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Cellulose Insulation Manufacturers Association
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Air Flow Through House - Cellulose Seals All Seasons

Air/Vapor Barriers with
Cellulose Insulation


Air infiltration and exfiltration has a major impact on the R-Value that insulation delivers and, as a result, on the thermal performance of an insulated dwelling. 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).

Dense insulation inhibits movement of air from warm to cold (interior to exterior) in winter and cold to warm (exterior to interior) in summer. 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 potentially are more efficient at inhibiting air movement. Cellulose and foam insulations both provide excellent sealing of houses, better limiting airflow not only through the insulating material, but also difficult to insulate areas such as gaps around electrical boxes, wiring and plumbing.

Controlling Moisture

A common misconception is that all insulations require vapor barriers in all situations. Research has shown that this is incorrect. CIMA does not recommend the use of vapor barriers with cellulose insulation, except in circumstances of exceptionally high moisture levels, such as an indoor pool facility, or very cold climates. That's because cellulose is the only insulation that actually manages moisture.

Moisture moves by two transport mechanism: 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 issues in buildings. Cellulose impedes the movement of air generated by wind, stack effect, and mechanical imbalances within buildings. 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 further blocked by primers or paints 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.

This is a key distinction between cellulose and other types of insulation and an important benefit for homeowners when making insulation decisions.

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

DOE website diagram of double wall construction with cellulose insulation and vapor barrier
(Click to enlarge graphic)

DOE suggestion of a vapor barrier in double wall construction may not be necessary when using cellulose insulation in warmer climates. Verify with your contractor.