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What is preferred, rainscreen or rigid foam insulation?

bsandersga | Posted in Green Building Techniques on

If I have only about 1 1/2 inches to work with, which is preferred; rainscreen and no exterior rigid foam insulation or exterior rigid foam insulation and no rainscreen? This wall could be sheathed with:

1/2 inch plywood, 1/2 inch rain screen and then 5/16 inch Hardie panel.

Or, it could be finished with:

1/2 inch plywood, 3/4 inch foam board, and 5/16 inch Hardie panel.

I know that exterior foam insulation AND a rainscreen is the best option, but I am restricted by door trim and an electrical mast that I do not want to move.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Robert,
    The answer depends on many factors, including your climate and the roof overhang. It also depends on the size of your wall studs.

    3/4-inch-thick foam isn't enough to keep your wall sheathing above the dew point in winter in a cold climate (especially if your house has 2x6 studs).

    A rainscreen installation matters more in a wet climate than a dry climate, and it matters more on a house with a stingy roof overhang than one with a wide overhang.

  2. bsandersga | | #2

    I did leave out some important information, didn't I? This house is in Athens, Georgia (mixed humid environment) and the wall in question has 2X6 studs filled with damp sprayed cellulose. The roof overhang is 16 inches, and the wall is 8 feet tall.

  3. GBA Editor
    Martin Holladay | | #3

    Robert,
    In your climate zone (climate zone 3, I presume), thin foam won't do any harm, so the 3/4-inch foam would work.

    However, Georgia is humid, and your energy bills are low because your winters are mild. Personally, I would include the rainscreen and skip the foam -- but ultimately, this is a judgment call.

    Everybody else from Georgia who is reading this, remember: it's better to include both -- the rigid foam and the rainscreen gap.

  4. user-757117 | | #4

    Robert,
    Just curious...
    What do you have on the interior side of this wall?
    Is there a continuous air barrier - maybe airtight drywall?

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