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Moisture sources in houses

Source: National Research Council Canada, Richard L. Quirouette


If there is a significant amount of moisture produced within a house, a means of removing that moisture must also exist. Normally, water vapour is removed by air change, either by natural air leakage through the building envelope or by mechanical ventilation systems. It can also be removed by the use of dehumidification equipment but residential dehumidifiers have a limited capability for moisture removal during winter. Many of the current problems of high humidity and condensation in houses may be addressed quite effectively by controlling moisture sources rather than by using a dehumidifier or by increasing the air change rate.

Moisture Balance

To put moisture input into perspective, consider first just how much water is contained in the air of a typical two-storey house, in this case, one of the Mark XI energy research project houses in Orleans, Ontario.1 This house has a gross inside space volume of 460 cubic metres. If the air in the house were maintained at 30% relative humidity, then the total water content would be 2.6 kilograms or approximately 2.6 litres. If the air were completely saturated, that is, 100% RH, it would then contain 8.7 litres. While this may suggest that the indoor humidity level is an indicator of the amount of moisture coming in, it unfortunate is not. Conditions in a house are rarely static, that is, there is almost always some moisture being added to the indoor spaces and some moisture which is lost by ventilation or air leakage.

Furthermore, a constant humidity level may be maintained whether the input rate of moisture is high or low. Figure 1 shows two containers which hold water. Imagine that the containers represent the indoor spaces of two houses of equal volume. The extent to which they are filled is the same as the percentage of moisture present in the air of each of the houses.

Figure 1

The first container (A) is filled to 30% of its capacity. This situation parallels what was described above, that is, the house contains a fixed quantity of water vapour relative to its capacity and equal to its relative humidity. Container B is also filled to 30% of its capacity, and provided that the quantity of water entering the top of the container is equal to the quantity of water draining out the bottom, then the level of water remains constant, regardless of the rate of moisture input and loss. This is what is meant by moisture balance, that is, if the humidity level of a house is constant, then the moisture coming into the house must be equal to the moisture going out.

Humidity and Ventilation

Suppose we want to determine the rate of ventilation required to maintain humidity at 30 percent. in the house all winter. The relationship between water intake and overall ventilation can be illustrated by a simple graph (Figure 2) . This graph shows the relationship between the intake of water in liters per day and the rate of leakage or air change in liters per second, and the resulting indoor humidity data to external conditions. External conditions should be known because the air also provides steam at home.

Figure 2 Indoor Humidity Balance (Outdoor air – 18°C/100% RH)

If we assume that the total moisture input to the house is 7.4 litres per day, then 16 litres of outdoor air per second at -18°C will be required to maintain the RH at 30%. On the other hand, if the total moisture input is 18 litres of water per day, then an air change rate of 39 litres per second will be required to maintain the indoor humidity level at the same 30% relative humidity.

Thus, the indoor humidity level of a house is a function of the moisture sources on the one hand, the ventilation on the other, and the rates of each. Simply put, it means that you cannot determine what ventilation rate is required unless you know the total moisture input to any house.

Since ventilation is described in one of the later papers, this paper will examine the various sources of moisture which must be considered for control of the humidity level of houses.

Sources of Moisture

There are many sources of moisture which can produce water vapour in a house. Among these are humidifiers, people and their activities, construction materials, basements and crawlspaces, the seasonal storage effect and rain penetration. Each of these is independent of the others, that is, moisture from one source, such as the occupant, has no link to moisture from the basement and vice versa. But all these sources have a direct influence on the moisture balance of the house.


There are many types of residential humidifiers.2 These may be classed as humidifiers for central air systems and those for non-ducted applications, the portable humidifiers. Those used for central air systems include the pan type, the most common type of system for oil and gas as well as electric furnaces, the wetted pad type, sometimes referred to as the power humidifier, and the atomizing type, which is not frequently used.

Free-standing or portable humidifiers may employ any of the previously described means of evaporation, however, the wetted pad and blower fan are quite popular. The moisture output rates of humidifiers vary with the model and the make, the location, as well as the indoor temperature and humidity, and the air movement in the room. However, regardless of the type and size, when there is a high humidity problem in a house, one must search for a humidifier and shut it off.

People and their activities

It is commonly thought that household occupants and their activities are generally the cause of high humidities and thus the cause of many condensation problems. There is no doubt that, in some cases, this may be true; however, recent findings from a major study undertaken by Canada Mortgage and Housing Corporation suggest that this is the exception rather than the rule.3

In another project done some years ago, household occupancy was studied to determine the moisture production by people and the input rate of moisture for several types of household activities.4 Consideration was given to the activities of a family of four; it was found that although the activities of the residents may vary, the amount of water vapour produced by metabolic processes such as respiration and perspiration will average about 0.2 litres per hour or five litres per day. This is 1.25 litres per person per day.

A number of activities were also investigated including bathing, showering, cooking, clothes washing and drying, and floor washing (Figure 3). Each of these activities contributes moisture, however, the average increase in moisture input was 2.4 litres per day over the five litres contributed by the occupants.

Figure 3

There are four other sources of moisture that are linked to today’s lifestyle and are worth noting. These are the use of unvented gas appliances, the indoor garden, baths, saunas and hot tubs, and the use of firewood.

Gas appliances

In the study previously mentioned, it was also reported that unvented gas appliances released moisture. The gas refrigerator, for example, was found to release 1.3 litres of moisture per day. The kerosene heater also gives up a significant amount of water vapour. Its contribution, however, is better related to fuel consumption. The amount of water released by a kerosene heater is slightly more than one litre per kilogram of fuel consumed. Similar rates can be assumed for natural gas and propane.

House plants

The watering of plants and their subsequent emission of moisture was also studied and it was found that plants in general release about 0.5 litres of water per average size plant per week. If the household has a greenhouse with 25 to 30 plants, this may release about two litres of water per day.

Baths, saunas and hot tubs

A recent social trend in new and existing households is to add recreational facilities such as whirlpool baths, saunas, and hot tubs. All these devices generate and release moisture inside a house. Hot tubs in particular should definitely be covered when they are not in use.


With the era of energy conservation, there has been a revival of the wood stove. One cord of soft wood brought into a basement to dry would release about 130 litres of water with a 10% change in the moisture content of the firewood. The same cord in hardwood lumber is approximately twice as heavy, and would release more than 250 litres per cord. Considering that a typical house may use about three cords or nine face cords, the total moisture release may approach 800 litres during the course of a winter. If the heating season lasts six months, then it may be assumed that the firewood is releasing moisture at a rate of approximately five litres per day.

The Paradox

When one examines the total moisture input by a family of four and their activities, it is interesting to note that few sources contribute as much as the occupant. If all of these moisture-producing activities were to occur in the same day, and included clothes drying indoors, floor washing, cooking, and the drying of firewood, the combined load would approach 18 to 20 litres per day.

Figure 4 Indoor Humidity Balance (Outdoor Air – 18°C/100% RH)

When we consider even an above normal moisture input rate from an occupancy and equate this to the average ventilation (Figure 4), we are left with a significant gap between the average air exchange rate of the house and the average occupancy moisture input. These simply do not equal a high humidity condition.

It means one of two things. The assumed natural ventilation rate of the problem house is considerably lower than the average, that is, it is an airtight building, or the occupant contributes only a portion of the total moisture input and there are other sources to consider. It is quite likely to be a measure of both, as recent field investigations have already begun to reveal. However, since airtightness and low air change rates are well discussed in the following articles, we will focus here on the many “hidden” sources of moisture which affect the indoor condition of any house.

Construction Moisture

The typical house is constructed from lumber that is usually quite wet, concrete, which requires substantial water in its fabrication, and numerous other products including sheathing, insulation, air and vapour barriers and cladding materials. Concrete and lumber may contribute significant amounts of moisture after completion of construction.

Framing lumber

Using the same two-storey Mark XI house, it has been calculated that the total weight of framing lumber used to construct the partition walls and all floor joists of the first and second floor would be about 2100 kilograms. If the lumber used in the construction had a moisture content of 19% (and this would not be unusual), and if it eventually dried out to 9% moisture content, it would release over 200 litres of moisture. This moisture is given off to the interior of the house and mixed with other sources of moisture.

Concrete foundations

Most new houses are built on concrete foundations. Assuming that the foundation of the sample house is approximately 2.5 metres high, and 0.25 metres thick, and that it has 35 metres of perimeter wall, the foundation would contain 22 cubic metres of concrete. The basement floor contains about four cubic meters of concrete, for a total of 26 cubic metres. In a general mix of concrete, one cubic metre requires 210 litres of water or more during the mix, but with hydration, eventually retains slightly less than 120 litres of water. This concrete therefore releases 2340 litres of water during the curing process. This water would be released within the first two years and probably most of it within the first year.

When lumber and concrete are drying they may contribute from 2000 to 3000 litres of water to the indoor space, depending on the size of the building, the moisture content of the framing lumber and the surface area of the concrete which is exposed. Assuming an 18 month drying period, this represents from four to five litres of moisture per day, a significant contribution compared to the occupancy-generated moisture. It is not surprising therefore, that many complaints of high humidity and condensation problems appear in the first two years after construction.

Seasonal Storage of Moisture

There is another phenomenon which can augment the moisture input rate during the condensation season. This is the cyclical storage and release of moisture from furnishings and various construction materials inside the house. Given that most houses are vented in the summer, the warm humid air from the outside will impose a high water vapour pressure on all materials inside the house. Including some rainy days when the outside humidity is near 100%, the outdoor humidity level may very well hover in the range of 60 to 90% for several months during the summer. Thus with ventilation, it is quite likely that the indoor conditions of the house will also be at fairly high humidities, but because of the warm summer temperatures, there will be little or no condensation occurring anywhere within the building envelope except perhaps on cold surfaces in a basement. However considerable moisture may be stored within the building structure.

When winter conditions arrive, the indoor humidities will be much lower. This is because air leakage and ventilation carry away most of the indoor moisture, leaving a humidity level which is usually in the 30 to 40% range; this can cause much of the hidden moisture to reappear in the indoor air.

Framing lumber, plywood, furnishings

Figure 5 Equilibrium Moisture Content of Wood and Concrete

If the outdoor humidity during summer were around 75%, then the moisture contents of cellulose and wood furnishings would increase up to 11% (Figure 5). In comparison to this, if the indoor humidity were lowered to 30% during winter, then materials and furnishings would tend to give up the stored moisture and try to reach a new equilibrium moisture content (at about 6%). This is a 5% change in weight and would release 105 litres of water vapour during the 16 winter period. This stored water would be released at a rate of about 0.9 litres of water per day, assuming a four-month decay period until spring and summer conditions arrive once more.

Concrete behaves somewhat like wood, except that its percentage change in moisture content is slightly less for a given change in humidity level. However, it may be more important, because the total weight of concrete far exceeds the total weight of lumber in a typical small house.

From our previous example, it was determined that the two-storey house had about 26 cubic metres of concrete. While Figure 5 shows a potential change of about 3% from summer to winter, even a 1% change will have a significant impact on the moisture balance of a house. Twenty-six cubic metres of concrete may absorb as much as 600 litres of water during the summer (1% change in weight) to be released again in winter at the rate of about five litres per day. Again, this is not an insignificant amount when compared to the occupancy-generated input.

Combining the moisture released from the lumber, gypsum, furniture and concrete of a house, these sources are releasing from three to eight litres of water per day from seasonal storage only. The rate will, of course, depend on many factors, but the most significant are the temperature and humidity levels of the summer for a particular geographic location and the exposure of wood or cellulose products and concrete within the house.

Seasonal moisture storage and seasonal release of moisture are not quite linear. In fact, a substantial portion of the stored moisture is released rather quickly in early fall, when the outdoor temperatures and indoor humidities are falling rapidly. This is the usual cause for condensation complaints around this time.

Basements and Crawlspaces

A house basement should be compared to a warm sponge on wet ground. Many of the more serious moisture entry problems appear in the basement. Moisture enters the basement by diffusion, by the capillary action of water, by air leakage through block walls and through cracks and joints of concrete walls and floors, and by flooding and water drainage problems.

Concrete walls, even after lengthy years of drying, emit moisture to the inside by the process of diffusion, whereby water vapour migrates through the concrete from a wet condition on one side to a drier condition on the other. A DBR research project that is still in progress has investigated a few house basements and it appears that two to three litres of moisture per day may be diffusing inward through the basement walls and floor of an average size house. This will depend on the time of the year and the degree of wetness surrounding the concrete surface, as well as the height of the water table below the basement floor.

If a concrete floor slab is partly resting on water, the water may move up through the concrete by capillary action to the near surface of the floor. This could be within three to five millimetres of the floor surface. From this point, the water will vaporize and diffuse with little resistance the rest of the way.

During a recent investigation of a high humidity problem in a brand new bungalow, a plywood subfloor over a basement slab was found to be near saturation less than one year after construction. The plywood floor had become a large evaporative surface, to cause a high humidity condition in the house. The humidity level had been recorded at 50 to 60% during the months of February-March of 1983 and produced much condensation on the windows. It was eventually found that the perimeter drain tile system was blocked at the sump pump pit. After clearing of the blockage, water gushed into the pit from under the floor slab at about six gallons per minute for nearly six hours. A follow-up check (one year later) revealed that the problem has since cleared up.

Concrete blocks are particularly good conductors of moisture because of the greater concrete pore size and the hollow core structure of the block. When a concrete block wall is visibly wet for several feet above the basement floor all around the perimeter of an average size basement, there may be as much as eight to ten litres of moisture evaporating from this surface area per day if the indoor humidity conditions are maintained at 40% or less. If there is any visible water on the floor near the wall, this rate will be substantially increased.

Crawlspaces are another important source of moisture. If the ground surface of a crawlspace is exposed, it may release as much as 40 to 50 litres of moisture per day. If the air in the crawlspace is allowed to find its way to the inside of the house, it can result in serious condensation damage to many surfaces and, in particular, find its way to attics and roof spaces. It’s important to maintain crawlspaces covered with a suitable vapour barrier such as plastic film, roll roofing and preferably a screed concrete slab wherever possible.

For the unfortunate tenant or resident who has a flooded basement, 60 square metres of exposed water at five to ten degrees Celsius lower than the ambient air temperature would vapourize at the rate of six litres per hour, assuming that the air above were vented rapidly enough to maintain humidities below 40%. The ventilation required is about five air changes per hour during a typical winter period. In such cases however, the rate of evaporation is offset by higher humidity levels in the house and it usually results in serious condensation problems and damage of all types.

Not so obvious, but of some importance, is air leakage into the basement from around the basement floor perimeter wall joints, through cracks, and around drains. This air may also contain a significant amount of moisture. The moisture input rate would be at a maximum during the coldest part of winter.

This was found during a study of radon gas emissions in basement areas. Under a low pressure difference of ten pascals, water vapour was entering the basement along with radon gas, in the leakage air. During the winter, when a stack effect is at work, the basement area would be under a slight negative pressure with respect to the outside. Thus, outside air may seep down through the soil or around the exterior part of the foundation through window wells, or by drain pipes from eavestroughs to find its way into the perimeter drain tile, become wet, and enter the basement as cold but saturated air. If this finding is generalized, saturated cold moist air could be trickling into the house all winter long.

Rain Penetration

Rain penetration is an age-old problem. However, it is still as mysterious as ever and we are not certain that moisture problems in wall cavities are due exclusively to condensation of moist air leaking out and not from rain penetration.

When rain water appears on the inside of a room it is generally a sign of a much larger problem. Because most walls are constructed with the interior cavities flashed to the outside, most rain which penetrates the cladding should be drained to the outside. But is it? Following a long rainfall, moisture may be retained in various pockets, and may soak many parts of the building envelope.

Figure 6

Depending on the direction of the prevailing wind and the outside temperature, moisture in rain-soaked walls may saturate infiltration air; this generally occurs near the foundation wall junction (Figure 6), and may occur even if there is no wind. Stack effect, although not strong, will cause air to infiltrate at the lower portions of the building and in the process, unsaturated cold air will become saturated, thus bringing small but perhaps significant amounts of moisture into the building.

Figure 7

In an experiment which was conducted just recently to verify the use of water vapour as a tracer gas for measuring air change rates,5 a fan was used to pressurize a small building (Figure 7a). The fan was adjusted to a rate of approximately 60 L/s. The humidifiers that were used to maintain the indoor humidity at 40% responded appropriately by injecting 1.2 kilograms of water per hour to sustain the humidity level. However, when the fan system was reversed, that is, in a depressurization mode (Figure 7b), the humidifiers were not called upon to moisturize the air. Yet, the humidity level remained constant at about 40% during a six- hour test period.

The only reasonable conclusion was that although outside air was being drawn into the house, an equivalent 1.2 kilograms per hour of moisture must have been supplied from the structure (i.e. wet walls) and perhaps from the basement area.

The rain screen

Rain penetration control is the objective of the rain screen. However, recent investigations of the performance of the rain screen suggest that very few wall design and construction techniques actually produce a proper rain screen. The rain screen must be considered as a system and not merely as a vented cladding. The cavity behind the cladding has a very important function in relation to rain screen performance. For the rain screen to be pressure-equalized, the cavity pressure must rise or fall with the wind pressure on the face of the building. To obtain a pressure-equalized cladding, the surfaces and the materials which define the cavity must be airtight and as rigid as possible, so that the cavity volume remains as stable as possible. The cavities behind the cladding must also be appropriately compartmentalized around the building.

Figure 8

If a rain screen wall system has a cavity behind the cladding that connects all around the outside of the building (Figure 8a), then even if the inner wall is airtight, and regardless of the number and size of vents in the cladding, it may still be subject to severe wetting and a significant accumulation of rain in the cavity. This is because the wind-induced negative air pressures on two or three elevations of the house induce a negative pressure in the cavity. This results in a large pressure difference on the cladding of the windward side and forces rain directly through the cladding into the cavity. If the inner wall is airtight then the air and water penetrating the cladding to the cavity would deposit the moisture in the cavity, while allowing the air to circulate and exhaust from openings on the leeward side. If however, the air finds a passage through the inner wall, water may penetrate directly to the inside with the infiltrating air. The need for near-perfect airtightness of the wall cannot be overemphasized but compartmentalization is also a necessary component of the rain screen principle (Figure 8b)

Rain penetration through a cladding may never be stopped entirely, however, if greater care were taken with flashing detail, and more importantly with airtightness, rainwater penetration through a wall would be reduced.


When all the various sources of moisture are considered, the total moisture input in a house is a combination of that contributed by the occupant and his activities, but also from a number of less obvious sources such as construction moisture, the seasonal storage effect, basements or crawlspaces and rain-soaked walls.

High humidity conditions and excessive condensation appearing in a house are caused by the total moisture input from all sources, but those sources not contributed by the occupants should be corrected first before deciding that extra ventilation is the only way to lower or control the indoor humidity level during winter.

Figure 9

Finally, when considering the probable moisture input rate in a new house, the following guide is suggested to a probable moisture load for the first, second and subsequent years of operation of a typical new house (Figure 9). In the first year, the total moisture input from occupants and other sources may average 20 or more litres per day during winter. As the building materials dry out, the total moisture input rate may drop to 15 litres per day during the second year and settle eventually to a rate of about ten litres per day in the third and subsequent years.

Hence from a third to half of the total input of moisture is generated by sources other than the occupant and his activities. It is not practical to suggest that the occupant lifestyle has to change, except in special circumstances, but it is feasible, and it will be necessary to address the many other sources of moisture to control humidity levels in new as well as retrofitted houses.


  1. R.L. Quirouette, Conception et construction du projet de recherche sur l’économie d’énergie Mark XI, Division des recherches en bâtiment, Conseil national de recherches Canada, Note d’information de recherche sur le bâtiment no 131F, Ottawa, janvier 1980.
  2. Humidifiers, In ASHRAE Handbook, 1983 Equipment. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. ISSN 0737-0687, Atlanta. 1983.
  3. Moisture Induced Problems in NHA Housing, 3 Parts. Canada Mortgage and Housing Corporation. Report prepared by Marshall Macklin Monaghan Limited, Cat. No. NH20-1/2-1983-1E, ISBN 0-662-12662-9, Ottawa, June 1983.
  4. S.C. Hite and J.L. Bray, Research in Home Humidity Control. Research Series No. 106, The Engineering Experiment Station, Purdue University, Urbana, November 1948.
  5. R.L. Quirouette, Water Vapour as a Tracer Gas for Measuring Air Change In Houses. Division of Building Research, National Research Council Canada, DBR Paper No. 1085, NRCC 21002, Ottawa, 1983.