Vapour check not needed with cellulose. 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 that cellulose is the only insulation that actually manages moisture.
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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. 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. This is a key distinction between cellulose and other types of insulation and an important benefit for homeowners when making insulation decisions.
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