The Condensation Problem - Here Are The Causes And Cures

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The Condensation Problem - Here Are The Causes And Cures

Ser

NA7110

N21h8

no. 11

c. 2

BLDG

NATIONAL RESEARCH COUNCIL

CANADA

DIVISION OF BUILDING RESEARCH

HOUSING NOTE NO. 11

MERE

ARE KME CAUSES AMIP CURES

-_

BUILDING PESEaIRCH

-

I_IBPA,ipY

-

REPRINTED FROM

CANADIAN BUILDER, VOL. XIII, NO.

7

JULY 1963,

P.

34

OTTAWA, JULY 1963

The condensartiion probllem

-

here are

the causes and cures

The problems of condensation have become increasingly important to builders.

This fact was recently recognized by the Winnipeg House Builders As- sociation following requests from its members for information on the cause and prevention of condensation on windows.

A direct request by the Association to the Division of Building Research, National Research Council, led to the findings reported on these pages.

The DBR will make quantities of this note available to interested as- sociations following its issuance in the Housing Note series.

BY H. 8. DICKENS, Housing Section, Division of Building Research, N.R.C.

Excessive condensation of moisture on windows is a source of increasing concern in Canadian homes in winter. It not only creates a nuisance by limit- ing visibility and by wetting adjacent wall and floor surfaces, but in more severe cases it can cause rotting of wood and deterioration of paint and plaster. To the builder the problem takes on a special significance because the home- owner frequently assumes that window condensation is a fault of construction. He does not readily appreciate that his own living habits are of prime im- portance, nor that a well built house is often more vulnerable to excess mois- ture problems than one that is loosely constructed.

Fortunately most cases of window condensation can be controlled by the occupant once he understands the basic causes of the problem. It is the purpose of this paper to discuss these causes and to provide suggestions for controlling excessive moisture in houses.

What is condensation?

Condensation problems arise because air can hold only a limited amount of water vapour, an amount that varies with temperature. When air at a cer- tain temperature contains all the water vapour it can hold it is said to have a relative humidity of 100 percent. If, at the same temperature, it contains only one-half the water vapour it is capable of holding then the relative humidity is

50 percent. If the temperature changes but no water vapour is added or taken away, then the relative humidity will also change and will increase as the temperature falls. The relative humidity will continue to rise with falling tem- perature until the dew-point is reached -that is, the temperature at which the relative humidity becomes 100 percent. Any further decrease in tempeature will force some of the vapour to condense as water (when the temperature is above freezing) or as frost (when the tempera- ture is below freezing). Air cooled by contact with the cold surfaces of win- dows will therefore deposit some of its water vapour on the glass or the metal sash whenever it has more water vapour than it can hold at its new tempera- ture.

Condensation may occur on either the inside surface of the inner window or the inner surface of the outer window.

The first case indicates that there is too much water vapour in the air for the weather conditions prevailing at the time. The second case indicates air leak- age outward around the inner window, and will occur even when the amount of water vapour in the air is quite low. This latter form of condensation is more prevalent on up-stairs storm windows and the down-wind side of the house.

Condensation usually occurs first on windows because they have the lowest temperature of any of the interior sur- faces in the house. It seldom appears on walls because they are normally warmer, although occasionally conden- sation may occur on cold spots such as nail heads and in the corners of out- side walls and closets where the insula- tion value is reduced and circulation of warm room air is restricted. In extreme cases this has led to mildew and thc growth of mould.

Humidity levels in

houses in winter

Humidities should be controlled so that little or no condensation appears on the inside surface of the glass. With double glazing this still permits quite high rela- tive humidities except during the most severe weather as indicated in the follow- ing which shows the maximum humidities that can be tolerated if condensation is to be avoided in cold weather.

Outside Air Desirable Maximum Temperature, OF. Rumidlty, %

-20 20

-1 0 25

0 30

10 3 5

20 40

If only single glazing is used, much

Summary of

findings

Condensation will occur on

any surface that is below the dew-

point temperature of the air in

contact with it. In most houses in

winter, the inner surfaces of the

windows are the coolest surfaces

and condensation will usually

occur there before it appears on

other surfaces. When condensa-

tion becomes excessive the mois-

ture load must be decreased or

the ventilation rate increased. T o

reduce the moisture load, humid-

ifiers should be cut off, automatic

dryers vented, clothes drying in-

doors or other activities that

produce unusually high quantities

of moisture avoided, and the

structure checked for other pos-

sible moisture sources. Ventila-

tion is generally the most effec-

tive means available to the house-

holder for reducing humidity and

should be obtained by opening

windows or operating exhaust fan

as soon as significant window

condensation appears.

lower humidities will produce condensa- tion (less than 12% at 0°F); storm win- dows can he installed. however, to pro- vide the thermal equivalent of double glazing and thus permit these higher humidities to be maintained.

In practice, condensation will occur first over the lower part of the window because the glass surface temperatures are not uniform, being lower at the bot- tom than at the top. Condensation at the base of the window and also at the sides tends to be more severe with metal sash and with some special units such as fac- tory sealed double glazing where the method of assembly results in increased heat transfer at the edges. Drapes or other window coverings can contribute to the problem by restricting the flow of warm room air over the glass surface.

The householder need not measure the humidity directly, he can simply use the windows as a guide to the proper humid- ity level within the house. As soon as objectionable condensation occurs on the inside surface of the window, steps should be taken to reduce the relative humidity by controlling the moisture sources or by increasing ventilation.

It is a common belief that for health reasons there should be a lot of mois- ture in the air during the winter months. There is, however, no conclusive evi- dence that either the health or the com- fort of most people will be adversely affected if humidity is kept at a level that will prevent excessive condensation on the interior surfaces of double win- dows.

Sources of moisture

The principal sources of moisture in a typical home are the household ac- tivities, which vary with the living hab- its of the family. Some idea of the quantities of moisture released by these activities in a family of four is given in Table I.

These figures show that approximate- ly 15 to 20 Ib (1% to 2 gallons) of moisture per day may be introduced into a house with four occupants under nor- mal living conditions and that this can rise to as much as 40 to 50 Ib (4 or 5 gallons) per day on washdays. This may be increased considerably by an effici- ent humidifier, although fortunately for many homes the simple pan-type hu- midifiers commonly installed on fur- naces are relatively ineffective. If they were not, there would be considerably more cases of excessive moisture in houses. A gas range in the kitchen will also add moisture to the air from water vapour which is one of the products of combustion when gas is burnt.

When a high relative humidity is a problem, steps should be taken to con- trol the moisture sources. Although there

TABLE

I

Moisture Produced by Various Household Activities for

a

Activity Moisture Produced (

Ib.

Cooking

( 3

Dishwashing

( 3

Bathing

-

-

Clothes washing ( p e r w e e k )

4

Clothes drying indoors o r with

unvented dryer ( p e r w e e k )

26

Floor mopping ( p e r 100 sq. ft.)

3

Occupants (family of four per d a y ) 12

is usually little that a householder can do to alter the normal cooking and bathing habits of his family, the weekly laundry should be dried outside or in a well ventilated space inside the house. Automatic dryers and gas ranges should be provided with an exhaust vent to the outside and humidifiers should be dis- connected.

Severe cases of condensation have oc- curred in buildings constructed over an

improperly drained and unprotected

crawl space. As much as 10 gallons of water per day may be evaporated from exposed soil beneath a building if the ground is wet and the surface is not covered with a vapour resistant mem- brane.

In new houses considerable quantities of moisture may be released from the various materials used in construction. An estimated 700 gallons of water are used in plastering alone in a six-room house. This will add considerably to the total moisture load during the initial period of occupancy.

Importance of ventilation

If a house were completely air tight so that no outside air could enter, the continued addition of moisture to the house air would quickly raise the hu- midity to the point where condensa- tion would occur. In practice, of course, no house is that tight, and some air will always infiltrate under wind ac- tion. In winter such air will tend to lower the humidity within the house. This occurs because cold air holds very little moisture, and when brought into the building displaces an equal volume of heated room air at a much higher moisture content.

It is the reduction in ventilation that has been largely responsible for the prevalence of condensation problems in modern homes. Older homes were gen- erally more loosely constructed and the resulting high air leakage rate was suf- ficient to prevent excessive humidities.

Modern homes are smaller and empha- sis on weather-stripping, storm windows, caulking and insulation has led to a much lower natural air leakage rate, with the result that there is a greater tendency toward excessive humidities. But these excessive humidities can be prevented by increasing the amount of outside air entering the house.

Ventilation is often the only effec- tive means available to the householder for removing moisture. Dehumidifiers may also be used, but they are not normally a practical solution. Exhaust fans in the kitchen and bathroom are useful for drawing off moisture from cooking and bathing activities before the vapour can circulate throughout the house. Windows are normally relied on for general ventilation and whenever possible the windows nearest the source of moisture should be opened. Opening the fireplace damper or opening a base- ment window as well as a window in a

bedroom above are ways of ven-

tilating without creating uncomfortable draughts. Draughts can also be avoided and more positive ventilation assured by installing a special ventilation duct con- nected to the return air system of the furnace. This is the subject of a sepa- rate note.

When outdoor temperatures are

moderate rather than very cold, a higher ventilation rate may be required to con- trol humidity to the same level in the house because the outside air at higher temperatures already holds more mois- ture. This explains why humidities in houses tend to be higher in spring and fall than in winter, and why in Vancou- ver in winter, with its relatively mild temperatures but high outdoor humidi- ties, more ventilation is needed than in Ottawa to maintain the same humidity. (This paper is a contribution from the Division of Building Research, Na- tional Research Council, Canada, and is published with the approval of the Di- rector of the Division.)