Air/Vapor Barriers & Cellulose Insulation
A common misconception is that all insulations require vapor barriers. However, 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. The reason is cellulose is the only insulation that actually manages moisture.
Moisture moves by air movement and diffusion. Air movement is more significant, and is the primary cause for moisture related building failures. 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 summer 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