All homes, through various connections to the outside, allow some amount of air leakage. The air exchange rate is highly variable from house to house, and within a given house, depending on construction details, environmental forces (wind and air temperature), and the use of exhaust fans, dryers, etc. Homes must have some minimum amount of air exchange in order provide oxygen for people and appliances, control humidity, eliminate odors, etc. While some homes experience problems due to insufficient air exchange, most homes leak more than necessary most of the time. Very few homes have the ability to truly control ventilation rates, which is only possible with a very tight house and some form of mechanical ventilation such as exhaust fans or heat recovery ventilators.
Older houses, in particular, tend to be very leaky. While the rate of air leakage can vary greatly, it is not unusual for all the air in older homes to be replaced once each hour. ASHRAE Standard 62.2 recommends a minimum ventilation rate of 1 cfm per 100 sq. ft. of floor space, plus 7.5 cfm per bedroom, plue one bedroom. For example, for a 2,500 sq. ft. home with three bedrooms, it would be (2500/100)+(7.5*4) = 55 cfm fresh air. Very energy efficient today may have leakage rates as low as 0.05 natural air changes per hour, using controlled mechanical ventilation to ensure occupant health and comfort.
Reducing air leakage from the house envelope and ductwork is typically among the most significant improvements that can be made to reduce energy use, as well as improve comfort, health, and building durability. In winter, less cold, outdoor air would replace heated air, reducing drafts and cold areas. In summer, more hot, humid air would be kept out of the house. Pollen, dust, and radon entry can be reduced. The potential for structural damage resulting from moisture being carried into walls or attics with leaking air would be decreased. Another potential benefit associated with reducing air leakage, as with most other upgrades such as improved insulation or windows, is the ability to downsize heating and cooling equipment when it is replaced.
Factors to Consider
One factor to consider is the ultimate level of tightness desired. With more extensive air sealing, some form of mechanical ventilation may become necessary. A general rule is that exterior-vented bathroom and kitchen fans, equipped with timers, can provide adequate ventilation for homes with 0.20 to 0.35 natural air changes per hour. Mechanical ventilation provides location-specific control of moisture and odors and lower energy use. However, this approach is more expensive initially due to the additional time and material needed to tighten the house and the cost of any exhaust fans or heat recovery ventilators.
The present leakage rate of the house may also affect your decision to implement any air sealing. Obviously, the more leaky a given house, the greater the potential benefits.
Ventilation - Ensuring adequate house ventilation is important. A blower door can be used to determine if a particular house meets the standard(s) mentioned above for average air leakage/ventilation rates while also ensuring it is reasonably tight to minimize energy use.
Typical leak locations - Building air leakage typically occurs at several common locations. Penetrations through the ceiling plane, for ducts, pipes, chimneys, etc., are common and are the most important holes to seal as they tend to be relatively large and the stack effect ("hot air rising") tends to drive air out of the top of the house. For homes with naturally-drafted combustion appliances, air leakage from these holes may cause backdrafting. Other typical leakage locations are around windows and doors, at the bottom of walls where they meet the floor, and the first floor/foundation wall joints.
Moisture - Moisture is produced by a number of sources including cooking, bathing, plant respiration, human activity, and combustion within the living space. When a home is tightened, the average level of moisture (humidity) in the home almost always increases. Controlling sources of moisture can be critical in a tight home to prevent condensation on windows, and within walls during cold weather. Sources of excess moisture may be the result of any of the following:
- Rainwater - check condition of roof, siding, caulking, gutters and downspouts, drainage of water away from home.
- Plumbing leaks.
- Cooking and Bathing - are there properly vented exhaust fans that can be used during, and up to 30 minutes after, cooking and bathing?
- Clothes drying - is the dryer vented to the outdoors?
Venting/Drafting - Proper drafting of combustion appliances is an important health and safety issue. Drafting patterns can be greatly affected by air sealing. To verify proper drafting, tests can be performed which simulate worst-case house depressurization. These tests generally involve turning on exhaust fans, opening and closing interior doors, and operating combustion appliances in various combinations to determine if and when backdrafting occurs.
Material Options and Testing
Reduction of leaks in a house involves sealing holes and cracks with a variety of materials including caulk, expanding foam, and sheet metal or other sheet materials (plywood, foam). Densely installed insulation can also be effective in reducing air leakage through walls and ceilings. In new construction, or retrofit where framing is exposed, polyethylene can be installed as an air and vapor barrier. With proper installation, drywall and housewraps (such as Tyvek or Typar) can act as a good air (not vapor) barriers.
Equipment is available that allows us to measure the leakiness of buildings and ductwork, as well as locate the source of leaks. Called blower doors and duct blowers, these pieces of equipment pressurize or depressurize houses and duct systems, allowing measurement of air leakage and identification of specific leakage sites.
A number of other tests can also be performed using this and related equipment. These include testing for proper flow of "return" air, proper operation of exhaust fans, and proper drafting of combustion appliances. This last test is very important, as backdrafting of gas- and oil-fired furnaces, boilers, and water heaters can pose a serious health risk, and is more likely in tight houses.
These tests can usually be performed in 2-4 hours and are completely non-destructive. Observing some of these tests can be very interesting and informative. Therefore, it is strongly recommended that you be present to observe these tests if they are performed.
Consider having air leakage testing performed early in a remodeling project. The results of the tests may help you decide what approaches to take in regard to wall retrofit, window replacement, and duct improvements. Decisions on the need for, and sizing of, new heating or cooling equipment may be affected by the results of leakage tests and any air sealing work subsequently performed. If testing is performed by a contractor offering air sealing services, the air sealing can be performed at the same time as the testing. Strongly consider having air leakage-related tests performed after sealing the house or ductwork, especially tests for proper drafting of combustion appliances.
Compared to retrofit methods, reducing air leakage is much easier in new construction. If you are adding any new rooms to the house, you should consider including some special air leakage detailing. This detailing is easy to do and adds minimal first cost, while saving energy, increasing comfort, an extending the life of the house.
Sealing all horizontal and vertical construction joints, such as where two exterior walls meet or where a wall sits on a floor, should be considered standard practice.
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