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Can a Home Be Too Tight?

Sometime during your crusade to air seal those energy-robbing cracks and holes in your building structure, you may ask yourself, "Can I take air sealing to the extreme and harm my indoor air quality?"

The answer is yes, absolutely, but probably not, and it likely depends on the age of your structure.

Newer structures:  Making a structure too tight is much more likely with newer structures built under the more stringent modern building codes because these structures are required to be air sealed, to one degree or another, as part of the construction process. 

Older structures:  When retrofitting older structures built before energy efficiency was a big issue, it is difficult to make the building envelope too tight; generally speaking, these structures have many air leakage points in the building envelope and it can be difficult to get to all of them.    

For healthy indoor air quality, the U.S. Environmental Protection Agency recommends a rate of .35 ACH (natural air changes per hour).  That is, just over 1/3 of the air in your building being replaced every hour, or 3 hours for a full air change.  In a structure with an air-change rate lower than this, it will begin to feel like you’re in a plastic bag.  It can feel stuffy, moisture can build up, and smells and toxins can accumulate.

When a structure is air sealed for maximum energy efficiency, it can often have an air change rate of below .35 ACH.  The solution in this situation is strategically placed passive ventilation, such as windows with fresh air vents, or mechanical ventilation such as an added bathroom fan, whole house ventilation system, or heat recovery ventilation system (which captures the heat from outgoing air and transfers it to incoming air to minimize heat loss during the heating season, and vice versa during the cooling system).  

Ventilation standards like ASHRAE 62.2 are largely designed for newer, very energy efficient buildings that have very little air leakage.

The key to getting the right balance between energy efficiency and good air quality is to conduct air sealing in conjunction with periodic blower door testing.  This will allow the professional air sealer to assure the right balance between closing down leaks for energy efficiency and allowing or adding fresh air from the right sources and in the right volumes.  In achieving energy efficiency and good indoor air quality, the mantra is "seal tight, and ventilate right."

Don’t assume your structure will allow good indoor air quality just because the building envelope is already leaky.  A well-ventilated structure will prevent the buildup of pollutants, like dust and mold, and keep them from reaching unhealthy levels.  However, when ventilation is from a leaky building envelope, there is no assurance that indoor air pollutants will be properly eradicated.  First, the sources of air into the structure could be from low, unhealthy areas, such as crawl spaces or garages.  Second, during periods of calm weather with no wind, for example, air can sit stagnant in a leaky home for days.  

The best way to ensure that a home is both healthy, safe, and energy efficient is to air seal the home as well as possible, and to pair air sealing efforts with increased, targeted passive or mechanical ventilation to make sure that air cycles in and out of the house at a healthy and consistent rate.

The best thing you can do to ensure that your home is as energy efficient and as healthy as possible is to talk to your energy auditor about where you should focus your home improvement efforts.