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Devil’s triangle and cathedral ceiling insulation

bab1488 | Posted in Energy Efficiency and Durability on

I’ve been reading this forum for a long time now and am at the point where I’m beginning the work.

I have a 1.5 Story Cape. Roof is vented, I am unable to build an unvented roof at this time.

I have derived a plan to insulate the kneewalls and sloped ceiling but am concerned that my plan may lead to a moisture build up. Any advice would be greatly appreciated.

Sloped Ceiling (Roof Sheathing Down):
1) Continuous and Sealed Ventilation Chutes from the Unconditioned Triangle to the Attic Space
2) Fiberglass Batt Insulation
3) Continuous Layer of XPS Board Across rafters Taped at Joints)
4) Finished Wall Board

Do I need a spacer between layer 3 (XPS) and layer 4 (finished sheetrock)?

Kneewalls (Outside to Inside)
1) XPS Board Adhered to back of 2×4 Studs
2) Fiberglass Batts in Studs
3) Finished Sheetrock

Will the XPS board being adhered to the rear of the studs trap moisture in between the sheetrock and XPS leading to the mold/rot inside the wall cavity or is this an acceptable way to boost the insulating value and airtightness?

Sorry for the length, please let me know if any more information is needed to advise.

I appreciate any help.

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Do I need a spacer between layer 3 (XPS) and layer 4 (finished sheetrock)?"

    A. No, as long as the person installing the drywall doesn't mind screwing through the rigid foam layer. You can do it -- I've done it many times -- but some drywall contractors prefer to have 1x3 or 1x4 strapping, 16 inches on center, on the interior side of the rigid foam, so that the drywall can be directly fastened to wood.

    You might consider switching from XPS (which is manufactured with a blowing agent that has a high global warming potential) to a type of rigid foam that is more environmentally friendly. Especially if you end up installing strapping (furring), it makes sense to choose foil-faced polyiso. Foil-faced polyiso is easy to tape, and you'll get a boost in the R-value of the assembly from the foil facing adjacent to the air space.

    Q. "Will the XPS board being adhered to the rear of the studs trap moisture in between the sheetrock and XPS leading to the mold/rot inside the wall cavity?"

    A. No. Remember that in cold climates, thicker rigid foam is safer than thinner rigid foam. For more information on determining the thickness of the rigid foam layer, see this article: Calculating the Minimum Thickness of Rigid Foam Sheathing.

    Your post does not mention the need to provide air-sealed blocking under the bottom plate of your kneewall (between the floor joists) and above the top plate of your kneewall (between the rafters). For more information on the need for air-sealed blocking in these locations, and for further hints about the work you are undertaking, see this article: Two Ways to Insulate Attic Kneewalls.

    For more information on insulating sloped ceilings, see this article: How to Build an Insulated Cathedral Ceiling.

  2. bab1488 | | #2

    Thank You very much Martin.

    I'll was wondering about the PolyIso v XPS.

    I did put blocking between the joists below the kneewall and proceeded to air-seal there already.

    I appreciate the response and all of the other information put out on this site.

    Thank You for your help!

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