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Rigid foam on inside of wall?

Greg Krantz | Posted in Energy Efficiency and Durability on

Hello,

I am working on a Habitat for Humanity SF home where we are trying to meet the thermal bridging requirement of the new ENERGY STAR version 3 the most cost effective way possible.

The cheapest way to meet the requirement is by doing advanced framing, but we may not be able to do advance framing because the house is within 1 mile of the ocean, in the 110 mph wind zone, and the building inspectors are giving us mixed signals whether they will allow it.

Another way to meet the requirement is to use at least R-5 insulated sheathing (we are in climate zone 5 – Massachusetts). It would be cheaper and easier to install the rigid foam on the inside of the wall, behind the drywall, because we wouldn’t need to install furring strips. It would also look better because the vinyl siding wouldn’t be wavy between the strips.

I have come across several builders doing this in New England and they have been happy with the results, but I talked to another person who said it may cause moisture issues.

Would you recommend installing the rigid foam on the inside of the wall? Could it lead to moisture problems?

We want to build a durable home, so any advice you have would be appreciated. Thanks.

Greg

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Greg,
    R-5 exterior sheathing works well in Massachusetts, as long as your studs are 2x4s. If you frame your walls with 2x6s, the minimum R-value for exterior foam sheathing is R-7.5. More information here: Calculating the Minimum Thickness of Rigid Foam Sheathing.

    Q. "Would you recommend installing the rigid foam on the inside of the wall?"

    A. My own advice: exterior foam is much better than interior foam, because the exterior foam keeps your stud bays warm and dry, and because the exterior foam addresses thermal bridging at the rim joists and partition intersections.

    Although interior foam is not as good, there's nothing wrong with it. Be sure that your local Energy Star program approves of your details before proceeding, however.

    Q. "Could it lead to moisture problems?"

    A. If your detailing is poor, almost any insulation choice can lead to moisture problems. To avoid moisture problems in walls, you need good rain management and flashing details (which is why exterior foam with rainscreen strapping works so well -- you get the advantage of the rainscreen) and you need a good air barrier (so that moisture-laden interior air doesn't sneak into your wall cavities).

  2. Greg Krantz | | #2

    Good stuff. This helps a lot - thanks!

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