Air / Vapor Barrier

Air Barriers

Air infiltration is almost as important as R-Value in the thermal performance of a building. Infiltration of outside air means that heating and cooling systems must expend more energy to compensate for the infiltration.

Insulation inhibits the movement of air from warm to cold (interior to exterior) in winter and cold to warm (exterior to interior) in summer, by trapping air between its fibers or pores. The higher the density of the material, the better it is at inhibiting the movement of air. Both cellulose and foam insulations have a much higher density than fiberglass, so both do a much better job at inhibiting air movement. Fiberglass insulation is air permeable, allowing outside air to flow into the building through cracks, seams between building materials and through the insulation itself. Cellulose and foam insulations both seal houses better, limiting airflow not only through the insulating material, but also around difficult to insulate areas such as gaps around electrical boxes, wiring and plumbing.

Field test have shown that cellulose insulation can provide a building envelope seal that is 36% tighter than a fiberglass batt insulation seal.

(Sources: Consumer Update - Insulation Effectiveness Bulletin #4, A Public Service Publication Of CIMA)

To prove the point, Canadian engineers conducted a test on a new cellulose-insulated home for air tightness. First the engineers measured its air leakage after a polyethylene air/vapor barrier and before siding were installed. The engineers then slit the polyethylene air/vapor barrier in approximately 20 places and retested air leakage of the building. There was absolutely no change in measured air leakage. Additional Smoke Pencil Testing at the slits showed “not a breath” of air leakage.

Vapor Barriers

A common misconception is that all insulations require vapor barriers to protect from moisture. At FTI, we don’t recommend the use of vapor barriers with cellulose insulation except in circumstances of exceptionally high moisture levels, such as an indoor pools or spas. The reason is that cellulose is the only insulation that actually manages moisture.

Moisture moves in two ways: air movement and diffusion. Of these two, air movement is the more significant, accounting for over 98% of the total, and it is the primary cause for moisture related problems outside of floods and leaks. As noted above, cellulose impedes the movement of air. By blocking the movement of moisture-laden air, cellulose reduces moisture movement to manageable levels within the building. Any remaining moisture is diffused within the cellulose insulation, and will be further blocked by primers or paints used on the interior surfaces.

A vapor barrier is not only unnecessary but also can be potentially harmful, especially during the summer months in air-conditioned buildings, when warm, moist air passes through wall assemblies and condenses on the outside of the cool vapor barrier. The hygroscopic nature of cellulose insulation allows it to manage and wick moisture from areas of greater to lesser concentrations, thus preventing damaging amounts of moisture from accumulating.