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Community and Q&A

Polyiso on cathedral ceiling

RLvm9uvwsW | Posted in Energy Efficiency and Durability on

I own a log home in Southern Ohio with the primary section having a cathedral ceiling and smaller part conventional truss built roof.

Recently I hired a contractor to tear off shingles and replace with standing seam metal. I thought it would be a good opportunity to upgrade the R-value of the cathedral part of house. Starting from the inside…..rafters, T&G oak flooring for ceiling, felt, 2×10 on end with fiberglass insulation in between (looks like 1-2 inch space above insulation already in place), 3/4″ poplar decking, felt, 3 inches of polyiso with osb unvented nailboard, felt, metal roof. Continuous ridge vent now on entire length of roof. Polyiso joints taped on surface but not sure on edges.

After finishing roof, I noticed small amounts of moisture streaming down from under metal rakes covering both sides with 24 inch overhangs when outside temps recently dropped to around 20 degrees in evening. I was concerned about possibility of my new roof getting “too hot” since my contractor warned me of this from the beginning of job. He consulted an architect on this matter and it was decided that I needed to get airflow between top of fiberglass and poplar decking. He felt it was necessary to install continuous eave vents on both sides of cathedral roof as well as soffitt intake vents on all overhangs.

I was hesitant to do this, based on information found on this website. I really believe he applied conventional ventilation experiences to a different animal here, but oh well. Plus he was inexperienced with installing rigid foam to roof.

My feeling is that what is needed is to check the seal on the edge joints of the foam insulation boards under the rakes and eliminate at least the eave vents and maybe the soffitt vents as well. I am looking for some professional opinions before I make next move. thanks,
Chip Young

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  1. GBA Editor
    Martin Holladay | | #1

    Your roof assembly includes several errors, including:

    1. Your ceiling is not airtight.

    2. You made an insulation sandwich with cold air flowing between two layers of insulation.

    First of all, to address the need for an airtight ceiling: a layer of T&G boards topped by asphalt felt is not airtight. Although you have included more airtight layers near the top of your roof assembly, it is essential to provide an airtight ceiling so that warm, moist interior air can't get into your fiberglass batts. You failed to do that. However, it may be possible to fix your roof assembly without addressing this unfortunate flaw.

    If I understand correctly, you have installed 8-inch-thick fiberglass batts with a ventilated air space above the batts, and then you have installed 3 inches of polyisocyanurate above the ventilated air space. So cold exterior air is flowing in the middle of your insulation layers. That makes no sense.

    (If I have misunderstood your roof assembly, please let me know.)

    If you intend to insulate your roof with a combination of fiberglass batts under the roof sheathing and polyiso above the roof sheathing, you can't have ventilation in the middle of the sandwich. You need to either choose an unventilated roof assembly, or else you need to create ventilation channels above the polyiso.

    To fix your roof, you'll need to block off the air intake locations near the soffit, and also block off the air exit locations near the ridge. You'll probably end up using spray polyurethane foam for this air-sealing work. This solution will only work if you have good access to all of the ventilation channels, and if the spray foam installer is conscientious.

    For more information on this type of roof assembly, see How to Build an Insulated Cathedral Ceiling.

  2. homedesign | | #2

    Martin, you nailed what's wrong
    I wonder/doubt if your suggestion is going to solve "the moisture problem"
    even if "he" seals off all the edges and vents
    I think there is nothing to stop warm moist air from invading a 3-D network of voids...

  3. GBA Editor
    Martin Holladay | | #3

    You're right that an airtight ceiling is desirable but missing. Like you, I pointed that out in my answer.

    My hope is that if -- and this is a big if -- Chip can do a good job of air sealing the air inlets and air outlets, the temperature of the underside of the roof sheathing will stay warm enough. That way there won't be a condensing surface. (After all, there is a 3-inch thick layer of polyiso above the roof sheathing, providing R-20.)

  4. homedesign | | #4
  5. GBA Editor
    Martin Holladay | | #5

    All air movement in building assemblies is 3-D. I haven't seen a 2-D problem yet.

    Roof sheathing failures, rot, dripping, and mold occur when warm interior air can contact cold sheathing. The whole point of putting 3 inches of polyiso on top of the sheathing is to keep the sheathing warm (above the dew point in winter) so that no problems are created when warm interior air hits the sheathing.

  6. RLvm9uvwsW | | #6

    So to be clear I need to close eave, soffitt intake vents AND seal off ridge vent. I don't have a spray foam contractor in area.

  7. GBA Editor
    Martin Holladay | | #7

    There are always contractors willing to travel -- the only question is whether you want to pay what they will charge.

    If you want to do the work yourself, you can buy two-component spray foam kits at most lumberyards for $400 to $600.

    There are other ways to fix your roof if you don't want to use spray foam. You can disassemble your roof from the top, expose the rafter bays, seal up the ventilation openings with rigid foam caulked in place, and then install dense-packed cellulose in your rafter bays. Then reassemble all the layers, including the polyiso, paying attention to air sealing at each layer.

    Of course, that is much more intrusive than the first solution I suggested.

    Depending upon the configuration of your soffit and ridge, it may be possible to expose the air intake areas and air exit locations sufficiently to allow you to plug each opening with solid materials and caulk, if you want to avoid the use of spray foam.

  8. RLvm9uvwsW | | #8

    I'm striving for as air tight as possible. As far as sealing the ridge, is simply applying spray foam into the ridge cavity to certain depth to provide seal satisfactory or should we also blow insulation into area between fiberglass and underside sheathing to fill channels?
    Chip young

  9. GBA Editor
    Martin Holladay | | #9

    If you can block the airflow through each rafter bay at the ridge (as well as the soffit), that should do the trick.

    However, if you can fill the gap between the top of the fiberglass batts and the roof sheathing with insulation (for example, dense-packed cellulose), that would be even better. (You'll still need to do the air sealing, though, even if you find a way to fill the air gap with insulation.)

  10. homedesign | | #10

    Chip, can you post some photos? before/after
    did you take any photos during the roof re-build?
    can you describe your ceiling/roof before the re-build?
    did you have ridge vent before?

    what is 3/4" poplar decking? is this something like 1x6 with a gap?

    you say the polyiso joints were taped on the surface...did you mean the OSB part of the nailbase was taped on the surface?

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