w i n d o w surface at 3 2 O c2n o n l y r e t a f n

0 - 3 7

p e r c e n t moisture, w h r e a s t h e r e

w a s

0.74

per c e n t present i n i t i a l l y , and condensation on t h e w i n d o w surface

must

occur,

If

the v i n d a w surface l a above

32'

the condensation w i l l appear as dew; if below 32',

it w i l l

appear a s frost,

The s w f s c e s a? nost cannon materials have

an

af_Cinitg f o r r ; a t c r m o l e c c l ~ s . i ! o l e c u l ~ r f o r c e s or

a t t r z c t - i o n ~ \ l f l lh o l d w a t e r molecules to the surface,

The

thickness of t h e w a t e r f L h 2ornied, and t h e r e f o r e t h e m o u n t

of

water h e l d

in

equilibrium ractl? the surroundcling atmosphere is r ~ v - g h 1 ~ p r o p o r t i o n a l to r e l z t i v e

hmiei

ty,

D a t a on the m o i s t w e contefits

o f

varSolrs common

rnzt e r i a l s

in equilibrium w i t h

t h e

atmosphere a t v a r i o u s

r e l a t i v e h m i d i t i e s a r e availzhle. Wood, far e x a m r ~ l e , ~ a i T 1

have a m o i z t w e c o n t e n t o f

5

p e r

cent

by w e i g h t at 2 C p a r cent

~ e l ~ t i v e

humldTty. T b j s increases to

10

p e r cent

at

50 p e r c e n t r e l a t T v e humidity, and to 30 p e r c e n t z t r e l a t i v e

humidities v e r y

c l o s e

to 1CQ pep cent.

It

w i l l o f course

c c n t a i n oven more water thsn L l i s

when

f u l l y saturated. Plost p l ~ n t a n d

aninal

aroducts ex.hib5.t similar ~ r o p ~ r t i e s * IuIascnry

materials a l s o ~E!TF: on

water

f r o m

t h e

a i r ,

Significant dimensional changes t a k e p l a c e

in

many

m o t c r i a l s u s e d in buildings with change

in

r r ~ o i s t w e

con t e n t . Again, wood providez a r ~ ~ e l l - k n o w n example,

It

exhibits s'lrFnBa,ges o f a b c c t 0,1? 2 , and

4

p e r cent in the Scngitudinal, r a d i a l

and

tangential directTans respeetive-

ly

on a change f'.'rcn

air-dry

at

12

to 15

p e r c e n t moisture

c ~ n t e n t

to oven-dry. Simillar ex?ansions trike p l z c e rfxcn the

r n c i s t w e content 5s r c ~ { ~ a r e d . . Most w o o d . - r i b r e products,

including

papers,

w113

exhibit n o T s t u r e c > ~ n n s i a n consistent w i t h the b a s i c ~ . - o o d properties t o a degree dependent

o n

the

f i k re orientation 2nd srrnncement, A l r ~ o s t all y l a n t and a n i m a l products exhibit similar

expansion

and contraction

w i t h changes

Zn

m o i s t u r e c o n t e n t , Less g e n e r a l

ly

recognizsd are the d h ~ n s i o n a l ckanges w h i c h can occur ir? - ~ l r : , ~ ' : ~ y m a t e r i a l s a s a

result

of changes in r n o i s t ~ z e c c - - ' -

-_>,

- -

_water

is

either

an

essential

or

a c o n t r i b u t o r y

f a c t o r

in

a l m o s t

all

c a s e s o f

brezkdown

of building m a t e r i a l s resulting from chemical

changes

such as t h e rusting o f

s t e e l ,

p h y s i c a l changes such as the palling of

mascnry

by frost

a c t i o n , 07 b i o l o g i c a l processes such as t h e

r o t t i n g

o f p l a n t

and

animal

products,

In

all c a s e s these e f f e c t s a r e

ninin5zed or eliminated if a l o w l e v e l of moisture

content

c a n be maintained,

X f f e c t s

of

R e l a t i v e Humidity

upon

People

There

is, curlnusly

enough, little concZusive

e v i d e n c e t o show

that

elther

high

o r

low

r e l a t i v e

humidities

a r e of t h e m s e l v e s d e t r i m e n t a l t o the h e a l t h of normal

geople,

It

may be predicted, from the known p r o p e r t i e s of 1 - ~ a t e r , t h a t l o w humLdities

wdlL

l e a d t o dry !lair and skin

and to i n c ~ e a s e d e v a p o r a t f o n r a t e s f r o m t h e membranes of

t h e resniratory system, and t h a t h i g h h u ~ i d i t i e s will

wro2uce

t h e opposite

effect,

Evzporation from t h e

s w f

ace

of

t h e body is one of + 5 e rnechanf sms emnloyed by the

human body to regulate i t s tem~erature.

If,

however,

the

e v a p o r a t i r n r a t e tends t o be e i t h e r high o r low b1. v i r t u e

of low or

high

humidLty of the surrounding

a l r

the Cody

i s a b l e

to

make caapensating adjustments w i t h o u t , as f a r

as is known, any ill efzects under normal conditions. EvLdence on Yne e f f e c t s of krumtdity upon the

s p r e a d of air-borne bacteria is also inconclusive.

Cornfort, as o p p o s e d

to

h e a l t h ,

is

a subjective reaction. P e o p l e

a r e

c o m f o r t a b l e

or

uncomfortable if

they

t h i n k they a r e .

It

seems r e a s o n a b l e t o assuxa

thct

dryness

o f t h e hair, s k i n , and nasal passages ~ e s u l t i n g f r o m

low

humidity,

or

a wetness of t h e s k i n resulting f r o m h i g h

humidifv

a r e

i n v o l v e d

in

s e n s a t i o n s

o f

discomfort attribut- ed to extremes of humidity,

It

may be n o t e d , however, t h a t conditions w3ich

a r e

normal

t o one p e r s o n may be abnormal, and t h e r e r o r e uncomfortable 50 anather,

The

well-known

ASSAE Comfort C h a r t which has been

in

use f o r many y e a r s i n d t c a t e s t h a t a s the r e l a t i v e

humidity

f s increased, t h e temperature o f t h e air*

must

be

lowerg8 t o p r o v i d e t'ls same comfort sensation.

A

temperature of

74

F

at

10 p e r cent r e l a t i v e humidits

i s

shown to p r o v i d e t h e szme c o m f o r t a s a t e ~ e r a t u r e o f 7 0 w i t h a r e l a t i v e

3 m i d i t y

of

50 p e r c e n t .

It

has

been a ~ g u e d t h a t humidifying a house w i l l result in a s a v i n g in

f u e l

since t h e temnperatwe can be

loprered r ~ i t h o u t s a c r - i f ice

in

c o m f o r t , C ~ l c u l a t i o n w i l l s y o w , however, t h a t

with

normal r a t e s o f a i r l e a k a g e

in

houses tbe

e x t r a

r u e 1

r e q u i r e d to e v a ~ o r a t e t h e w a t e r for humidification w i l l

usually

offset t h e szzving t o be made by t h e t e m p e r a t u r e

reduction

p e ~ r n i t t e d

by

the

comfort c h a r t ,

A

research papep p r e s e n t e d r e c e n t l y before the

ASHAE

r e p o r t s no change in sensations of' comfort over the

range of r e l a t i v e humidity f r o m 30 to 80 p e r cent, at l e a s t

f o r c o n L 5 t i o n s at tqhich no free

sweat in^

occurs. This

i n d i c a t e s t h a t the influence of r e l a t i v e humidity may

be

over- emphasized by the Comfort Chart, The b a s i s f o r the Comfort

C h a r t is

shortly

to be re-examined through an e x t e n s i v e r e s e a r c h program,

It

has

long

been recognized thai

the

r e l a t i v e

humidities

in

Canodian

buildin~s h e a t e ? to comfort

temperatures a r e u s u a l l y quite

low

in

w i n t e r , The o u t s i d e

a i r

I-fhich e n t e r s a bui.1 ding in w i n t e r has a l o w moisture c o n t e n t , and when h e s t e d provides low r e l a t i v e humidity unless substzntial quantities o f w a t e r a r e e v ~ p o r a t e d

within the b u i l d i n g , This mzy be v e r i f i e d by r e f e r e n c e to t h e process shown by the line

F

G

of Figure 1, which

shews % h o t a i r from outside at 2 0 ' ~ and 100 p e r c e n t r e l a t i v e

humidity

r ~ i l l

when

h e a t e d to

75%'

have only 12 p e r cent r e l a t i v e :mrnidity. The m o t s t w e c o n t e n t

will

have

to

be

increazee f r o m 0,2 to

0,74

p e r cent to p r o v i d e

40

per cent r ~ l a t i v e

humidity,

The r e l a t i v e humidity

w i t h i n

a b u i l d i n g w 5 l Z

depend cn t h e moisture c o n t e n t of t h e outside a i r , the

v e n t i l a t i o n

r a t e ,

the rate at w h i c h moisture i s l o s t

t h r o u g h t h e building enclosure and the rate at w h i c h m o i s t - u r e is s u p p l i e d to the air within t h e b u i l d i n g ,

These

f a c t o r s m a y v a r y widely from one case

to

another,

The average January temperature in Vancouver is

~ S ' F

w h i l e t h a t

for

O t t a w a is +10'~.

The

corresponding

o u t d o o r r e l a t i v e hwnZdPties a r e about;

the sane,

88

p e r cent,

at each l o c a t i o n . However, when outtioar ~ . i r at t h ~ s e c o n d i t i o n s

i s

mimed t o

7S0F

t3e

resulting

humidity f o r

Ottewa

is only

r.$

per c e n t while t h a t f o r Vancouver is 25 per cent,

In

many

p u b l i c buildings h a v i n p high v e n t i l a t i o n r a t e s and l o w rates of moisture s u p ~ l y , the relative humidities produced w i l l . be

c l o s e t o

these

values,

'd?um the

v e n t i l a t i o n r a t e

Is

at t h e m i n i m w l l r e q u i r e d of

10

c f a p e r person, the r n o i s t u r e

rate

r e s u l t

in

'nuruictitics s f

60

r?sr c e n t

fop

I ~ c n c o c v e r 2nd

1;s

n e r cent

f o r 0ttal:x.

It

rngy bbe n o t e d t h a t n ~ b l i c b u i l d i n g s

in

Vancouver may n o t r e q u - i r e huvidiflcatlon even ir_ w i n t e r , an2 that

h ~ ~ i d i k s s s n a y ~ i s e at times a b o v c 50 g e r c e n t at m o d e r a t e

to low ventilation r a t e s ,

In

d w e l l i n g s thc situ2tioa i s s o m m h a t d i f f e r e n t

t h a n in

~ u b l i c bxildings s i n c e ventilation rates a r e

frequenklg reduces to a nlinimwn

in

o r d e r to conserve fuel

and t h e r e are substantial sources of m c i s t u r e . T h e a v e r a g e Canaciian dr.relling + r i l l '.lcLrc a transmission h e a t l o s s of

about

0.04

Btu

p e r cubic T o o t o f b u i l d i n g v c l - m e per degree

ternperatme d i f f e r e n c e

between

indoors and outdoors, Vent-

i l a t i o n (or infiltrstian) w i l l add to t h i s to the extent

45

p e r cent for e a c h h o u r l y air change. The s p a c e p r o v i d e d per o c c u p a n t

will

f o r many

houses

be a b o u t 2000 c u b i c feet, and

the hourly air change reqcired

f o r

v e n t i l a t i o n

at

the accepted

m i n i m u x o f 10 cfm p e r p e r s o n w i l l be cnly 0 . 3 , Tne advantages

in

fuel economy o f reducing the v e n t i l a - t i o n

r a t e

a r e a p p a r e n t ,

It

has

been d e t e r ~ ~ i n e d t h a t the a v e r a g e family

of

f o u r w f L l produce by i t s a c t i v i t i e s w i t h i n a house about

0 , 7 pounds of w a t e r vapour p e r h o u r normally, b u t t h e t this

fireg r i s o t o a s much a s 2 pounds p E r h o w on w a s h days,

These q u a n t i t i e s may be used to calculate t h e v e n t i l a t i o n

r a t c s at r~r'nricln

40

per cent r e l a t i v e humidity will b e m a i n t a i n e d

w i t h i n

the

?touse

for

t h e a v e r z ~ ; e

January

c o n d i t i o n s at Vancouver 2nd at O t t a w a .

It

i s f o u n d that

far

Vancocver the ventilation

r e t e must

b e 0.40 2S.r c h a n f ~ e c per hour nc.rmz!llg, a ~ d 1.1

on

wash days and for O t t a : . ~ a t h e r a t e s

must

be 0.25 and 0,F.S a i r

changes p e r h o w . Since o n l y

0.3 air

changes p e r hour are

r e q u i r e d f o r f r e s h c s s

it

may b e concluded! t h a t many houses a r e a d e c j u a t e l ~ 1 1 a ~ 1 i d . i f fed by norrrlal mcist-ure soEpces

w i t h i n t h e house,

Tf

t h e v e n t i l a t i o n r a t e s a r e

minimum,

On

days on which exceptional

amounts o f

r n c i s t w e are r e l e a s e d within t h e house

it

may be necessary t o i n c r e a s e

the ventilation r a t s

fnr above t h e ninimm to p r e v e n t t h e r e l a t i v e humidity

from

-

rising above 40 per cent,

T h e cases just considered a r e

for

a v e r a g e modern

house:, t i g h t l y constructed, and u i t h a r e l a t i v e l y

low

v o l u m e

per occ'llp~nt, Older, l a r g e r houses, multi-storey, h a v i n g =any

100s

e-Ti

thing ~ a i n C o i . ~ s , and 3 ~ o v i d i n g Large volume p e r o c c ~ p a n t w i l l have unavoidably I ~ i g h v e n t i l a t i o n r a t e s and may have

relatively small n a t u r a l m o i s t u r e sources, Rere, t h e extreme

low btwnid.ities may approach t h o s e c a l c u l a t e d f o r p u b l i c :

T o r Vancou-~er and

5;

p a x a cent f o r O t t a ~ * i a , _and m a y

be

even

l o ~ ~ j e r

f o r

p e r i o d s

o f

extreme

l o w t

em>erature. Feglec

ting

n a t w a l m a i s t ~ r e c o u r c e s , .r: ~ u c h a s

9.6

gallcns

of

v:.rater

p e r day w i l l have t o be,added t o rrtaintain

40

p e r cent relative humidity a t

7s F

f o r

each 10,000 cubic f e e t oP

b u i l d i n g v o l u ~ e

f o r

one a i r change p e r hour w i t h c u t d o a ~

air

6

szturatcd at 20

F,

k n ~ r i l e houses

of

twenty o r g l r r e ! gears ago had v e r g

low humidities unless humidifiers w e r e u s e d , it is p r o b a b l e

that

2 l a r g e p r o p o s t j o n of the h o u s ~ s constructed within the l a s t t c n y e a r s h a v e s-ficient humidity p r o v i d e d by natural. sources w l t h i n the house,

Upper

Limits

of Bumidity in i J i n t e r

L i m i t s a r e p l a c e d

on

t h e r e l e t i v e humidities v ~ h l c h c s n b e c r . r r i e d in Canadian b u i l d i n g s

in

w i n t e r by

the

occurrence of condensation, The occurrence of v i s t b l e candens

at

ion on t h e room-side s ~ r f a c e s

of

windows, walls,

f l o o r s , and c e i l i n g s depends upon t h e co~binstion o r t h e

r e l a t i v i :

humidity

maintained a n d the extent of the cooling produced a t t h e surfaces in question. Visible condensntion

a l t h o u g h cornyon cn v:indows in w i n t e r i s n o t a s t r c u b l ~ s o m e as c o n c e a l e d c o n d e n s a t i o n which can ~ C C U T in r o o f s , attic spaces, and in w a l l s , Water vapour can & i f f u s e through

i n t e r i o r finishes o f w a l l s and t h r o u g l ~ ceilings, or m a y be

carried by a i r l e a k a g e ,

until it

encounters cold surfaces

and i s c o n d ~ n n c d , When the surfaces on w11ich t h i s condensa- tion occurs ere brlow

freezing

f o r l o n g p e r i o d s of t i m e t h e

condensation

accumulates

as frost,

It

is n o t impossible

(in

the c o l d e r a r e a s o f ~ a n a d a ) , with p o o r v z p o u r c o n t r o l in

a w a l l or ceiling t o c o l l e c t a s

much

as one pourad or frost

p e r square foot of w a l l or roof a r e a ,

This

provides

one

g a l l o n o f w a t e r p e r stu.d s p a c e

in

a frame w a l l , w h j ~ ? rr-ay

be r e l e a s e d o-Jer a few h a w s upon a r i s e

in

-t;emper:-t!rrre.. There is l i t t l e p o i n t in attelrrpting to f o r c e t h e

r e l a t i v e humidity in a building zbove the paint at which visible ccndensotion begins to occur, s i n c e the moisture

added will be transferred t o t h e c o l d s u r f a c e s and % r i l l

accumulate

t h e r e . The u p p e r lirnfts imposed o n r e l a t i v e humidities if windornr c o n d e n s ~ t i o n i s t o be avoided a r e

Outdoor

Temperature

Sin@

e Illlndow Double Window

Wind

Nu

Wind Wind N o Wind

It

nay be n o t e d

that

double windows p r o v i d e higher i n s i d e surf ace temperatures m d t h e r e f

o r e

permit hZgher humidities t o be carried,

Houses

can now be reasonably w e l l constructed,

w i t h i n s u l a t i o n , v a p o u r b a r r i e r s , and a t t i c ventilation, s o

t h a t

it

i s p o s s i b l e to c a r r y humidities u p to - h o s e produc- i n g window c o n d e n s a t i o n , w i t h o u t o t h e r m o i s t u r e dlf'f i c u l t i e s ,

150st public and commercial b u i l d i n g s a r e net cons true t e d t o t h e same s t a n d a r d s o f I n s u l a t i o n and v a n o u r c o n t r a 1 as wood frame houses and cannot

t o l e r z t e

the same r e l a t i v e

humidities without difficulty, Indeed much contemporary

b u i l d i n g d e s i g n employfng much glass and m e t a l . in w a l l s

is accentable only if t h e

inside

r e l a t i v e h m d d i t i e s

a r e

allowed- t o remain at a low l e v e l

in'

cold weather,

The humidity in

o l d e r houses and

in

most public

and cormnercial buildings

not

s p e c i a l l y designed f a r p r o p e r

c o n t r o l of

w a t e r vapour s h o u l d p r o b a b l y not ~ x c e e d 20 p e r cent

a t temperatures below z e r o . F o r

ineoor

humidities of

SO

pey

cent a t sub-sera o u t s i d e temperature, insulation and vapour

b a r r i e r s

must

be used, Low r e s i s t a n c e h e a t p a t h s p r o v i d e d

by metal

or

s o l i d masonry at columns, at t h e edges of f l o o r and r o o f s l a b s and arcund v.Jindows

m u s t

be avoided, or

given

s p e c i a l treatment to e l h i n a t e cold

room-side

s u r f a c e s , Double windov~s a r e essential,

Industrial

buildings

such

a s

textile mflls

in

which

hwniditles

o f

70

t o 80 p e r cent must

be c a r r i e d r e q u i r e still g r e a t e r preczutions in design, Windows m a y h a v e to b e eliminated, s p e c l a l vapour barriers,

u s u a l l y a p p l i e d as the i n s i d e surf ace,

will

b e required,

and

h e a t may have to be introduced a t c e r t a i n portions of the

structure

t o maintain adequately high surface temperatures,

R o o f s

f o r

such conditicns

present a

s p e c i a l problem.

It i s

clear

that

m z n y problems and much expense a r e

involved when

Conclusion

While I t w o u l d be d e s i r a b l e t o a v o i d

htgh

m o i s t m e c c n t e n t s to

p r e v e n t

deterioration

of

t h e b u - i l d i n g

structure,

and a"t;ihe

same

t i m e to p r o v i d e

constant humidity the

Fear

round t o minixize

d i m e n s i o n a l and o t h e r chztnges

in

m t e r i a l s

and

ru~nizhlngs

witbin

a b u i l d i n g , this

is

d i f f i c u l t t o achieve mder C s n a d i m

~ - 1 i n t e r

c o n d i t

i

a n s . Even

noderat

e

humidities inside buildings can produce wetting by condensa-

t l o n

on

and

in

the building

structure,

because o f the large

t e m p e r a t u r e gradients w Z t h i n w a l l s in rdinter. T h e b e s t

answer

in

many c a s e s where higher humidities are not; necessary r i i L L b e to allow the humidities to f 2 l l t o l o w l e v e l s . In

the case

of b u i l d i n g s having l a r g e moisture s o w c e s steps may have to be t a k e n t o ellmTnate water v a p o u r f r o m t h e n t o

a v o i d condensation. Consideration should

z l w ~ y s

be

g i v e n

to

t h e difficulties which m a g he c r e a t e d

in

any g i v e n

b u i l d i n g b e f o r e humidification i s added. )ken

r e l a t i v e

humidities

in

the i n s d i m

t o

high range m s t be

carried,

the buildings

must

be specially designed and constructed,