Homeowner Forum / Construction / June 2006

Since you are building a new structure and have expressed an interest
in getting it right, you perhaps ought to go beyond the immediate
question of type of insulation. I assume you are interested in more
than getting a low bid. Choice of insulation should be a part of
complete wall design, and that includes location and post-construction
integrity of the vapor barrier; proper installation and flashing of
windows; integration of siding, housewrap, and any rainscreen
provisions. It all works together, so all of the wall components should
be designed together. Spend a lot of time researching the subject on
sites like this and others.

One problem with some types of insulation, such as fiberglass, is that
they collapse and become far less effective if they get wet, and that
is where proper installation of housewrap, windows (with respect to
flashing - web sites claim 90% of windows in the country are not
installed and flashed correctly), and vapor barrier come into play. If
your vapor barrier is installed diligently, then punctured all over the
place during wiring, plumbing, and sheetrock installation, then in
winter you have moisture moving through the wall assembly, across a
temperature gradient until it hits the dew point and you have
condensation.

As you search further, you'll see comments about the effectiveness of
insulation being diminished by as much as 50% by air infiltration on
cold and windy days in winter. In the case of fiberglass insulation,
even with no air infiltration high temperature gradients in the dead of
winter can result in small circulation currents within the batt, due to
differences in air density on the warm and cold sides of the batt, and
any movement of what should be dead air within the insulation cuts into
its effective R value. Blown cellulose is tighter and far less prone to
this, and foams are even better in this respect.

A case can be made for foam insulation, but it costs more. You'll see
discussions of both open-cell foams, such as Icynene (R about 3.6/inch,
but less costly) and closed cell polyurethane (R 6.5 to 7 per inch, but
more money). Both are vapor retarders, both will block infiltration of
air in bulk, and both will give more total R value in the wall than
will fiberglass.

Finally, if you wind up with a well-designed wall that is
energy-efficient and protects the insulation from moisture from both
inside and outside, you'll have a tight house, and that in turn means
you'll need fresh air for the occupants. That calls for a heat recovery
ventilator and appropriate ducting to serve the whole house. Don't let
the builder tell you that the house "needs to breathe a little." That's
an ill-informed way of saying the house may have to be deliberately
leaky and energy-inefficient to compensate for moisture getting into
the wall cavity from improper design and construction of the wall. The
problem with making the house "tight, but not too tight" is that on
cold windy winter days the house will leak far more, on mild windless
days it will leak very little, and in any case you'll never know
whether the leakage (expressed as air changes per hour, or ACH; 0.3 to
0.4 is a good working number) is too much or too little, and whatever
it is you'll have absolutely no control over it. Put in the ventilator,
and you can dial up whatever air change rate you need for comfort,
health, and humidity control.

Do your homework. Don't rely on the builder to get it right, because
far too many of them just do things the way they've always done things.
There are plenty of sites providing good advice on building science
these days, and a number of forums that regularly toss around ideas on
wall design. You'll also see discussions of the horror stories that
result when the wall wasn't done right for the environment around the
house.

Enough soapboxing. I'll let others comment on what I've said. I'm sure
there will be plenty.