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How does closed cell spray foam help control moisture in commercial designs?

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Moisture control and closed cell spray foam on the exterior - continuous insulation

Icynene's Building Genius Blog continues to look at closed cell spray foam insulation's role in moisture control in exterior wall applications/continuous insulation.  If you missed the previous post, read it here.

Airtightness

During an exterior insulation application, medium density spray foam products have the benefit of creating a continuous insulation and air barrier layer in one step. This means that moist outdoor air that gets past the cladding system will come against a barrier preventing it from moving further within the wall. The result is a wall system interior that is drier. For wood framed wall systems, this means less chance of wood rot and mold growth to occur. 

Continuous Insulation

A layer of exterior continuous insulation dramatically increases the effective thermal performance of a wall since most thermal bridging is eliminated. Furthermore, the more insulation used on the exterior the closer the remaining (inboard) part of the wall will be to the temperature of the living space. So, during cold outdoor conditions, a large portion of the wall will be dry, for the reasons noted above, and warm. During hot outdoor conditions, a majority of the wall will be dry and cool. Such environmental conditions translate into a wall under less long term temperature and moisture “stress”.

An additional benefit of having exterior continuous insulation in a cold climate located wall is that drying of the wall towards the interior can be increased according to the building codes. As noted in Figure 3, the building code allows a wall, with the R-value of exterior medium density spray foam (or insulated sheathing) achieved per climate zone and wall framing type, to contain a Class III vapor retarder instead of a class II vapor retarder. Since a Class III vapor retarder is less water vapor permeable than a Class II counterpart, drying of a wall towards the interior will occur much easily.

Water Vapor Permeance

Even though they can become a Class II vapor retarder (at noted thicknesses), all of Icynene’s medium density spray foam products would allow an assembly to dry more easily than equivalent thicknesses of all faced/skinned rigid foam insulation boards (either aluminum or foil based) and some house-wrap alternatives. Therefore, spray foam can provide a wall with greater drying potential (to the outdoors) which should result in enhanced wall durability and longevity. If having a low water vapor permeance material on the exterior side of a wall assembly is a concern, note that the lesser the thickness of medium density spray foam the greater the drying potential of the wall.

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