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Will my home be too airtight?
By Richard Rue - Taken from ICF Builder Magazine
You may have heard people say that a house has to breathe
in order to be healthy. So when we talk about creating an
airtight home, or structure, that may sound like a bad thing,
but in reality it is critical to achieving ultra energy-efficiency.
So, you ask, how do I overcome my fears – especially
when my contractor is telling me that I need to bring in “fresh
air,” or I am reading about Energy Recovery Ventilators
(ERV) or Heat Recovery Ventilators (HRV)’s?
One of the true secrets to energy efficiency is how airtight
you can make the structure. This is also one of the areas
that is most misunderstood about energy efficient construction.
Basically, you want to tighten the structure down as tight
as you can possibly get it, but your still going to have a
certain amount of air leakage through the windows and the
At Energy Wise, our rule of thumb is that if you have at
least 10% glass to wall ratio. In other words, if you have
4,000 sq. ft. of wall, you have to have at least 400 sq. ft.
of glass—and it’s operable glass not fixed glass—that
you have enough natural leakage in the house. Assuming, of
course, that you don’t have anything excessive in there,
like a commercial dryer or something of the sort. In nearly
every case, if you meet the 10% glass-to-wall ratio, you will
have enough natural leakage into the house that you don’t
need to use an any sort of air-to-air heat exchanger.
I talk to people every day that are concerned about the airtightness
of the houses they’re fixin’ to build, and whether
or not they’re going to suffocate themselves, and they
want to get my advice on whether or not to put in an ERV or
HRV system. I have to explain to them that with the way that
houses are constructed today, even if we did everything perfect,
and got it as tight as humanly possible, there would still
be enough leakage through the windows and doors that they
would not need, in most cases, an HRV or ERV system.
The reason that happens is because the average house that
we see will have somewhere between a 20% to 30% glass-to-wall
ratio. And as long as you have that 10% minimum, there’s
enough natural leakage in the house that you will not need
any kind of additional mechanical ventilation system.
If you install an air-to-air heat exchanger, especially in
hot and humid or polluted climates, you’re just asking
for trouble. In fact, mechanical ventilation is a real problem
if you put it into an Energy-Wise engineered house. I get
real tickled when they call them fresh-air-to-air heat exchangers,
because the air in Dallas, Texas certainly isn’t fresh—and
in most parts of the country, it’s the same situation.
It’s not fresh air. And not only is it polluted air,
but it’s very humid air in most areas of the country.
Why bring in additional outside moisture or polluted air?
There’s no need. There’s enough leakage through
windows alone to make up for any air exhausted through bathroom
or kitchen venting. Don’t bring polluted, hot, humid
outside air into a structure just because somebody thinks
that you might be building this structure too tight.
If there is a situation where it’s too tight, we will
come back and will alert the homeowner that they have too
much fixed glass, and that they should look at some kind of
air exchanger. What we like to recommend in those situations
is “ventilation on demand.” What ventilation on
demand is, you’re going to monitor the air quality with
a CO2 sensor, and that CO2 sensor is designed so that if it
detects a buildup of CO2 to unacceptable levels, then it will
send a signal to a mechanical damper that is tied to the return
of the unit and open that damper up to allow air to come in
and “freshen up” the air. Then the mechanical
system cuts off. You never have air just automatically coming
in, which most systems are designed to do.