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Community and Q&A

Can ccSPF be applied to the top side of a beadboard ceiling with a hip porch roof overhead?

jedi | Posted in Energy Efficiency and Durability on

I am currently replacing a small roof over an enclosed porch. The porch was once just a three season room that stayed cold during the winters and had no issues. Years ago (10 or more) baseboard heat was added to allow the room to be heated minimally. Ice dams have been pretty bad the past several years and have finally waranted repair. The roof has never been vented – it didn’t need to be. Of course I found that the roof boards needed to be replaced and, while removing them, discovered that the rafter tails were in bad shape too. Also, the homeowner was surprised to see that there was any (in this case cellulose) insulation above the t&g beadboard ceiling. Obviously the ceiling leaks air terribly. We are considering soffit and roof/wall juncture vents.

I am in the process of replacing all the rafters and don’t want to just replace to roof to have the same things happen again. Adding more cellulose won’t help. Can air sealing and insulating be done properly on top of a beadboard ceiling? There’s no easier time to do it. Also. should we still plan to vent the roof?

The ceiling joists are 3 5/8″ x 1 3/4″ @ 2′ o c. There is approximately 6″ from the top exterior edge of the wall plate to the top edge of the rafters. The hip roof meets the wall of the house with about 5′ of level roof. I’ve read conflicting opinions on venting this juncture.

Thanks for any advice. Can’t seem to attach any photos – too large.

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  1. jedi | | #1

    Make that 7" from top plate to top of rafters.

  2. Expert Member
    Dana Dorsett | | #2

    Since you're in the process of replacing the roof deck, the right place to insulate would be at the roof deck, but not necessarily with ccSPF and NOT vent the roof, which can be both difficult & problematic.

    What is your climate zone?

    How deep are the rafters?

  3. GBA Editor
    Martin Holladay | | #3

    There are lots of options here. As Dana notes, an unvented roof assembly with rigid foam above the roof deck might be the easiest way to go.

  4. GBA Editor
    Martin Holladay | | #4

    If you decide to try to install spray foam against the existing ceiling from above, you need to make sure that the expanding foam does squeeze through cracks between the boards. One way to do that is to install strips of cardboard between the ceiling joists (on the attic side of the ceiling boards) before installing the spray foam.

  5. jedi | | #5

    Climate zone 5. 2x6 rafters - replaced the 2x4 rafters. I can't easily do rigid foam above the roof sheathing because the window sills are only 4" above the roof framing. I'd have to pull off the new rafters and lower the ledgers. That would lower the pitch further - it's 3/12 now. Also, the fascia and soffit would need a good deal of work. They're in great shape still despite the water infiltration.

    Is it just not possible to air seal a ceiling like this well? I was thinking I'd have the foam come up over the tops of the ceiling joists as well as the top plate of the wall. Wish I could attach a photo or two.

    Thanks guys.

  6. jedi | | #6

    Good tip with the cardboard. Thanks.

    If we went the route of spraying the top of the ceiling, should the spray foam carry up on the wall of the house below the ledger boards? The sheathing is 1 x 6 t&g and quite gappy.

  7. GBA Editor
    Martin Holladay | | #7

    Yes, it would be a good idea to try to establish an air barrier with spray foam on the exposed part of the wall. That will limit heat flow from the wall to the colder attic.

  8. Expert Member
    Dana Dorsett | | #8

    In a zone 5 location as long as 40% of the total center-bay R is above the roof deck the roof deck won't load up with moisture unless you keep the wintertime humidity in the room pretty high.

    With conservative derating, 3" of polyiso is good for about R15 in a zone 5 climate, which would be enough to allow you to install R23 batts in the 2x6 rafters. If you're going to cheat the above-deck R to less than R15, install unfaced R20s in the rafters and install a sheet of MemBrain or Intello Plus on the underside of the rafters.

    You could just air seal it and cross your fingers, but there would be no drying path without venting it. And in a snowy climate and a low pitched roof butted into a wall is nearly impossible to get right. If you dropped back to 2x4 rafters you'd likely be able to get R20 above the roof deck and R15 between the rafters, raising the roof level only 2-2.5" from where you currently have it set.

    IRC code-min in zone 5 is R49, but with R15 above the roof deck and R23 you'd be at R38, with R20 above/R15 between it would be R15, Still, the ice damming potential will plummet despite the lower center-cavity R, since you have an R15-R20 of thermal break over the thermally bridging rafters, and no vent holes near where it hits the wall of the house leaking heat to start the whole ice-damming problem.

    There are lots of tools for reducing the resolution or other qualities of a JPEG to reduce the file size, and it may help the discussion.

  9. jedi | | #9

    Ok. Thought so.

    Should we consider venting the roof now? Any experience with venting the top of a hip porch roof? I once installed a vent for a shed roof that met up to a vertical wall.

    Lastly, if we spray the ceiling - is it ok to lap onto the top side of the soffit? It is level with the top plate of the wall.

  10. jedi | | #10

    I'll see what I can do with the photos resolution. Smart phones...

    There's no way to get above the ceiling once the sheathing is down. I'll think hard about adding rigid foam up above. I agree it would be best. Always last minute with what I'm working on.

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