Condensation: Causes and Control
Condensation - Causes and Control
Whenever moist air comes into contact with a cooler surface, condensation is likely to occur. The cool surface may be the underside of roof sheathing or the inside of wall sheathing in winter, or the underside of a sub floor in summer when the building is air conditioned.
The only requirements for condensation are moist air and a cool surface. In the winter, the moisture content of the indoor air (usually measured as relative humidity or vapor pressure) is important, as is the temperature of the surface on which this moisture could condense. The amount of moisture in the air outdoors is also sometimes a factor.
Condensation can be controlled three ways:
- Reduce the amount of moisture initially in the air;
- prevent the moisture from reaching a cold surface by introducing a vapor retarder;
- carry it away by ventilation.
Condensation Explained
Water stays in the air as vapor as long as the temperature of the air and the amount of water are such that the air can hold it. The amount of water in the air, relative to the amount which the air can hold, is called "relative humidity."
Warm air can hold more water vapor than can cold air. Thus, as air with a given amount of water vapor in it is cooled, the relative humidity will rise until a temperature known as the "dew point" is reached. At this point relative humidity becomes 100%, and some of the moisture will condense as "dew." If moist air contacts a surface at or below its dew-point temperature, condensation will occur on that surface.
Water vapor in the air produces "vapor pressure," which is a measure of moisture concentration. Air with high vapor pressure tries to escape or seek equilibrium with air of lower vapor pressure. The vapor can escape either with a flow of air through cracks or openings in the building shell, or without it by direct penetration of building materials. "Vapor permeance" is a measure of the ease with which vapor can penetrate solid building materials. Materials with low permeance are rated as "vapor barriers" or, more properly, "vapor retarders."
Changes in construction due to energy-saving features have tended to increase moisture levels within today's houses. Washers, dryers, cooking, showers, indoor steam rooms and swimming pools are sources of water vapor within houses. In older houses, air infiltration around doors and windows, and often directly through cracks in the walls, more or less automatically eliminated condensation. With the tighter, energy-efficient houses being built today, control of condensation must be planned.
Condensation Control
The first step in the control of condensation involves reducing excess moisture inside the home. Vent clothes dryers to the outside and not to the attic or crawl space. Install range hoods over cooking stoves, and operate them when any appreciable amount of steam is being generated. Install exhaust fans in bathrooms and vent them to the outside and not to the attic (consider wiring the fan so that it goes on automatically with the bathroom light).
Methods of moisture control vary with location in the house. For attics and enclosed cathedral ceilings, the simplest form of control involves ventilation. A ceiling vapor retarder is recommended in conjunction with ventilation for cathedral ceilings. With today's ever increasing amounts of insulation and "tighter," construction, a ceiling vapor retarder may not be so necessary for attics when adequate ventilation is provided. Its omission would allow vapor to more easily travel through the ceiling and out through the attic vents. It is important, however, to seal or avoid penetrations for electrical ceiling fixtures, which can allow mass movement of moist air into the roof cavity or attic.
For walls, ventilation is impractical, and condensation controls will generally take the form of vapor retarders. Vapor retarders in walls, and at other locations, should always be on or nearest the winter warm side in order to block vapor before it reaches a portion of the construction with a temperature below the dew point. (In hot, humid climates, a wall vapor retarder is sometimes omitted. Check local practice in these areas.) If vapor is allowed to penetrate a wall, and temperature reaches the dew point within the wall, the vapor may condense and cause trouble.
Wood floors are seldom so cool as to cause surface condensation of vapor from within the house. Structural panel floors bonded with exterior adhesives have sufficiently low vapor permeance (1 perm or less) to prevent excessive indoor moisture from escaping into the crawl space, when penetrations or openings are adequately sealed. This is particularly important when insulation is applied to the underfloor area.
Use a vapor-retarder ground cover to prevent introduction of moisture from the ground beneath the house to the crawl space or interior. This is easy in crawl-space houses, where a layer of 6-mil polyethylene over the ground in the crawl space is usually all that is required. It is more difficult in basement houses, where vapor retarders should be installed under basement floors and outside foundation walls.
Condensation in the crawl space is unlikely in winter when a ground cover is used and adequate drainage is provided around the foundation to prevent moisture accumulation in the crawl space. Thus, foundation vents may be closed during the winter for energy savings. Closure of vents in winter for energy saving is particularly effective when foundation walls, rather than the underside of floors, are insulated. This technique is also more effective than floor insulation for preventing summer condensation, particularly when the building is air conditioned.
In modern basement houses, ventilation is usually inherent with forced-air heating systems. Ventilation and air movement should be given separate consideration when heating systems are used that do not provide air circulation, such as baseboard heaters.
Ventilation Requirements
Minimum ventilation requirements are usually covered in building codes. The required net free area of vents can be found by multiplying the value in the third column of the table by the appropriate floor or attic area of the building. Note that these are *minimum* code requirements, which have been found to be adequate under most normal residential circumstances. However, ventilation in excess of these minimums may be necessary when unplanned water is introduced by venting an appliance, such as a dryer, into the space, or by misdirected surface or rainwater. Care should also be taken to provide adequate extra attic vent area when moisture-laden air is introduced to the attic by "whole-house" fans. In such cases, attic vent area should be increased in accordance with manufacturer's recommendations. Attic ventilation strategy should also consider location of vents to minimize dead air spaces.
Ventilation in excess of minimums may be necessary in high occupancy structures or in structures that contain moisture producing activities such as commercial kitchens or laundry facilities. It is the responsibility of the building owner or builder, architect, or engineer to determine the amount and location of ventilation to assure satisfactory performance.
Ventilation Check List
It is sometimes necessary to inspect an existing building for adequate ventilation where there are signs of unusual moisture. When checking for ventilation, be sure to note the following information:
- Area of floor and attic to be ventilated.
- Presence of ground cover and ceiling vapor retarder.
- Signs of moisture accumulation, including decay, water stains, blistered paint, water standing in crawl space, rusty fasteners or mold growth.
- Quantity, size, type, location, and condition of roof and foundation vents. Measure vents to be certain of size and check vent manufacturer's data for their net free area.
