How does air tightness affect IAQ, and does a house wrap make the home too airtight?
“House wraps like Tyvek® do make a home tighter. There are many factors, however, that affect a home’s IAQ. The types of building materials; climate; amount of natural ventilation from open doors and windows; and the driving forces like wind and mechanical systems all play a part in the IAQ of a home. The tightness or ventilation rate is only one factor.
“Leaky homes can have bad IAQ due to a variety of non-air change-related issues, such as offgassing of building materials, excessive moisture, occupant activities, etc. So one should never rely upon the cracks and gaps in a home to provide adequate ventilation. The reason for this is that ventilation dependent upon air infiltrating through cracks and gaps is very unpredictable and depends on driving forces like wind and temperature differences.
“These driving forces are less during the spring and summer months and therefore there is less ventilation. During the winter months there could be substantially more air infiltration than is needed for adequate ventilation, driving up energy costs.
“The best building practice is ‘build tight, ventilate right’ with controlled ventilation. Build the walls tight, but vapor breathable. Having tight walls helps to keep unconditioned air from scavenging away heat, helping the insulation do its job to help make walls warmer. Tighter walls also reduce conditioned air from exfiltrating out through the walls where it can carry damaging moisture into the wall system causing mold, etc.
“Tyvek also helps prevent humid outdoor air from transporting moisture into the wall cavity.”
Do energy-efficient homes inherently have poorer indoor air quality?
“No, this is a myth. Many people believe that, because a home is energy efficient, it is tighter and therefore has poorer IAQ. The fact is, there are several factors that affect the indoor air quality of a home. These include:
“For example, if you bake brownies you can still smell them even if you open every window in the home, which would seem to provide plenty of ventilation. You still smell the brownies because of the source strength and emanation rate.”
What are the major pollutant sources in a home?
How does the air exchange rate of a home affect indoor air quality?
“The air exchange rate, rated in air changes per hour (ACH), is only one factor in determining a home’s indoor air quality. The basic idea is that outdoor air infiltrates into the home and dilutes the stale air. While infiltration helps maintain better IAQ, it by no means ensures that a home will have adequate IAQ, because the reality is the air exchange rate in a home is determined by many driving forces like:
“Relying on the air leaks in a home is a very unreliable form of ventilation because most of these driving forces are dependent upon weather conditions. When the driving forces are great, like a cold, windy day, a home may have two to 10 times the fresh air it needs to have good IAQ. In the spring, summer, and fall, when there are few driving forces, most homes do not have enough air changes to provide adequate ventilation.
“Natural ventilation also assumes that the outdoor air is cleaner than the indoor air, which is not always true.”
How do you know if it is necessary to mechanically ventilate a home?
“Check local and state codes. Some building codes require mechanical ventilation. If it is not already required, then perform a blower door test.”
How do you properly ventilate a home?
“Follow the guidelines in the Lawrence Berkeley National Laboratory (LBNL) report Recommended Ventilation Strategies for Energy-Efficient Production Homes. This booklet gives several examples of how to design a whole-house ventilation system.
“In this report, LBNL recommends independently ducted multi-port supply ventilation in all climates except cold because this strategy provides the safety and health benefits of positive indoor pressure as well as the ability to dehumidify and filter ventilation air. In cold climates, it recommends that multi-port supply ventilation be balanced by a single-port exhaust ventilation fan, and that builders offer balanced heat recovery ventilation to buyers as an optional upgrade.”
A copy of the report is available at http://enduse.lbl.gov/Info/LBNL-40378.pdf.
Publication date: 07/01/2002