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Building Materials News, 20, p. 7, 1964-04
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Humidity and buildings
Hutcheon, N. B.
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C A N A D A
D I V I S I O N O F B U I L D I N G R E S E A R C H
-.*l'
HUMIDITY
AND BUILDINGS
BY
N . B . H U T C H E O N
E u l L D l ; r ; l t ' j . f " : : t l l - l . l t T r \ r ' /-AUG
rs tg6t
N A T I O I I A L R E S E A R C H C O U N C T LA h ! A ! . Y 7 -
E D
P R I N T E D T E X T D U P L I C A T E D F R O M B U I L D I N G MATERIALS NEWS. v o L . 2 0 . A P R | L 1 9 6 4 T E C H N I C A L P A P E R N O . I g g O F T H ED I V I S I O N O F B U I L D I N G R E S E A R C H
OTTAWA
S E P T E M B E R 1 9 6 5
N R C A r 5 2 P R I C E I O C E N T S3c,4(68L
I n c r e a s i n g
a t t e n t i o n is now being given to the provision of modified and
con-t r o l l e d r e l a con-t i v e h u r n i d i con-t y i n buildings for cercon-tain occupancies.
N e w c o n s i d e r a
-t i o n s a r e -t h u s i n -t r o d u c e d i n
t h e s e l e c t i o n o f
t h e h u m i d i t i e s
t o b e carried, which
m u s t b e r e a l i s t i c a l l y
a p p r a i s e d in terrne of need or desirability
a s w e l l a s o f
t h e p r o b l e m s
w h i c h may arise in the provision of thern.
T h e s e v e r e w i n t e r
c l i r n a t e i n Canada rnakes it necessary to give special attention to theproblerns
a s s o c i a t e d with increased indoor humidities during cold weather, not only in
the provisions
which rnust be made so that hurnidities
can be rnaintained,
but
a l s o in the design of the building enclosure in order to avoid accelerated
deg-r a d a t i o n a n d i n c deg-r e a s e d m a i n t e n a n c e .
T h e p a p e r d i s c u s s e s t h e s e v a r i o u s c o n
-s i d e r a t i o n -s
a n d indicates in general terrns what must be done when increased
i n d o o r r e l a t i v e hurnidities rnust be carried in winter.
O n s r o c c u p e d e p l u s en plus ) I'heure actuelle de rnaintenir dans les rnaisons
d e s h u r n i d i t 6 s r e l a t i v e s rnodifi6es et contr6l6es pour certains usages. De
nou-v e l l e s consid6rations sont ainsi introduites dans Ie choix des hurnidit6s b
rnain-t e n i r .
C e s h u r n i d i t 6 s
d o i v e n t 6 t r e e v a l u 6 e s d e f a g o n r 6 a l i s t e e n f o n c t i o n d e s
b e s o i n s o u d e s conditions d6sirables
a i n s i q u r e n fonction des problbrnes qui
p e u v e n t se faire jour b cause dreIIes.
L e s 6 v b r e c l i m a t d t h i v e r a u G a n a d a f a l t
q u t i l e s t n 6 c e s s a i r e d e p o r t e r u n e a t t e n t i o n s p 6 c i a l e a u x p r o b l b m e s a s s o c i 6 s
a v e c l e s hurnidit6s accrues b lrint6rieur
d u r a n t l e s p 6 r i o d e s f r o i d e s n o n s e u l e
-m e n t e n ce qui concerne les rnesures b prendre pour que les hurnidit6s
puis-s e n t 6 t r e r n a i n t e n u e puis-s rnaipuis-s aupuis-spuis-si en ce qui concerne la conception depuis-s bdtirnentpuis-s
e u x - r n 6 m e s a f i n d t 6 v i t e r l a d 6 g r a d a t i o n a c c 6 1 6 r 6 e e t I r e n t r e t i e n a c c r u .
L t 6 t u d e
p a s s e en revue ces diverses consid6rations
e t i n d i q u e e n t e r r n e s g 6 n 6 r a u x c e
q u i doit €tre fait lorsque des hurnidit6s relatives
a c c r u e s
s o n t r e q u i s e s
b
I t i n t 6 r i e u r
e n h i v e r .
HUMIDITY
AND
BUITDINGS
by N. B. HUTCHEON
*
This paper wN presented at the Symposium on Contolled Humidity in Buildinis, at thA Construction Induslry Exhibition in MonteaL Aue.,
November 26, 1963
A building volume of 10,000 cubic feet, about that of the average house, contains about 750 pounds of air. Asso-ciated with this amount of air there will be from 2 to 12 pounds of water vapour, amounting on the average to about I per cent by weight of air. This water vapour con-tent of the air, though relatively small, can be an import-ant part of the environment provided for people, goods and processes.
Relative Humidity
Most common materials have an affinity for water, so that their moisture content is influenced by the moisture in the air surrounding them. This important effect, along with several others, is primarily related not to the absolute moisture content of the air but to its.relative saturation or relative humidity.
There is a definile limit to the amount of water vapour that can be held in the air at any given temperature. As the temperature increases, the capacity of the air to hold water vapour increases markedly. Several important con-siderations follow immediately from this. Air at a low temperature outdoors in winter when fully safurated, that is at 100 per cent relative humidity, contains very much less water vapour than can be held at room temperature. For example, the saturation moisture content at -20oF is O.026 per cent while that at 73o F is 1.76 per cent or 68 times as much, the relative humidity being 100 per cent in both cases. When such outdoor air is brought in-doors and warmed to room temperature without the addi-tion of water vapour, the amount of water vapour which it contains is only lr/z per cent of that which it requires for saturation at its new temperature. That is, it will be at lL/z per cent relative humidity. This explains why many buildings, ventilated with outdoor air in winter, but with-out intentional humidification may have relative humidi-ties of only 5 or 10 per cent.
There is always some gain of moisture within every normal building. This may come from people, from wash-ing, cookwash-ing, and drying of clothes in the case of dwell-ings, and from a variety of wet processes carried on in other buildings. It does not follow that because there is no intentional humidification the indoor relative humidity will always be low; some buildings have ample moisture supplied from internal activities and many require in-creased ventilation to reduce the relative humidity even in winter.
Condensation in Buildinge
One of the problems that arise in winter with too high a relative humidity indoors is that of condensation. This can be demonstrated by consideration of the reverse case to that involved in ventilating with air drawn from out-doors in winter. Air which is at 50 per cent relative hu-midity at 73oF has 0.88 per cent moisture content by weight and is 50 per cent saturated. But this moisture
r , . A s s - i s t a n t D i r e c t o r , D i v i s i o n o f B u i l d i n g R e s e a r c h , N a t i o n a l R e s e a r c h C o u n c i l , O t t a w a .
F i g u r e I
- 1 0 0 l 0 2 0 3 0 4 0 5 0 O U T S I D E A I R T I M P T R A T U R E , " FR e l a t i v e h u r n i d i t y a t 70 oF at
w h i c h c o n d e n s a t i o n o n i n s i d e s u r f a c e g l a z i n g o c c u r s f o r v a r i o u s o u t -s i d e a i r t e m p e r a t u r e -s .content is sufficient to saturate air at 54oF which, because of its lower temperature, requires less moisture for satura-tion. Cooling of air at 73oF, 50 per cent RH to 54oF will therefore produce saturation, that is, the relative hu-midity will be 100 per cent. When cooled below this point condensation must occur, since the air can no longer hold its original moisture content. The excess moisture will be rejected as dew on the cooling surface, or as frost if the surface is below the freezing point. The temperature at which an air-vapour mixture will become saturated upon cooling is called the dew-point temperature, and this is another way to describe its moisture condition. Naturally, as indoor relative humidities at a given temperature are decreased, the dew-point temperature also decreases, since the air must then be cooled to some lower temperature to produce saturation. It is apparent tlen that under win-ter conditions when exwin-terior walls and windows are cooled below room temperature the indoor relative humidities may have to be limited in order to avoid condensation on
-20 -10 - i l 0 1 0 0 * R . H . o f I n l e r i o r A t m o s D h e r e l 5 t r t P H 0 u t 3 l d 6 S l l l l A l r 0 u t s l d e
2-4 2 . 2 2 0 r ' 8 t . 6 t.4 t . 2 t.o . E . 6 . 4 . 2 F
-o=
e d o TF 2 z o F=
cool surfaces. On the other hand. when it is known in ad-vance of construction that a particular relative humidity must be maintained, the design of the building can be adjusted appropriately. o 30 40 50 60 70 75 80 90 loo T E M P E R A T U R E . F
2
S i m p l i f i e d
p s y c h r o r n e t r i c
c h a r t s h o w i n g p r o p e r t i e s
2 0F i g u r e
a n d p r o c e s s e s o f a i r a n d w a t e r v a P o u r rnixtur e s .Cooling and Dehumidification in Surnrner
The situation in summer is potentially one requiring both cooling and dehumidification. In Ottawa, for example, the combination of high temperature and high relative humidity outdooors may occasionally reach a limiting value represented by an outdoor dew point of almost 80oF. This corresponds to a moisture content of. 2.2 per cent and re-quires during conditioning to 75oF and 50 per cent RH, the removal of water to the extent of 1.3 per cent of the weight of ventilating air being processed. Added moisture sowces within the space can add further to this dehumidi-fication load.
The Control of Relative Humidity
The relative humidity within a building may be allowed to vary naturally, without the aid of intentional humidifi-cation or dehumidifihumidifi-cation. It can then vary from low values in heated buildings in winter to high values in sum-mer, as determined largely by the moisture content of the ventilating air. The presence of significant moisture sources within the building will always result in increased relative humidities, which can in part be offset at times by increased ventilation. With high moisture sources in winter or summer, condensation can take place within buildings and will itself provide a limit on conditions if allowed to occur.
Intentional humidification and dehumidification can be employed separately as required or as integral functions of a complete air-conditioning system. In either case de-cisions must be made as to the levels and the precision of control of relative humidity to be provided, and for guid-ance in this it is necessary to consider the influence of
relative humidity on people, materials and processes, as well as upon the enclosing construction.
Relative Humidity and Materials
All porous materials as well as most natural organic materials have large internal surfaces which exert forces of attraction on water molecules. Consequently these ma' terials have natural moisture contents which vary with the relative humidity of the atmosphere to which they are exposed, provided that there is the time and the opportun-ity for any necessary adjustment in moisture content to take place.
Many natural organic materials become hard or brittle at low moisture contents so that control of moisture con-tent at moderate or high levels by the maintenance of high relative humidities may be necessary in processing themn as in the case of cigar- and cigarette-making. In other cases it may be necessary to reduce relative humidities to inhibit the growth of fungi or bacteria or to prevent chemical changes such as the corrosion of metals.
Significant dimensional changes take plaee in many materials with change in moisture content. The best known are those which take place in wood, of the order of O.1,2, and 4 per cent in the longitudinal, radial, and tangential directions respectively on a change from air-dry at 12 to 15 per cent moisture content corresponding to about 65 per cent RH, to oven-dry conditions. Most wood-fibre products, including papers, will exhibit moisture expan-sion consistent with the basic wood properties. Almost all plant and animal fibres experience appreciable moisture changes with changing relative humidity and undergo sub-stantial dimensional changes of the same order as those in wood. Less generally recognized are the dimensional changes that can occur in masonry materials as a result of changes in moisture contenL
The cycling of relative humidity, leading to changes in moisture content and to corresponding dimensional chan-ges can, when repeated many times, produce degrading physical efiects in some materials. Combinations of ma-terials having different moisture response characteristics, when put together at one relative humidity and then changed to another, may warp or in extreme cases may be ruptured as a consequence of the dimensional changes which are produced.
Thus three distinct requirements for relative humidity control within buildings may arise because of humidity effects on materials:
1. Low relative humidities may be required in order to slow down or prevent degrading chemical or biologi-cal changes. Close control of the level of humidity may or may not be necessary.
2. High relative humidities may be required to promote moderate to high moisture contents which are desirable in handling or processing. Close control of the level of humidity may be necessary.
3. Maintenance of a constant relative humidity may be desired in order to minimize degrading or other"urise undesirable dimensional changes during processing, use, or storage. Closeness of control required will depend upon the response characteristics of the materials in-volved and the degree of dimensional control desired. It may be noted that a conflict in requirements may exist. Low humidities may be desired for preservation, while some comprornise may be necessary if the materials must
also be handled. This is the case in libraries, where it may be highly desirable also to maintain constant relative hu-midity in order to avoid degrading dimensional changes in rare books and papers.
Relative Ifumidity and People
It has long been accepted that thermal sensations of comfort are aftected by relative humidity and that for the same comfort a higher temperature is required to offset a decreased relative humidity. The research work on which these conclusions were based is under review lnd it is now evident that the earlier conclusions drawn from it apply mainly to the thermal sensations experienced over limited periods of time after leaving one room for another at a different relative humidity. This effect will be well known to those who have occasion to move from one con-trolled temperature room to another at the sarne temper-ature but difierent humidity. It is due in large part to the thermal effects involved in the rapid readjustrnent of the moisture content of clothing to the changed relative hu-midity. For conditions of prolonged exposure to the same relative humidity at temperatures within the comfort zone, most people engaged only in light work will detect little change in thermal sensations of comfort over a wide range of relative humidities.
It has been argued that, with increased relative humidity, lower temperatures may be maintained with the s:rme com-fort but with consequent savings in heating costs. This argument may be contested on two counts. The comfort conditions are not likely to be equivalent for prolonged occupancy, and the energy cost to evaporate the added water required for humidification will often more than offset the saving to be gained by reducing temperature.
Arguments favouring the reduction of'relative humidi-ties to moderate levels indoors under summer conditions would appear to be somewhat more valid. Although the eflects of relative humidity upon the comfort, or more properly the degree of discomfort, of people engaged in moderate to high levels of physical activity have not yet been carefully re-examined, there is little reason to doubt that relative humidity has a marked influence under con-ditions of sweating. It seems reasonable to assume that reduced relative humidity may prqvide comfort over a wider range of physical activity for the same temperature in occupancies of mixed activities, as in the case of people seated in a ballroom while others are dancing.
There may well be valid arguments against low humidi-ties, on grounds other than thermal comfort since they undoubtedly lead to increased evapciration from mem-branes of the nose and throat, and to dry skin and hair. There is little conclusive evidence, however, of serious haz-ards to health despite the fact that millions of Canadians have lived and worked under low
hu-midity conditions for winter after win-ter.
It seems logical to assume that, in general, extremes of humidity are un-desirable and that relative humidities should be kept within a broad range of 30 to 70 per cent. There is, however, no firm basis for establishing such lim-its so far as the health and comfort of most people are concerned.
Electrostatic Efrecte and Humidity
It is well known that electrostatic charges capable of producing large and often unpleasant sparks when dis-charged can be generated by walking or sliding on a carpet in winter. This comes about by a separation of elec-trical charges when materials of high electrical resistance are brought inti-mately into contact by sliding one over the other. Dfficulties may be en-countered in the handling of paper sheets, fibres and fabrics which, when charged, may be attracted or repelled by one another or by other bodies de-pending on the nature of the charges. Control of dust or lint may also be a problem when it is attracted and clings to oppositely charged objects. Other diffculties may be introdu0ed when explosive gas mixtures are present since these may be ignited by acci-dental electrostatic spark discharges.
r o 2 0 ' * f l r . t t i o m F i g u r e 3 E l e c t r o s t a t i c c h a r g i n g o n
f a b r i c s a t d i f f e r e n t r e l a t i v e h u r n i d i t i e s .
The water layer which is held on most surfaces becomes sufficiently thick as the relative humidity is increased so that it prevents the accumulation of such charges. An in-crease of relative humidity to 50 per cent or more will usually be necessary to reduce or eliminate undesirable electrostatic effects with many materials; wool and some synthetic materials may require still higher humidities.
= 3 m
! zmo
o-o Colbn Drill o . . . o i l y l o n T a f t e h q- - -6 Propylon q - - - - - e W o l
Electrostatic charging may, however, be a maximum at a relative humidity of from 25 to 35 per cent under some conditions and even higher in a few special cases so that increasing the relative humidity from a low value may ac-tually increase the dfficulty unless it is carried sufrciently beyond the value at which the effect is a maximum. Elec-trostatic effects are sometimes so serious, as in the case of hospital operating rooms where explosive mixtures of anaesthetics are used, that they may determine the hu-midity levels to be carried. The unpleasant shbcks exper-ienced by guests in hotels may even be cause for serious consideration of increased humidities. The reverse situa-tion may be encountered in electrical or electronics work where a requirement for the highest possible electrical re-sistivity may call for the maintenance of low humidity.
Relative Hunidity and Aud.itoria
A very interesting humidity efiect is involved in air absorption of sound waves. It is found that air absorption is quite substantial for the higher frequencies and is a maximum in the relative humidity range of 15 to 20 per cent with absorption increasing as the frequency increases.
0 o . o o . o o . o o . o l o . 0 r o o Practieal Coneiderations
It might be concluded from the discussion thus far that a constant relative humidity of 50 per cent maintainer! the year 'round would be ideal for some conditionri Gr occu-pancy and a good compromise in others. Indeed, this is a commonly adopted value for many laboratories and workrooms in which constancy of relative humidity is a major requirement. But there is a great difference in the air-conditioning equipment required, and therefore in the cost, to maintain relative humidity reasonably constant, in comparison with the more usual comfort conditioning in which temperature is controlled but relative humidity varies over a range depending on the load and system char-acteristics. Reasonably constant relative humidity can be provided, but only at a price. The more closely the rela-tive humidity must be held, the greater the over-all cost will be.
It must not be assumed that close humidity control is dependent only upon the equipment provided. The charac-teristics of the system and of the whole building must be taken into account. For example, it is useless to provide sensitive humidity contxol capability if temperature is not also closely controlled and held constant throughout the work area. Since a LoF change in temperature at 73o causes an RH change of 3 per cent of the RH involved, the air temperature variations throughout the area to be controlled must also be held within appropriate toler-ances. In addition, variable radiation efiects from cold walls, hot sources, and solar efiects which cause the tem-peratures of irradiated objects to differ appreciably from air temperature may have to be eliminated. Finally, the air infiltration as well as the air distribution must be care-fully controlled, since this may cause variations in the water vapour distribution throughout the space, thus nulli-fying the efiects of close temperature and humidity control at the humidistat locations. The importance of these mat-ters increases as the control precision increases, and in the extreme they may dictate many features of the enclos-ure, layout, and use of the enclosed space.
The level of the humidity to be maintained also has im-portant practical implications. Dehumidification in sum-mer to provide control at 50 per cent RII can be obtained with chilled water. If the relative humidity is to be held much below this value, however, chilled water systems can-not be used because of freezing, and other types of de-humidification equipment must be considered. On the other hand, problems may be encountered in the design and operation of the building when the relative humidities to be carried during the coldest weather are much above 20 per cent. For these reasons it will often be practical to adopt a compromise and to maintain 50 per cent RH in summer and some lower relative humidity in winter, thus easing both the dehumidification and the building prob-lems.
A steady increase can be expected in the number of buildings that must be constructed to provide a constant indoor humidity of 50 per cent or higher both winter and surnmer. These will probably include hospital operating and clinical rooms, resealch laboratories, and libraries, museums, and art galleries housing valuable collections. In some of these cases a reduction in the winter level of relative humidity may be accepted to ease the building de-sign problem. I F o rl i F z
=
9 B E U o I z o F t z U F F o r o 2 o 3 0 4 0 5 0 6 0 7 0 R E L A T I V E H U M I O I T Y , P E R C E N TF i g u r e 4
E f f e c t o f r e l a t i v e h u m i d i t y
o n a i r a b s o r p t i o n o f s o u n d
a t d i f f e r e n t f r e q u e n c i e s .
( R e p r o d u c e d
w i t h p e r r n i s s i o n f r o r n I ' A b s o r p t i o n o f
S o u n d i n A i r i n t h e A u d i o - F r e q u e n c y
R a n g e r r , J o u r n a l , A c o u s t i c a l S o c i e t y
o f A r n e r i c a , V o l . 3 5 , J a n u a r y 1 9 6 3 .
The effect will be of little importance
for speech
but may
merit consideration
in the case of large halls or auditoria
which are required to have very good acoustic properties
for musical performances. An increase in relative humidity to 40 per cent provides a marked reduction in absorption, and at 50 per cent the effect may for practical purposes be considered unimportant.
( l ) P a p e r M i I I
( Z l O p e r a t i n g R o o r n
( 3 ) C o r n p u t e r E q u i P r n e n t
F i g u r e 5
T h e s e a c t i v i t i e s r e q u i r e b u i l d i n g s c a p a b l e o f c a r r y i n g i n c r e a s e d
r e l a t i v e h u r n i d i t y i n w i n t e r .
( I ) P h o t o - D B R / N R C ;
( Z l P h o t o
-O t t a w a C i v i c H o s p i t a l ;
( 3 ) P h o t o - D i v i s i o n o f M e c h a n i c a l E n g i n e e r i n g , N R C .
But it is quite apparent that there is a growing demand for increased relative humidities in winter in all kinds of buildings designed primarily for human occupancy. This stems partly from the more widespread acceptance of air-conditi,oning and probably also from the improved hu-midity conditions which are now possible, and widely maintained, in Canadian dwellings.
It is of the greatest significance that buildings in greatly increasing numbers will be required to carry higher rela-tive humidities in winter. This requires the most careful consideration of designers and builders alike, since, with-out an enlightened design approach, buildings will either experience ierious degrading effects with increased main-tenance or reduced life or will be changed to operate at
tential difficulties in increasing the humidities in
commer-followed will now be reviewed briefly.
Builfing Design for HumiditY
It is fust of all essential to arrange that all interior sur-faces be kept at temPeratures above the dew-point tem-perature of ihe desired indoor conditions under the lowest iustained winter temperatures to be encountered. Windows
F i g u r e 6
I n d o o r r e l a t i v e h u r n i d i t i e s
i n w i n t e r i n e x c e s s o f t h e
t i r n i t s e t b y c o l d w i n d o w s u r f a c e s l e a d
t o c o n d e n s a t i o n .
become necessary to eliminate windows or to use double
vide close control in interior enclosures only'
There are usually many other parts of the enclosing construction in addition to windows which are weak points thermally in today's designs. Paths of high thermal con-ductivity leading to reduced inside surface temPeratures occur where columns, spandrel beams, and the edges of slabs and cross-walls intrude into or through the exterior wall. Attempts to insulate these members on the inside will usually not be successful, at least for extreme cases; the proper place for.insulation is on the outside of them.
The second set of principles relates to control of water vapour flow into the wall in order to avoid condensation and serious degrading effects within the enclosure. Make the entry of water vapour into the wall from the inside as
upper portions of a building since' due to chimneY action in winter, air leaks in at lower floors and outward at upper floors of anY heated build-Ing.
Careful attention should be paid to all possible dimensional changes aris-ing from shrinkage, creep' tempera-ture and moistempera-ture effects throughout the enclosure and the portions of the building frame associated with it' Cracks on the warm side of the wall which will allow air leakage should be anticipated and adequate sealing pro-vided at the outset at strategic points' Particular care should be taken at all critical sections such as the junction of wall and roof' and below window sills to ensure that adequate protec-tion against vapour and air flow is provided.
AIR
A i r l e a k a g e f r o r n a h u r n i d i f i e d
o n e w i n t e r .
6
F i g u r e 7
b u i l d i n g c a u s e d t h i s d a r n a g e i n
It will be found, when these prin-ciples are applied, that strong con-tinuous exterior cladding rigidly fas-tened at frequent intervals to the building frame is not necesary, and indeed is not a good way to construct an exterior wall which must meet the thermal, vapour, and air leakage re-quirements of a humidified building. A structurally self-sufficient interior wythe can be insulated on the outside and the insulation made continuous across the outer faces of the frame, slabs, and partitions. This greatly aids in
meeting the requirements already set out. It is then de-sirable to consider the use of exterior cladding designed on the basis of what is now being called the "open rain screen" approach. Positive control of rain penetration can be achieved, diffcult sealing problems can be obviated, the exterior cladding relieved of major wind loads, and the
thermal problems associated with the necessary ties greatly
relieved. Conclusion
There is no doubt whatever that failure to follow proper principles in the design of humidified buildings will result in many problems and much expense. The record of difr-culties is growing rapidly and there are increasing num-bers of buildings of recent construction in which the rela-tive humidities are being reduced in winter to levels below those intended. This is a waste which can be avoided, since it is not necessarily difficult or expensive to design build-ings to carry almost any level of relative humidity satis-factorily, provided that the principles are understood and the design of the enclosure is approached in a rational wav.
