Cellulose

Cellulose insulation, either spray-applied or dense-packed in walls, is very effective at sealing buildings against air infiltration. And air infiltration is almost as important as R-value in the thermal performance of a building. Scientific studies have confirmed anecdotal reports and "conventional wisdom" about the superiority of cellulose at tightening buildings. Research shows cellulose to be up to 40% better than fiber glass at controlling air infiltration. Infiltration of unconditioned ambiant air means that heating and cooling systems must expend more energy to compensate for the infiltration. Many authorities believe insulating a building with cellulose makes air barriers (housewrap) unnecessary. Canadian engineers tested a new cellulose-insulated home for air tightness, then slit the polyethylene air/vapor barrier in about 20 places and retested the building. There was absolutely no change in measured air leakage. Smoke pencil testing at the slits showed “not a breathe” of air leakage. No attempt was made to tighten the exterior sheathing and the siding had not yet been installed when the test was performed.