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Vented cathedral ceiling

user-7174702 | Posted in Energy Efficiency and Durability on

I’ve got a half cathedral ceiling, 12′ high.  I’ve insulated the 7 bays it with 2″ XPS with a 2″ air gap between the XPS and roof sheathing.   It’s as well sealed as I could get it.

The rafters are 11″ in depth so I have 7″ left to fill.  My options are more rigid foam, Roxul or a mix of both.  I have quite a lot of green and pink XPS and foil faced Thermasheath sitting around.  I’m thinking it’s best to avoid the foil faced foam as I want to maintain some drying to the interior.  Does that make sense?

I’m also a bit concerned about cobbling a puzzle of XPS pieces together.   It will not have the continuity of the first XPS layer but I hate all this rigid going to waste.

I’ll achieve R33-40 under the air channel depending on my mix of rigid and Roxul.   I could add depth to the rafters if needed, to get up to R50 or so.  I will definitely have a ceiling fan. Heating is both electric baseboard and woodstove insert.  This is upstate NY which is climate zone 5.

Looking for opinions.

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  1. Expert Member
    Dana Dorsett | | #1

    At 2" the XPS runs about a half-perm, which isn't much. There'll be more drying into the channel via the path through the rafters than through the foam. It would be better on all accounts if it were 1" and about 1 perm, with only a 1" path through the rafters. At 1" thickness or less using foil faced stuff is fine.

    R30 rock wool batts are 7.25", so compressing it to 7.0" would be about R29. The 2" XPS should be derated to R9, the performance waranteed by the manufacturer- it may be less than that after 25 years, but it'll be at least R8.4. So the 2" XPS + 7.0" rock wool will deliver ~ R38-ish performance.

    Cutting in 2" foam + 1x Bonfiglioli strips on the rafter edges would increase the cavity from 7" to 9.75". See:

    A standard 12" nominal thickness R38 designed for 2x12 framing compressed to 9.75" would deliver about R34 performance. A 10.25" thick high density R38 designed for 2x10 framing compressed to 9.75" would run about R36. With either you'd hit mid-40s for center-cavity R when added to the 2" foam. See:

    With the thermal break from Bonfiglioli strips on the rafters a mid-40s at center cavity would likely deliver code-minimum performance on a U-factor basis.

  2. user-7174702 | | #2

    By 1" path through the rafters do you mean an air channel of 1" height? If so, why is that better than 2"?

  3. Expert Member
    Dana Dorsett | | #3

    No, I mean vapor diffusion through the 1" of rafter-wood from the fiber insulation side of the foam to the vent channel side instead of 2".

    That is better because path that is half the length can pass twice the amount of moisture out to the vent channel, which if vented properly at the soffit & ridge would have the same humidity as the outdoors.

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