Q. I understand that fiberglass batt insulation can allow air to move through it and that this air movement degrades the insulation’s R-value. Two causes I’ve heard about are wind-washing at the eaves and convective loops that can start in a wall cavity on really cold days and wick warmth from inside to the outside wall surface. But recently I have taken pains to do a very good air-sealing job on everything I build, whether it’s an addition or a new house. I’m getting blower-door readings of less than 1.5 air changes per hour at 50 pascals. I also use baffles at the eaves, and seal the sheathing to the framing before insulating. Under these conditions — with air movement cut to a minimum — will fiberglass insulation perform at its stated R-value?

A. Patrick Dundon, an insulation contractor in Windsor, N.Y., responds: To perform at maximum R-value, fiberglass batt insulation must be installed perfectly. It has to be of consistent density throughout the wall cavity, which means that every piece must be cut to fill the cavity completely in all three dimensions. If you can do that in every...

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