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XPS Insulation

Northern_Insulation | Posted in Energy Efficiency and Durability on
I have torn out the sheetrock in my bonus room above garage (Roof Rake and Knee Wall) and cut a bunch of 2″ XPS pink board (R-10 for between the studs).  Because of the drafty soffit vent and hornets.
So far I have 1.5″ proper vent, 2″XPS (Great Stuffed in place) and put up R-19 Fiberglass (Unfaced).  Then plan on Strapping, Sheetrock, Latex Paint.
My questions are do I need a vapor barrier (Between Sheetrock and Fiberglass)? Or dry to the inside?  Or Start Over?
Is the 2″ XPS thick enough so the dew point is not in the wall here in Zone 6-Southern Maine?  Do I need more than 2″ XPS, and how much more?

I feel like I messed up the order of the XPS and Fiberglass.

So I started on the other side of the roof reversing the 2″ XPS (heated side) and Fiberglass (winter cold side).  
Is this a better method? 
Do I need a Vapor Barrier in this order of materials?
Can I do this same 2nd method to the ceiling (Between the collar-ties) creating a “foam box”?
Thank you.

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  1. user-2310254 | | #1

    I'm having a hard time visualizing your approach. Is it like Martin outlined in this article?

    1. Northern_Insulation | | #3

      It is similar only my roof is vented w/ 1.5 inch proper vent air gap and the XPS is cut into 14 inch to fit in the rafter bays and spray foamed in. I know I have a thermal bridge on the rafters but it is a retrofit and I did not want to move all of the electrical and lose the space in the room.
      On one side of the roof I put the XPS on the proper vent so it is on the cold side. I am thinking this is a mistake now because the fiberglass will be vapor laden.
      Thank you for the help.

  2. Expert Member
    BILL WICHERS | | #2

    I agree it's difficult to visualize, but I think I might understand what you have. It sounds like you did (from the inside out) drywall, 2" XPS, then fiberglass batts between the studs? Normally the XPS would go across the back of the studs on the cold side, with fiberglass batts between the studs and drywall on the interior attached directly to the studs.

    If you have a VENTED attic on the cold side of this wall, you should be fine. If you DON'T have a vented attic, you're probably still fine but sealing the wall will be more critical. I'd tape the seams of the XPS sheets, and then be very diligent about sealing any/all penetrations. Use the drywall as your primary air barrier (airtight drywall method, modified for your XPS installation).

    It sounds like you have a vented attic, and that means any moisture that gets through your wall assembly has a way to escape from the attic, which means you don't have condensation risk the way you do in a wall assembly that has structural sheathing on the cold side. You still want a good air barrier, but the vented attic is much more forgiving of less than perfect moisture control.


    1. Northern_Insulation | | #4

      Hi Bill,
      It is a retro fit. I tore down the sheetrock and put the 2" XPS between the rafters against the 1.5 inch ventilation. The XPS is between the rafters (cold side), then the Fiberglass (hot side) is also between the rafters. My sheetrock guy is insisting on strapping. My concern is the XPS will still be cold (Maine Zone 6) and condensate moisture in the fiberglass. I think I need to get the XPS to R-17.5 because it is a 2X10 to prevent this. 3.5 inch XPS then 4 inches of fiberglass.
      The other side of the roof I switched it up: the Fiberglass is between the rafters on the cold side (still 1.5 inch venting) and the XPS is between the rafters on the warm inside. I think this is smarter but do not know. Here in Maine Winters they say to put the XPS on the warm inside.
      Thank you for the Help.

      1. ThirtyWest | | #5

        There was an article that addressed the condensation on foam in site built baffles (foam to the cold side). As I recall, the article was followed up by discussion on that same topic and there seemed to be some disagreement as to whether or not any moisture that makes its way to the foam would simply dissipate through the rafters. I don't remember if there was a clear answer to that question. But I believe they said that if you do try that approach you'd have to have an excellent detail to inside in regards to air sealing on the drywall side..

      2. charlie_sullivan | | #6

        You are right about which is better.

        If you buy more foam, please don't buy more XPS. The gas used to blow the bubbles has a huge climate impact. Polyiso would get you more R-value per inch, avoids the climate impact problem, and it is easier to tape for a good air seal. Or, EPS is cheaper per R-value, and also avoids the climate issue.

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