Is open cell spray foam insulation a vapor retarder?

Open cell spray foam insulation is not considered a vapor retarder because it has a higher permeability to moisture compared to closed cell spray foam insulation. This means that open cell foam allows water vapor to pass through more easily, which can be beneficial in certain applications to prevent moisture build-up.

Typically, vapor retarders are defined as materials that significantly reduce the amount of moisture diffusion through them, usually measured by a metric known as the perm rating. Open cell foam generally has a perm rating above the threshold that qualifies it as a vapor retarder, thereby allowing for more vapor transmission.

In areas where moisture control is critical, like in basements or bathrooms, relying solely on open cell foam insulation could pose a risk of condensation and mold growth. It is also important to note that while open cell foam does not serve as a vapor retarder, it can still provide other benefits like thermal insulation and soundproofing, making it suitable for various applications when combined with appropriate vapor barrier systems where needed.