As I go about my work of providing home energy assessments and retrofits here in the
islands, I frequently come across clients that are concerned with houses that are “too
tight”. Building techniques over the past 20 years have certainly changed, and the fear
of an inadequately ventilated home is valid. It can also be misunderstood by many
homeowners… so let’s do a little myth-busting.
Houses need to breath, and so do you! Home energy savings and healthy indoor air
quality can easily coexist, as long as we strike a controlled balance between the two.
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Indoor air quality greatly affects our health. Carpets, pets, mold, soil gasses, wood
stoves and propane appliances can all contaminate the air. Unfortunately, many homes
rely on building deficiencies to exchange the air: gaps around plumbing and electrical
penetrations, inadequate seals around windows, holes in heating systems, and the
unsealed spaces around vents and outlets. Not only are these deficiencies erratic, they
also exchange the indoor air with air from potentially contaminated areas like your attic
or crawl space. I would personally rather avoid breathing the same air as the critters and
contaminants that inhabit those spaces.
We clean the air by exchanging indoor air with fresh air from outside. According
to the really smart folks at the American Society of Heating, Refrigerating and Air-
Conditioning Engineers, a good target for air-change is .35 times per hour (ACH). This
gives occupants enough air to breath, and yet retains valuable heating energy.
If your house is full of holes then you are losing energy through the roof system
(exfiltration) and introducing drafts and crawl-space air into your home (infiltration). In
the leaky house, warm, moist air that is leaving through the ceiling can also create mold
and rot problems in roof and attic systems. The energy efficient home creates an airtight
barrier to separate you from the elements. When the air inside your home stays inside
your home, your home uses less energy. “But what about fresh air” asks the proponent of
a not-so-tight house?
Rather than relying on Mother Nature, the healthy home uses mechanical ventilation
to provide fresh air. A bathroom or laundry exhaust fan on a timer will consistently
and reliably exchange your air. “That is why I open a window” you might say? That’s
fine, just know that you are no longer controlling the air exchange, Mother Nature is.
A healthy home ensures air exchange through mechanization, which is much more
consistent than our fickle human nature and the irregular whims of wind. Luckily, the
new exhaust fans are nearly silent and can be run on timers. You won’t have to think
about it or listen to it.
If you want to get really fancy, you can install a Heat Recovery Ventilator. This fan will
send out stale air and pull in fresh air through a ducted system, while retaining much of
the heat. These units are a great option if you want a really tight or super efficient house.
To determine the required run-time for the fan to reach a .35 ACH rate, we start, and end,
with a diagnostic test called a Blower Door. This determines the exact tightness of the
house. We then let the Blower Door guide us in air-sealing the home. When combustion
appliances like gas water heaters and woodstoves are present, it is important to run
combustion safety tests as well. These appliances can limit how tight the house can be
before problems will arise.
In the Home Performance industry we have a saying, “Build tight and ventilate right.”
We have to take the guesswork out of your home’s performance if you want to live in a
home that balances energy efficiency with health. Hopefully we have sufficiently eroded
the myth of the too-tight house and offered inspiration for you to create a meticulously
sealed, mechanically ventilated home. A healthy home… and an energy efficient home!
Please give a call (298-1313) or email with questions or comments. Building science is complex and I always enjoy a lively discussion!
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