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How to best insulate an unvented 2×6 hip roof with spray foam and foamboard to +-R60

musician235 | Posted in Green Building Techniques on

Greetings Insulation Enthusiasts,

If you were going to insulate a 2×6 framed roof deck as close to modern specs as possible what materials would you use and in what order would you apply them? I’m in a cold climate and will be removing the rockwool from the attic floor and pushing the insulation to the roof deck so as to heat and cool the attic. Would this set up work and where would you use the ice and water shield?

1. Sprayfoam 2X6 bays against roof deck (would you use open or closed foam if planning to add foam board on the opposite side of the roof sheathing?)
2. Roof sheathing
3. Six inches of taped foam board on top of the roof sheathing
4. 2X4 purlins to fasten roof and create venting
5. Metal roof

Any suggestions as to open vs closed cell foam and where to apply the ice and water shield are appreciated. I don’t have any room on the underside of the roof deck to add insulation under the rafters (ceiling will be low as is).

Thank you.

 

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Replies

  1. Expert Member
    Peter Yost | | #1

    hi there musician235 (be nice to have your name for Q&A):

    1. I would read this blog by Martin Holladay: https://www.greenbuildingadvisor.com/article/how-to-build-an-insulated-cathedral-ceiling .

    2. I would read this blog too: https://www.greenbuildingadvisor.com/article/how-to-insulate-a-cathedral-ceiling-with-mineral-wool.

    3. You will need to be careful with type and schedule for your fasteners for your topside rigid insulation.

    4. Ideally, pick your rigid foam insulation and your cavity insulation to support drying in one direction or the other.

    5. The title of your post is unvented hip roof and in your callouts you state: "2X4 purlins to fasten roof and create venting." Can you reconcile this?

    Peter

    1. musician235 | | #3

      Peter, thanks for the information sir. By 2x4 purlins I meant fastening 2x4’s “on the flat” on the exterior over the foam board to create a venting channel and to fasten the metal roofing panels but I was wondering if Ice and water shield would be necessary as a top layer directly over the foam before fastening the 2x4’s on the flat..

  2. Expert Member
    Dana Dorsett | | #2

    At 6" a continuous layer of 2lb roofing polyiso (R34-ish) the assembly could meet IRC code min on U-factor basis even without any cavity insulation.

    With 5.5" of rock wool under the roof deck (R23) and R34 above, about 60% of the total R would be above the roof deck, which is adequate for dew point control a the roof deck anywhere south of the coolest edge of DOE climate zone 7.

    A layer of Ice & Water Shield on the original roof deck is an adequate air barrier- no need for spray foam in the cavities. Use #30 felt or other semi-permeable WRB between the exterior side of the foam & purlins.

    >"5. The title of your post is unvented hip roof and in your callouts you state: "2X4 purlins to fasten roof and create venting." Can you reconcile this?"

    Venting under the steel roofing between the top-side of the foam and steel isn't the same as venting under the roof deck.

    1. musician235 | | #4

      Yessir, I was wanting to reduce/offer an escape for any roof leaks by venting between the steel roof and the top side of the foam, while also understanding the need for this style of roof to dry to the interior since the polyiso isn’t vapor permeable.

      In your opinion, would the lack of sprayfoaming undeneath the roof deck in favor of the rockwool with ice and water shield in between the roof deck and the polyiso foam boards still provide a tight enough airseal to stop the “stack effect?” Without the sprayfoaming or a vapor barrier under the rock wool how would one attain an airseal between the top plate
      the walls and the roof deck? Or is that possibly unnecessary. Thanks again for your thoughts.

  3. Expert Member
    Dana Dorsett | | #5

    Ice & Water Shield is a tried and true method of air & water sealing plank decked roofs. Stack effect is pretty weak, and Ice & Water Shield is "self healing" around fastener penetrations.

    It's not difficult to make reliable air dams in rafter bays to air seal at the top plates without filling the whole rafter bay with foam. An AIR barrier under the rock wool is also a good idea, but not a substitute for air dams in the rafter bays wall plates. Any rigid material foamed in place with can-foam or caulked with polyurethane caulk will work. Even stapling in cardboard air dam and spray-foaming it with sufficient closed cell foam for dew point control on R23 insulation would work too. The amount varies with actual climate, but most would be fine with 2" of HFO blown closed cell foam on a corrugated paper air dam and R23 rock wool in the rafter bays, no interior side vapor barrier.

    A true vapor barrier UNDER the rock wool is an actively bad idea in your stackup, since that creates a moisture trap. Even if you went with vapor permeable underlayment between the 6" of polyiso and the roof deck nothing is going to dry through 6" of polyiso in any reasonable amount of time. If you're going to use spray foam in the rafter bays instead or rock wool use open cell, to provide a drying path toward the interior.

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