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Critique my cathedral roof insulation (Part 2)

Kevin Lauscher | Posted in Plans Review on

You might have saw my earlier thread. Here it is:

A friend recommended I make a slight adjustment in order bring down costs a bit.

For price reference:

– 32 sq. ft. @ 2″ of XPS (pink foam) is – $51 a sheet. (for R-20 that’s $100 (2″x2))
– 100 sq. ft. of fiber batt is $90. (R-22, i think)

Anyway here is the secondary design. I really apologize for the awfully crewed drawings but I couldn’t find much other way to do it.

View post on

The idea is to put 1″ of XPS (while maintaining at least 2″ of airspace between roof deck and XPS..I have adequate softet vents ) above a layer of fiber batt (down to the ceiling joist). I will caulk/sprayfoam where the XPS meets the wood to airseal. Below the ceiling joist will go 4.5″ of poly iso. Polyiso will be air sealed at all joints (so no air barrier will be needed below polyiso). Followed by OSB or drywall. I may put tung and groove back or stick w/ drywall..not sure.

This design saves me quite a bit of money while providing similar R-value.

Questions I have…
– Will this design work? I figure I will make every precaution to install fiber batt properly.
– Can I use a 1/2″ of XPS on the top. I have quite a bit of insulation below so I don’t think I will have condensation issues.

– Any critiques are great. Thanks a lot everyone!


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  1. GBA Editor
    Martin Holladay | | #1

    Your design will work. as long as you pay attention to airtightness every step of the way -- with one modification: you don't want an air gap between the top of your fiberglass batt and the underside of your 1-inch XPS. Fill the entire rafter bay with fiberglass insulation -- from the underside of the XPS to the bottom of the rafters.

    If 1/2-inch XPS is sturdy enough to install without cracking or breaking, you can use it instead of 1 inch XPS.

  2. Kevin Lauscher | | #2

    Thanks a lot for the response. I just wanted to make sure I wasn't making any huge mistake. I have never built or done anything in my life.



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