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Exterior insulation only?

jeffnewman | Posted in Energy Efficiency and Durability on

I have a question about an old 1902 house with zero insulation. I am currently wrapping the exterior with two 2 inch layers of Poly Iso. the house has tongue and groove inside interior walls with dry wall over it. The exterior is cladding is wooden clap boards. I am going to airseal the existing wood clading with Zip system sheeting, tape all seams, add my two layers of staggered Poly iso foam, wrap with a house wrap (nonwoven) add my 1×4 strapping, and then the cladding. Should still put cavity insulation in the empty walls?I would need to do it from the outside and drill around the fire stops. Is this really worth it?

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  1. Aedi | | #1

    It depends on the climate zone. It probably isn't worth it below climate zone 5 though, 4" of polyiso is a decent amount of insulation. Better to pay close attention to your air sealing.

    If you are not using the zip sheathing as a WRB, use plywood instead, still taping the seams with zip tape. It'll save you a little money, while still being a very good air barrier.

  2. jeffnewman | | #2

    I was going to use Zip sheeting as my airseal closes to the house interior, followed by the exterior foam followed by my WRB with would be a house wrap, and next would be the rain screen or 1x4 for cladding attachment. I am in climate zone 4.

  3. GBA Editor
    Brian Pontolilo | | #3

    Hi Jeff,

    I'm still a little confused by your plan. Are you planning to strip the exterior down to the studs?

  4. jeffnewman | | #4

    Hi Brian,
    I was going to leave the clapboards ( wood siding)and install zip sheeting over the Clapboards. As long as the wall is plumb, i didn't feel the need to rip off all of the wood siding that has nails driven into some tenacious oak. If i did choose to remove the existing wood siding, i could then fill the cavity with insulation, but didn't know if it was necessary.

    1. GBA Editor
      Brian Pontolilo | | #5

      Hi Jeff.

      It sounds like there is no sheathing on the house, just clapboards over the studs?

      If you remove the siding you'll have the opportunity to insulate the wall cavities if you want to and I would think you'll end up with a more air-tight assembly when you install the ZIP Sheathing. Installing it over the lap siding will require close attention to detail and judicious gap filling for a tight exterior air barrier.

      As Aedi says above however, if your drainage plane is going to be at the WRB that you plan to install over the rigid foam insulation, you don't need to pay for ZIP, you can seal and tape OSB or plywood sheathing for an air barrier.

      I assume that you are including the housewrap because you plan to install outie windows. Just keep in mind that it is important to identify what layer is your drainage plane (WRB) and integrate all flashings with that layer.

      1. GBA Editor
        Brian Pontolilo | | #6

        You might find these articles helpful:
        Innie Windows or Outie Windows?
        Where does the Housewrap Go?

      2. jeffnewman | | #7

        Brian if you tape OSB for an air seal, is there any good tape as good as the zip tape without the cost of the zip tape?

        1. Expert Member

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