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Another cathedral ceiling and insulation

Sco44 | Posted in General Questions on

Greetings, I have a home in climate zone 4c that desperately needs some insulation, last summer I was seeing 90+ interior temps. And burn 4 plus cord a year to stay warm.

Built in 1954, the whole house is vaulted ceilings. All 2×8 cavities, 16″ oc. There is continuous soffit venting and no ridge vent. About 2″ batt insulation.

I plan to reroof to achieve the r value I want. My plan was roxul mineral wool r30 batts in the bay’s, resheet roof with zip sheeting, followed by 3″ polyiso.

I had been planning on closing soffit venting at the top plates with closed cell spray foam. When I met with the latest roofer, he didn’t agree with that and isn’t a fan of a hot roof assembly. He mentioned leaving soffit venting open, and adding a ridge vent. But with foam on top the roof, that just doesn’t make sense. So now, I just figured I would ask on here. I have read most of the articles on hot roofs, and foam on top of roofs.

Also, Tpo or pvc, which is longest lasting and most durable? The roof is a low slope 2-12 pitch.


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  1. GBA Editor
    Martin Holladay | | #1

    You are correct, of course, about the venting. You'll need to use spray foam above the top plates of your exterior walls to make sure that air from your soffit vents doesn't enter your rafter bays. With rigid foam above your roof sheathing, you definitely don't want to invite any outdoor air on the interior side of your insulation.

    On the question of "hot roof vs. cold roof," there is still room for debate. If you (or you roofer) want a so-called "cold roof," you can build one. The ventilation channels belong above the rigid foam, however, not under the lowest level of roof sheathing.

    This type of venting is optional. There are advantages to insulating above the rigid foam, but the final decision is up to you. (Most of the advantages have to do with reducing ice dams, which are probably not a major problem in your climate.)

    If you take your roofer's advice to build a "cold roof," here is how you would do it: After installing your 3 inches of polyiso, you would install a series of 2x4s above the polyiso, perpendicular to the ridge. These 2x4s are installed flatways, to create 1 1/2 inch deep ventilation channels. The 2x4s extend from soffit to ridge, and are usually installed either 16 inches o.c. or 24 inches o.c., with a 2x4 above each rafter.

    Above the ventilation channels, you would install another layer of plywood or OSB, and then roofing underlayment and roofing.

    I don't have enough experience with TPO roofing or PVC roofing to make any recommendations about which one to choose. You should ask your roofer about warranties, however.

    For more information on these issues, see these three articles:

    How to Build an Insulated Cathedral Ceiling

    How to Install Rigid Foam On Top of Roof Sheathing

    Combining Exterior Rigid Foam With Fluffy Insulation

  2. user-1072251 | | #2

    When the house is very hard to heat and cool, it is often (usually) related as much to air infiltration as insulation. You should have an energy audit performed - preferably by an independent - and have the house air sealed. You can add lots of insulation that makes little difference if the house is very leaky.

  3. Sco44 | | #3

    Thanks Martin, this pretty much sums up what I already have learned from reading on here. After meeting with multiple roofers, it seems they know little about the correct practices for insulting these types of roofs. I will move foward with my plan for an unvented roof assembly.

    As far as air leakage, I whole heartedly agree. However I have all new doors, and windows, all foamed in place. While I do only have 3" of mineral wool in my walls, i believe I have air leakage to a minimum. In this case I think that having almost zero insualtion in a low slope, black torch down roof assembly is the major cause of the issues.

    Thanks for the knowledge on this site!

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