Q. When replacing old double-hung windows with vinyl replacement units, what’s the best way to insulate the counterweight cavity?

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.