Styrofoam Moisture Problem?

mighty anvil

The design of wood stud walls in cold climates usually follows one of two options:

Option 1) The cavity dries to the exterior with polyethylene film on the inner face to control condensation on the back of the sheathing and the exterior material configuration is as vapor permeable as possible.

Option 2) The cavity dries to the interior with foam insulation acting as sheathing or covering structural panel sheathing to control condensation by raising the temperature of the back of the sheathing above the dew point.

Both options can work well but there is a third option combining some of the features of these options that does not always work well since it can allow the relative humidity of the cavity to exceed 80% which can result in fungal growth.

Option 3) Exterior foam insulation is too thin to prevent condensation in the cavity so polyethylene film is needed on the interior face but the exterior materials are not vapor permeable enough to allow effective drying to the exterior.

Features that can contribute adversely to this problem are: 2” or thicker foam insulation, un-perforated foil facings, OSB sheathing, and semi-impermeable cladding with no vent space (like stucco or EIFS).

Comments:
Option 1 requires no special design effort (other than for flashings) and is often the most cost effective.
Option 2 requires an accurate dew point calculation and possibly a great deal of expensive foam insulation.
Option 3 requires the design effort of option 2 with careful attention to the permeance and placement of materials. Since such decisions are often made for other reasons, the success of this option varies from OK to disasterous.

The presence of fiberglass cavity insulation and the choice of weather resistant barrier (cladding underlayment) materials both play minor roles in the success of the above options.

K2

Mighty Anvil goes away for a while and comes back retired. Congrats

mighty anvil

Ok, I work just enough so my wife doesn't give me a list of things to do. I guess that's semi-retired.

woodmagman

I would like to comment on this item "styrofoam over batt insulation" my understanding is that the V/B is applied to the warm side of the wall, being the inside of the house. The batt insultion is installed between the framing of the structure. The rigid foam is then applied over the exterior sheathing or over the exterior finish. This should not create any moisture lock if you do not chaulk or seal the joints of the styrofoam. The moisture can not be pulled through the structure from the inside as the out side tempeture become cooler. The batt insultion should not become moist, or promote mold.

mighty anvil

It depends on the climate, the permeability of the foam, etc. I would not rely on the joints alone of any sheathing material to provide vapor movement to the outside.

woodmagman

If you are creating a water tight envelope you will create problems. CMHC as great information on this exact process.