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HELP! apprehensive about adding exterior XPS insulation

user-1140127 | Posted in Energy Efficiency and Durability on

Hello everyone, this is my first post as an avid DIY homeowner. I have renovated multiple homes and I am a big fan of adding 2″ XPS insulation board to the exterior as part of a window and siding upgrade.

This “NEW” house just sent me a curve ball. I just discovered that my “NEW” 1982 home in upstate New York (zone 6) has 1/2″ foil faced polyiso between the studs and exterior sheathing.

I am very conflicted. Some say adding the 2″ XPS will prevent any condensation issues and not to worry about it, while others say that the sheathing will rot due to possible trapped moisture. I am inclined to add the XPS but I would like some more experience to chime in here.

My local code enforcement is a part time pencil pusher / rubber stamper with zero knowledge and even less interest in helping me find an answer. Thanks in advance.

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  1. user-1137156 | | #1

    I agree you would be courting disaster, any moisture that leaked into or condensed in or on the sheathing would be trapped between two vapor barriers (at least darn good retarders). Instead of impermeable foam this situation is ripe for rigid mineral wool (Roxul) exterior insulation. The 8 # density sheets are r 4 per " so you'd have to go a little thicker for equal effect. You probably should use furring strips under the siding as an extra drainage plane. The mineral wool has very high moisture permeability well over 30 perms so drying outward would be unimpeded.. I recently got a quote on 3" @ $1.07/sq ft, 4"@ $1.23/sq ft.
    hope this helps.

  2. GBA Editor
    Martin Holladay | | #2

    Jerry's answer is the correct one: in general, it isn't a great idea to sandwich wood or wood-fiber products between two layers of rigid foam.

    However, in remodeling situations, exceptions can be made. First of all, it's important to remember which direction the threatening moisture might be coming from. If you go ahead with your plan to add a layer of rigid foam on the exterior side of your sheathing, the idea is to add enough foam to keep the sheathing above the dew point during the winter. If you do that, then you don't have to worry about any condensation or moisture accumulation in the sheathing due to interior moisture. Since the sheathing will be warm, such moisture accumulation won't occur. All you have to worry about is rain penetration.

    If the exterior sheathing is dry when you cover it, it will stay dry, as long as you do an impeccable job of flashing. If your flashing skills aren't so great, you could be in trouble. If you understand flashing and WRBs, you might find the risk acceptable. Certainly, a ventilated rainscreen gap between the siding and the new foam would go a long ways toward reducing the chance of rain penetration.

    One last point: your plan to add R-10 foam sheathing in Climate Zone 6 is fine if your house has 2x4 studs, but it's somewhat risky if your house has 2x6 studs. If the house has 2x6 studs, switch to polyisocyanurate. More information here: Calculating the Minimum Thickness of Rigid Foam Sheathing.

  3. user-1140127 | | #3

    Thank you for the very quick replies. I have a couple follow up questions / comments. I have access to polyisocyanurate panels 2 1/4" thick with a permeable tar paper type facing. I believe they are intended for under a membrane roof system. Would these panels be acceptable to use in my situation? I do plan to use vertical 3/4" plywood furring strips over the insulation to fasten my siding to. This would leave a 3/4" vented drainage plane between the foam and siding. This seems logical to me but I want to make sure I am thinking this through thoroughly. I am also unsure of whether to use TYVEK house wrap under or over the polyiso panels. Thanks again for all your help.

  4. GBA Editor
    Martin Holladay | | #4

    Q. "I have access to polyisocyanurate panels 2 1/4 inches thick with a permeable tar paper type facing. I believe they are intended for under a membrane roof system. Would these panels be acceptable to use in my situation?"

    A. Yes.

    Q. "I am also unsure of whether to use Tyvek housewrap under or over the polyiso panels."

    A. See this article: Where Does the Housewrap Go?

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