We all know we need to promptly fix a roof that leaks water, but fixing homes that leak air through multiple tiny cracks in the fabric is just as important. Though the energy crisis of the 1970’s had us weatherstripping our doors and windows, it's only recently that we've begun to systematically seal the entirety of our homes against air infiltration, with special attention to floors and ceilings. The benefits in energy use and indoor air quality have been significant.
Why air-seal? A surprising amount of air moves through the average home on a more or less continuous basis. Multiple small gaps in floorboards and baseboards, in sheathing and sheetrock joints and at attic hatches and electrical and plumbing penetrations can amount in total to the equivalent of leaving a window permanently open, summer and winter. A consistent driver of air movement through these gaps is the thermal stack effect, which drives warmed air out through the roof and draws in an equal replacement volume through the crawl space or basement. Not only is this an energy-use issue: that replacement air entering through a typical vented crawl space can also bring with it excess humidity and the possibility of mold contamination.
This is not a problem if you're comfortable in a home that's hot and humid in summer and cold in winter, nor is it an issue if you're happy paying exorbitant utility bills and dealing with chronic respiratory problems. Otherwise it's something you need to address.
Doesn't the house need to breathe? No, but you do, and so do your combustion appliances. In a tight home fireplaces, woodstoves and gas appliances each need their own fresh air supply and exhaust vent. Naturally it's important to choose interior finishes and components that are not going to outgas toxic materials, so low- or no-VOC's and formaldehyde, and if your home is sealed to the very high standards of Passivhaus construction a mechanical energy-exchange ventilation system is a necessity.
A sealed crawl space can be an important component of a home with good air sealing. Insulating and conditioning the crawl space also helps the HVAC system perform well by keeping the air handler and ductwork within the conditioned enclosure of the home. Sealing air gaps at the top of the house prevents humid air from migrating into the ventilated attic space where it can condense on rafters and roof deck to cause mold and rot problems.
Did you know that your air conditioning system has to work much harder to wring the moisture out of the air in your home than it does to cool it? That's why preventing new humid air from continually entering your home can make such a big difference to your summer energy bill.