Build Tight, Ventilate Right
Build tight, ventilate right. It should be a builder's motto. However, most builders are skeptical. They often ask: Why should I go to all the trouble of extensive caulking and then poke a bunch of holes in the house? It's a good question, and there are several good answers. The combination of air sealing and ventilation improves comfort, saves energy, controls moisture, reduces indoor pollution, promotes ventilation and reduces callbacks.
Leaky houses are drafty and uncomfortable. Blocking air leaks improves comfort, customer satisfaction and your reputation for building quality houses. Proper ventilation keeps the indoor air pleasant and healthy.
Air sealing is one of the most cost-effective ways to save energy. In the typical house with no special attention to air sealing, air leakage accounts for about one-third of the heating and cooling costs. If a house is well insulated, the percentage of heat loss from leakage could be higher. Energy savings from sealing these air leaks could amount to hundreds of dollars per year.
"Once you put a lot of insulation in the envelope, air sealing is about the only thing left," says Bill Reed a builder of affordable homes in Portland, Oregon.
By reducing the amount of air leakage, you also reduce the potential for wood decay. As inside air leaks through walls and ceilings, it carries with it large amounts of water vapor generated within the home. If the temperature of wall or roof sheathing is below the dew point, the vapor will condense on these cool surfaces, supporting the growth of decay organisms (insects, fungi, etc.).
Air leakage carries much more moisture into building cavities than vapor diffusion through materials. Vapor retarders--such as 4-mil plastic, low-perm paint and the facing on insulation--block diffusion, but they have little effect on air leakage through holes and cracks. Reducing air leakage and providing mechanical ventilation are key strategies in extending the life of wood structures.
Reduces Indoor Pollution
Some indoor air pollution problems actually begin outside. Tight construction can keep these pollutants out. The best example of this is radon gas, which occurs naturally in the soil. A well-sealed floor system (wood framed or concrete) is the first line of defense, and a prerequisite for other measures, such as underfloor ventilation.
A growing number of people worry that building materials may be harmful. Tightness shields occupants from some of the potentially hazardous materials used in the structure, such as insulation and treated wood.
The indoor pollution problem is caused less by a lack of ventilation than it is by an abundance of harmful materials used in houses. "The real problem is that houses are being built with polluting materials and without provisions for supplying the occupants with fresh air," says John Bower in his book, Healthy House Building. Tight construction and effective ventilation improve indoor air quality.
"Natural" air leakage is not ventilation. Two natural forces drive air leakage: wind and the stack effect. The stack effect occurs when air on the inside of the house is warmer than outside. Being warmer, it rises toward the ceiling and escapes through openings. Meanwhile, colder air enters near the floor.
When there's no wind and outdoor temperatures are mild, the driving forces are absent. Without them, the windows could be open and air wouldnÕt pass through. On the other hand, high wind and cold temperatures could easily create too much air leakage, which wastes energy and causes discomfort. Since the natural driving forces are tied to the weather, people have little control. Leaving a house intentionally leaky so it can "breathe" just isnÕt reliable.
Automatically controlled, mechanical ventilation is essential. Modern, low-volume ventilation systems circulate from 80 to 200 cubic feet of air per minute through the house. Low-volume ventilation systems work best in tight houses.
Air flow follows the path of least resistance. In a tight house, that path is from the fresh air inlet to the stale air outlet. Leaky houses can defeat this ventilation system because, the path of least resistance could be around pipes, electrical outlets or under a wall plate. These short circuits interrupt the air flow needed for proper ventilation.
Reduces Callbacks and Liability
Some builders hope that ignorance is bliss. And ignoring some of these issues is one way to deal with them. But sometimes, problems refuse to be ignored. Moisture and indoor air quality are two issues where builders expose themselves by not taking adequate precautions.
"I'm concerned about liability," says Doug George, of Doug George Homes in Dover, New Hampshire. "WeÕre building 1.5 million potential lawsuits a year in this country." George builds homes based on proven principles of building science. His homes are deliberately protected with super-tight construction and controlled ventilation.
A common thread passes through all these reasons. It's control. By practicing tight construction and installing effective mechanical ventilation, you increase control over the indoor environment. Unless you build tight and ventilate right, people in the house are at the mercy of the weather.
Homes with combustion appliances also face a risk of carbon monoxide poisoning. Builders that specialize in tight homes know about these risks and take steps to prevent problems. "It amazes me that people will build a brand new home and then run out and buy a carbon monoxide detector," says Doug George of Doug George Homes in New Hampshire. George thinks this shows that people don't think enough about these problems when the house is designed and built.
What is tight?
Building tight can mean different things to different builders. Without intending to, you are probably building a tighter house than in years past. Doors and windows have better seals. Sheets of plywood and drywall cover most of the building. Housewrap is more common than building paper. Caulk and expanding foam are used more and more. All these items add up to a tighter house.
A few builders take tightening to an extreme by installing an air barrier system that can reduce air leakage to a trickle. But there's no need to worry, you can't build a house too tight--as long as you also include an effective ventilation system.
This article appeared in Energy Source Builder #37 February 1995