Air / Vapor Barriers with insulation

Air infiltration has a major impact on the real world R-Value that insulation delivers and, as a result, on the thermal performance of an insulated building. Infiltration of unconditioned ambient air means that heating and cooling systems must work harder to compensate for heat losses (in the winter) and heat gains (in the summer).

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 and seams between building materials. 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.  But what about moisture?

A common misconception is that all insulations require vapor barriers. However, we don’t recommend the use of vapor barriers with cellulose insulation, except in circumstances of exceptionally high moisture levels, such as an indoor pool facility. The reason is that cellulose is the only insulation that actually manages moisture.

Moisture moves by two transport mechanisms, 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 building failures.  As noted above, cellulose impedes the movement of air generated by wind, stack effect, and mechanical imbalances within the building.  By blocking the movement of moisture-laden air, cellulose reduces moisture movement to manageable levels within the building assemblies.  Any remaining moisture is diffused by the cellulose, 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 poly 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.

Note: Many building codes require an air barrier or vapor barrier be installed.  Work with your local officials regarding compliance. A Provider Member of CIMA can supply you with data regarding vapor barriers and cellulose.