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House Sheathing with XPS Insulation

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  • House Sheathing with XPS Insulation

    I am planning to remove the old cracked and faded vinyl siding from my bungalow, insulate with 2" of extruded polystyrene (XPS) and re-install new vinyl siding. Upon removing the siding, I found cedar lapped planking under it. I fear that because of the air gaps between the XPS and the cedar, the R-value would be diminished. My new plan calls for removing the cedar before adding the XPS. Upon closer examination, I found that under the cedar is 1/2" of tentest which is non-structural. Now my new plan #2 calls for removing the cedar and re-sheathing the exterior of the house with 7/16" OSB over the tentest, then adding the 2" of XPS and then install the new vinyl siding. The more I think about it the more sense it makes to install the XPS directly to the wall (leaving the tentest on) first and sheath over top of it. This would make it easier to install the siding. My concern now is whether the 7/16" OSB would provide enough structural lateral stability to the house with the nails going through the 2" of XPS (basically air) before hitting the studs. The normal friction of the sheathing against the studs would be lost.
    Does anyone know if this is common practice or should the sheathing be applied first? I welcome all hints, tips, and suggestions.

    PS - I'm in Northern Ontario with ccccold winter months - thus the reason for the extra XPS.

  • #2
    Re: House Sheathing with XPS Insulation

    OSB on the outside of the 2" XPS will not provide the necessary lateral structural support (piggy with the house of sticks comes to mind).
    If the cedar is in good shape I would leave it and put the XPS on top of it, extra sealed air gaps will actually increase the R value. If you are looking for a flat surface for the XPS you could strap the cedar. Make sure you tape all the XPS joints with tuc tape, for extra air penetration protection you could add a layer of tyvec and tape the joints.


    • #3
      Re: House Sheathing with XPS Insulation

      Wasn't this question here before????

      All your windows and doors will have to be furred out. And when you do this, the crack between the joining pieces will accumulate water and rot the wood out.

      I once read that dead air is a R-Value in itself. (windows)

      OSB on the outside of a 2" of extruded polystyrene, then siding. Thats a good chance for failure to happen. (sag)

      We are all trying to come up with ways to save on fuel. Unfortunately when we bought our homes, they were not done for a massive heat savings. For the amount of money and time to do what your planning, I would rip out my drywall, fur the wall out for another 2"s or what ever and sprayfoam all the walls. Before the drywall goes back up, I would put up reflective foil.

      Now that the house is sealed up, you'll need fresh air coming back in, more stuff to buy...

      I could go on and on, but something tells me to stop....
      Great Link for a Construction Owner/Tradesmen, and just say Garager sent you....

      A good climbing rope will last you 3 to 5 years, a bad climbing rope will last you a life time !!!