A.John Curran, owner of
RSI General Contractors, a roofing, siding and
insulation company in Syracuse, N.Y.,
responds: In our experience, the most
effective way to insulate the counterweight cavity
is to use blown cellulose. After the stops, sashes,
and pulleys have been removed, we tape over the
pulley hole, then drill a single 1-inch-diameter
hole in the center of the jamb (where it will be
covered by the new window). We then pump in
cellulose until it backs up and we know the cavity
is full.
The advantage of cellulose is that we can pack
the cavity tight without losing R-value, as with
fiberglass, or risking damage from expansion, as
with expanding spray-foam. The disadvantage is that
we have to lug a blower machine to the job.
Fortunately, we have a small one-bag unit, which is
relatively portable. These machines can be rented
or borrowed from home centers if you buy enough
insulation.
If we couldn’t put our hands on a
blower, our second choice would be to fill the
cavity with fiberglass batt insulation. We do this
by removing the counterweight access door and
pushing small clumps of insulation upward, and by
forcing even smaller pieces downward through the
pulley hole. Even though we lose some R-value, we
pack the insulation cavity tight to eliminate air
infiltration.
Until now, we’ve avoided using spray
foams because of the unpredictable expansion rate:
It’s almost impossible to completely fill
a blind cavity without overfilling and bowing out
the walls. We were also concerned that over time,
the foam would shrink and leave gaps. I’ve
been told recently that both Great Stuff (Flexible
Products Co., 800/800-3626) and Handi-Foam (Fomo
Products, 800/321-5585) now have two-part "
slow-rise" formulas that are much easier to keep
under control. A representative from Fomo Products
even told me that if I knew the volume of the
cavity, I could call their 800 number and they
would tell me exactly how long to squeeze the
trigger to dispense the correct amount of foam.
I’ll probably try it one of these days, on
my own house first.