Sources of moisture and its migration through the building enclosure

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Sources of moisture and its migration through the building enclosure

Rousseau, M. Z.

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Sources of Moisture and Its Migration

through the Building Enclosure

by M.Z. Rousseau

Appeared in

ASTM Standardization News

Volume 12, No. 11, November 1984

p. 35 -37

(DBR Paper No. 1322)

Reprinted with permission

Price $1.50

NRCC 24952

ABSTRACT

This paper d e s c r i b e s t h e v a r i o u s s o u r c e s of moisture i n houses

and q u a n t i f y t h e i r r e l a t i v e c o n t r i b u t i o n t o t h e moisture l e v e l s

i n t h e l i v i n g spaces.

It

a l s o d e f i n e s t h e mechanisms of

moisture t r a n s f e r a c r o s s t h e b u i l d i n g envelope and

i t

b r i e f l y

d e s c r i b e s how t o c o n t r o l them.

Ce document d C c r i t l e s d i v e r s e s s o u r c e s d 'humiditb dans

les

m a i s o n s e t i n d i q u e l e u r i m p o r t a n c e v i s - a - v i s

d e s t a u x

dlhumiditC dans

les espaces habitCs.

Les d c a n i s m e s de

t r a n s f e r t de l l h u m i d i t C 3 t r a v e r s l'enveloppe du b2timent e t l a

f a ~ o n

de c o n t r 6 l e r c e s d c a n i s m e s s o n t egalement examines.

Sources of

Moisture

and Its

Migration

through the

Building Enclosure

Madeleine

Z. Rousseau

Cause of Moisture

Damage

The deterioration of exterior cladding such as the buckling and warping of fiber- board siding, the peeling of paint, the ef- florescence on brick veneer and the spall- ing of brick and concrete block cladding is damage due to an accumulation of moisture in walls. Does the moisture come from inside the building as vapor until it reaches the dew point temperature location on its way out, or does it come from outside as rain or snow? It could be either or both, and in many cases it is dif- ficult to determine. In any event, whether the moisture comes from inside or outside, the major mechanism of moisture trans- port to the cavities is common, that is, air leakage (infiltration or exfiltration). The design and construction of proper control measures are, therefore, of prime impor- tance for the durability of buildings.

Moisture Balance

Every building operates under a moisture balance regime. The rate of moisture re- moval from the living spaces must equal the rate at which it is generated. As more moisture is generated, more moisture must be removed in order to maintain a constant humidity level. An interesting paradox is that field investigations some- times indicate that buildings experiencing moisture problems operate at quite low humidity levels. This can be explained by the moisture balance of the building. A constant low humidity level may be main- tained whether the input rate of moisture is high or low. The rate of moisture re- moval has to be proportional to it. Figure 1 illustrates this; the containers A and B

represent the buildings, and their liquid and their humidity level. Building A has no moisture input and no moisture re- moval, while building B experiences high moisture input and removal. The humid- ity levels of both buildings remain con- stant. This analogy demonstrates that the humidity level can be controlled by either reducing the sources of moisture or in- creasing the rate of moisture removal by means of dehumidification and ventila- tion. Unless the building is made airtight, exfiltration of moist air leading to conden- sation in the building envelope will be- come the undesirable way of removing moisture.

Moisture Sources

The most common sources of moisture in houses are people and their activities, plants, unvented combustion appliances, humidifiers, wet building materials, and the surrounding soil. The significance of some varies as a function of the occupcnts' lifestyle. Figure 2 gives approximate val- ues for the amount of water vapor re- leased per activity.

Other sources, like building materials, release moisture in proportion to their wetness and surface exposed to the inside. For an average Canadian house, it can be estimated that the lumber used at 19 per- cent moisture content can release a total of about 200 litres of moisture as it dries to average conditions. Concrete foundations can release 2,400 litres of water during the curing process. Soil surrounding the building can be an important contribut-

The buckling and warping of fiberboard siding (top) and the spalling of brick and concrete block cladding (bottom) are just two examples of damage due to an accu- mulation of moisture in walls.

RE

1 -Rate of Moisture Removal

ing source, particularly in the case of a crawl space, where as much as 40 to 50 litres of moisture per day can be released by exposed soil.

Given that most houses are vented dur- ing the summer, the high moisture con- tent of the outside air is partly absorbed by building materials and furnishings. This moisture is released later when the ambient humidity level starts to drop. Three to eight litres per day may be re- leased in the early fall.

Rain penetration also may be consid- ered as a source of moisture although it is

difficult to estimate its impact.

Mechanisms of Moisture

Movement

When the moisture has been generated in the living spaces, it may migrate to the outside through the building enclosure. In a cold climate, this induces a potential for concealed condensation and its ef- fects. Three conditions are required for the condensation to occur: some moisture in the air, a mechanism to move it

through the enclosure, and a surface be- low the dew point temperature of the air. In winter, it is more or less inevitable that the air in the building will contain more moisture than the saturation moisture content of the outside air and the outer parts of the building envelope will be be- low the dew point temperature of the room air. Thus, it is left to the designer to control the mechanisms that move the moisture into the building envelope, namely, air leakage and diffusion.

Air Leakage

Air can leak through the envelope when an air pressure difference acts on holes and openings that create an open path from the inside to the outside. The air pressure difference acting on an assembly of mate- rials is the result of a combination of the wind action, the mechanical ventilation and operation of combustion equipment, and the difference in temperature, be- tween inside and outside, the stack effect. The combinations of these loads provide quite complex distribution patterns of the air pressure difference over the enclosure.

The various paths for air leakage are usually the junctions between materials, components, and planes. This includes windows, doors, parapets, soffits, canti- levers, floor and wall junctions, and elec- trical outlets and fixtures, to list a few.

Since concealed condensation is pri- marily due to air exfiltration, one method to control condensation could be to re- verse the direction of the air flow to create air infiltration. Unfortunately, this solu- .

tion has its own problems. Air infiltration may cause condensation problems on in- side surfaces, the cooling of inside air, the lowering of the humidity level, an increase in heating costs, and some problems of com- bustion equipment backdrafting.

Airtightness

The most viable approach is to control the flow of air itself rather than its direction so that there will be neither infiltration

,

nor exfiltration. What are the require-

,

ments for the assembly of materials that I will stop air flow? Until recently, the em- phasis was placed on the requirement of the continuity of some selected elements over the whole enclosure and the air per- meability of these elements. The building community often interpreted this as

i

meaning only the sealing and stuffing of the flexible plastic membrane joints used originally as vapor barriers.

Recently, more thought was put into the quantitative performance criteria for what Canadians call an air barrier sys- tem. In other words, what range of air pressure difference should the air barrier withstand without losing its continuity, which is the major aspect of its durability. In order to stop air, the air barrier sys- tem must sustain the loads induced by the wind, the mechanical ventilation, and the stack effect. ~ s p e c i a l l ~ in high-rise buildings, the combination of these loads can induce some quite high outward pres- sure differences on walls and roofs. Therefore, the air barrier system must have structural strength and be directly supported by the structure of the build- ing. This applies equally to the elements of air barrier, the joints between materi- als, and the junctions between assemblies of materials. This is the most difficult part to achieve because it requires that design- ers and builders have a good knowledge of material properties, a good design of con- struction details, and a good quality con- struction. The connections between ele- ments are usually the most neglected and the weakest link in the chain.

FIGURE 2-Sources of Household Moisture PER BATH PER SHOWER 5 AVERAGE SIZE PLANTS

/

DAY DISHWASHING (3 MEALS) COOKING (3 MEALS) GAS REFRIGERATOR/DAY COOKING ON GAS STOVE FLOOR WASHING 12 "I2 CLOTHES DRYING INSIDE

/

DAY 0 1 2 3 4 5 6

WATER VAPOUR, litres

A relatively new requirement called for is rigidity. This has two purposes. The first is a matter of durability. A flexible material that is constantly flexing back and forth in a cavity may become brittle and crack. The second reason has nothing to do with stopping air leakage but with the control of rain penetration. The rain screen principle requires a pressure equal- ization in the compartmented cavity bounded by the back of the exterior clad- ding, the air barrier, and the horizontal and vertical boundaries to the width of the cavity. Since the pressure will vary as the volume of the cavity changes, it im- plies that the volume of the equalization cavity should be constant in order to keep the pressure variation to a minimum.

Just how still a material must be in or- der to be adequate has not been deter- mined, but clearly, an unsupported, flexi-

i

ble membrane is not stiff enough. In principle, the location of the air bar-

air and if the air does not move, neither will the moisture. The practicality of all possible air barrier locations is still under investigation.

Diffusion

There will be a movement of moisture to- wards the outside by diffusion in a cold climate. Diffusion is the movement of moisture through a material from a loca- tion of high moisture content to a location of lower moisture content. This is a slow process and occurs without any flow of air. Since thediffusion rate is a function of the permeability, its control can be achieved by the reduction of the material permeability. The selection of materials like plastic films, aluminum foil, or sev- eral types of paints and their installation on the warm side of the enclosure practi- cally eliminates the possibility of conden- sation due to diffusion.

I

rier does not have to be on the warm side

of the wall because the air will be pre-

Summary

vented from entering the cavity if it can- All of the moisture problems are related to not leave it. The moisture is carried by the three conditions: the indoor temperature

and humidity, the outside climate, and the design and construction of the build- 'ng envelope. In order to get an adequate moisture balance in the building, three factors can be adjusted. Moisture sources can be reduced or eliminated in some cases, ventilation can be increased, and the building can be designed and built to control built-in moisture flow through the envelope.

In order to control the concealed con- densation problems, there must be vapor and air barriers in the enclosure. Their characteristics are completely different but too often confused in the building in- dustry.

The vapor barrier deals with the water vapor, a gas that migrates because of a difference in concentration between in- side and outside. A material with a low permeability to water vapor installed on the warm side of the enclosure will con- trol this movement. The air barrier deals with moist air that moves in or out be- cause of the wind, theventilation, and the stack effect. To control air leakage, the air pressure difference must be resisted, and to this end, a continuous juxtaposition of building materials capable of withstand- ing the air pressure difference is required. This means that the materials used to pre- vent air leakage, the air barrier system, must be structurally adequate, of low air porosity, rigid, and assembled in such a way that continuity is maintained over the whole building enclosure.

- -

Madeleine Rousseau is a research of- ficer at the Division of Building Re- search at the National Research Council of Canada. From 1979 to 1981, she worked for the Quebec Ministry of Energy and Resources in Montreal, Que.

Rousseau graduated from the Uni- versity of Montreal in 1979, with a degree in architecture.

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