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EPS under siding

user-286028 | Posted in Energy Efficiency and Durability on

Hi all, I have a weekend cottage on a lake here in middle GA that was built in the mid-80’s that is in need of some serious TLC. One of the projects I’m planning on doing is to re-side the house with Hardiplank lap siding. The current siding is cedar lap on top of 1” foil faced Expanded Polystyrene (EPS). The corners are set with ½” plywood and ½” GP particle board for shear strength. Currently, there is no felt or house wrap between the sheathing and cedar siding. Due to 10 years of lack of maintenance and woodpeckers; the current cedar siding is in really bad shape. After I remove the cedar siding; I was considering leaving the EPS in place and going on top of that with ½”OSB, house wrap, then Hardiplank. I want to keep the EPS for several reasons including; saving labor time, keeping EPS out of the waste stream and the additional insulation. My question is; will I actually be trapping moisture between the EPS foil face and the back of the OSB? Or should I keep the EPS, install housewrap and proceed with the new Hardi siding? I’d like to add OSB for additional shear and nailing strength. I’m aware that I will need to make extensions to all my door and window jambs if I add the OSB. Thanks!

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Will I actually be trapping moisture between the EPS foil-face and the back of the OSB?"

    A. No. Needless to say, proper water management details require that your housewrap (the WRB) be properly installed, that your windows be properly flashed, and that the window flashing be integrated with the WRB. If you screw up your flashing details, it's always possible for your wall to get wet.

    An excellent way to keep your wall dry is to include a rainscreen gap between the back of your siding and the WRB. For more information on this topic, see All About Rainscreens.

    Q. "Or should I keep the EPS, install housewrap, and proceed with the new Hardie siding?"

    A. Your siding has to be well fastened. If you aren't fastening the siding to a new layer of OSB, you have to make sure that there is something solid to nail to. You may be able to attach the siding to the studs with long nails, but before you decide to do this, check with the siding manufacturer to see if this fastening method is allowed by the manufacturer.

  2. user-2310254 | | #2

    The sheathing should dry to the inside. But it would be a good idea to make sure that everything is properly sealed and flashed while you have the siding off. You should also consider installing vertical furring strips (1X4 inch boards, for example) to create a gap behind cement siding.

    It sounds like you are not demoing the drywall inside the exterior walls, correct?

  3. user-286028 | | #3

    Martin, thank you for your answer.

  4. user-286028 | | #4

    Steve, you are correct. I'm not removing the interior finishes. The interior walls are clad with 1x6 T&G pine run horizontally.

  5. Expert Member
    Dana Dorsett | | #5

    The t & g pine is extremely air-leaky, which makes it ever-more important to be meticulous about air-sealing.

    Your new OSB layer is probably going to make the most robust air-barrier, so be sure to tape the seams with the appropriate tapes, and caulk or can-foam the edges. Detailing the housewrap as a secondary air barrier would help too, but even though it's possible to make it the primary air barrier, it's long term air tightness is really dubious compared to well-sealed OSB.

  6. user-286028 | | #6

    Good thoughts Dana, thank you.

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