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Cellulose Insulation Manages Moisture Without Air/Vapor Barriers

Most construction industry professionals understand the basics of air infiltration.

Controlling Moisture

DOE website diagram of double wall construction with cellulose insulation and vapor barrier
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.

A common misconception, even among construction industry professionals, is that all buildings, regardless of the insulation used, require vapor barriers. However, CIMA does not recommend the use of vapor barriers with cellulose insulation, except in extremely cold climates, or in circumstances of exceptionally high interior 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 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 building failures. Cellulose insulation impedes the movement of air generated by wind, stack effect, and mechanical imbalances within homes and 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 will be further blocked by primers or paints used on the interior surfaces. Beyond this, cellulose insulation is a buffer, storing moisture safely in the cellulose fibers during wetting cycles and releasing it during drying cycles.

A vapor barrier over cellulose insulated exterior walls is not only an unnecessary expense 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 and businesses when making insulation decisions.

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

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