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?
Q: The specs for spray-applied urethane foam insulation say that it does not absorb water (hydrophobic). So why is it never used on the outside of foundations?
