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Thread: Cathedral ceiling insulating

  1. #1
    DIY Junior Member BillM18641's Avatar
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    Default Cathedral ceiling insulating

    We're remodeling our kitchen/dining room with a cathedral ceiling. We want the old-style beams showing. Its an old 1920's house and its a sloped roof over that area, no 2nd floor and the old beams from the original flat ceiling weren't supporting anything. How can i insulate it so the old fullsize 2x6's and 2x8's are still showing, or do I just have to insulate up in the soffit area and no insulation between the beams. I was thinking maybe 1" styrofoam insulation and then wainscote or pine strips over it to look like a wooden ceiling. Any ideas? I can add a few pics of the area i'm talking about if needed.

    Bill

  2. #2
    DIYer, not in the trades LLigetfa's Avatar
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    Pics may help. I'm not sure what you mean by beams cuz around here we call what I think you are describing, rafters.

    Around here we go for a minimum R40 and you won't get that with an inch of foam. SIPs placed on top of the rafters is generally how it's done here. You also have to keep in mind Summer cooling. The heat of the sun on the shingles may need to be dissipated for shingle longevity.

  3. #3
    Moderator & Master Plumber hj's Avatar
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    Here, you would have to enclose the roof rafters with insulation, and ventilation, between them. If you wanted visible "beams" below it, you would have to "surface mount" them on the underside. It would be the height of folly to NOT have any insulation above that area.
    Licensed residential and commercial plumber

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    DIY Senior Member dlarrivee's Avatar
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    Wow.

    So you basically want an uninsulated roof, so that the old lumber shows?

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    Retired Defense Industry Engineer jadnashua's Avatar
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    There are charts you can access online that show the minimum insulation - in NH where I live, today it is R49 on new construction, and you'd need to provide that on a remodel where you're tearing out existing stuff. That's about 5" of EPS foam, or about the full depth of the rafters you're trying to 'preserve' for view...so, since you should also preserve an airspace beneath the roof deck...this isn't going to happen! That room would be a major energy dump and uncomfortable most of the year without more insulation than what you are talking about. Then, the foam needs to be covered with a fire barrier...typically drywall. You might be able to preserve maybe 1/2" or so of it sticking out - those rafters might be full depth rather than today's machined, nominal sizes.

    There are fake beams (in foam), or you could scab some real ones (old lumber is available, but tends to be expensive) onto the existing ones (for looks only, not structural). The lower height at the sidewall might be a problem, depends on the original wall height.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Senior Member dlarrivee's Avatar
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    Jim since when is expanded polystyrene R9 per inch?

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    Retired Defense Industry Engineer jadnashua's Avatar
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    It isn't, I messed up...it's about R5/inch, so would need to be like 10" thick!
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Senior Member dlarrivee's Avatar
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    Basically the only option here would be poly-iso foam on the TOP of the existing roof sheathing, this would likely include lots of flashing and re-roofing details.

    Doing this properly is going to cost a small fortune. You might want to look into insulating traditionally and adding slivers of some nice looking wood for the effect.

  9. #9
    Moderator & Master Plumber hj's Avatar
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    The original "beams for the flat ceiling are not supporting anything", but they ARE holding the outside walls together so the roor does not sag and push them out. It takes some decorating expertise to make exposed trusses or structural members look like anything other than a "warehouse".
    Licensed residential and commercial plumber

  10. #10
    In the trades Dana's Avatar
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    Quote Originally Posted by dlarrivee View Post
    Basically the only option here would be poly-iso foam on the TOP of the existing roof sheathing, this would likely include lots of flashing and re-roofing details.

    Doing this properly is going to cost a small fortune. You might want to look into insulating traditionally and adding slivers of some nice looking wood for the effect.
    Ding ding ding ding dinnnngggggg...

    We have a winner!

    Short of that you could put 2" of XPS (R10) glued to the existing sloped ceiling with NO interior vapor barrier beyond that (2" of XPS is between 0.5 & 0.8 perms- sufficiently vapor-open for the roof sheathing to dry seasonally but not so vapor open that it would load up in winter. You would have to seal the edges with 1 part expanding foam and trim flush.

    When it's time to re-roof putting up 3" of fiber-faced roofing iso (~ R19) or a 3.5" Hunter Panel (or Atlas, et al) with a pre-applied OSB nailbase over the existing roof deck would finish it out to R30+, which even at full retail would still have a long term net-present-value in excess of the expense in a Scranton-area climate. (It would also then meet code for R-value on cathedral ceilings in PA, and EXCEED code on actual performance.)

    Nailbase panels come in vented or non-vented versions:



    But it's also simple enough to do DIY version with fiber-faced iso + 7/16" OSB. If doing your own vented version, use blocks of 2x furring rather than full lengths to get the cross-ventilation benefit- both the nailbase layer and the structural roof deck will stay drier. With 2' furring blocks with 1' gaps for x-venting, layed out 16" o.c. through-screwed 3/4" into the original structural roof deck with at least 2 screws per block, you can then screw the nailbase layer to the furring and have something that would meet wind-loading codes in PA. Sealing the seams of the iso with expanding 1-part foam as you go boosts the relative air-tightness of the assembly too.

    Vent the entire edge with 1.5" Cor-a-vent or similar to keep the rodents setting up a condominium complex in the vented cavity:



    Virgin stock roofing iso runs about 9-10 cent/R/square foot (~$55 for a 3"/R19 4x8 sheet, f.o.b. the distributors yard), but can often be had for 2-3 cents/R/square-foot as used-once reclaimed goods in good shape. There are multiple sources in my area, but the Insulation Depot has nationwide distribution, and will ship if you need a truckload. (For small quantities you'd have to bring your own truck to one of their warehouses.)

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