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Ice & Water Shield and closed-cell foam for an unvented roof?

Deck | Posted in Energy Efficiency and Durability on

I’m looking to add exterior insulation to a low-slope wood decking (2×6 T&G) roof.

I have come across details that use rigid foam, but the builder would like to use a purlin system to create a cavity for closed cell spray. My question is, if we go that route, would I still want a peel & stick air-barrier applied to exterior of decking and interior of the purlin/spray foam layer if the closed cell will act like an air barrier.

Bonus question, should the eaves be insulated to reduce ice-damming?

Thank in advance.

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  1. EricPadgett | | #1

    Where do you live? How deep with the purlin be? How deep will the foam be?

  2. Expert Member
    Dana Dorsett | | #2

    The thermal bridging of the purlins undercuts the thermal performance of closed cell foam. The tighter the purlin spacing the bigger that performance hit will be. Sprayed closed cell polyurethane also has a fairly hefty environment footprint, many times worse if it's blown with HFC245fa (the industry standard), though there are now some HFO1234ze blown closed cell polyurethane out there, but you have to hunt for it. (It's a bit more expensive too.)

    A continuous layer of rgid foam that isn't interrupted up by low-R purlins doesn't take the performance hit. A 6.5-7" of 2lb roofing polyiso (blown with low impact pentane) would hit an IRC code-max U0.026 U-factor even though it's only R38- R40, and would be BETTER at reducing ice dam potential than R49 between purlins. Laying 2x4 purlins atop the rigid roofing foam through-screwed to the rafters takes some pretty long screws.

    Depending on climate zone you can probably split the difference, put 5.5" of OPEN cell foam (blown with water, and only half the amount of polymer per R) for R20 in the cavities, and 3-4" of rigid foam above the roof deck, or some variation thereof, which is easier & cheaper to assemble. The ratio of exterior R to total R is important for keeping the structural roof dry, but that ratio is climate dependent, so, what is your climate zone?

  3. Deck | | #3

    Sorry, climate zone is 5b and the plan was to use 2x6 purlins. I should also mention this is an existing roof.

    I understand the point about thermal bridging. It's post and beam construction, so I've also been concerned about bridging through the beam tails and roof decking that extend as the soffit and eaves. It's an Eichler-type design, so there's a desire to maintain that 'look'.

    Best case is that we'd limit the system to one material (vs. a rigid - spray combo or sim.) and roughly the same depth (~5-1/2"). That said, a big concern with this is where to place the barrier/membrane.

    Thanks again.

  4. GBA Editor
    Martin Holladay | | #4

    Ice & Water Shield over the existing boards will act as an air barrier, which is a good idea. It will provide insurance, preventing possible air leaks between the purlins and the spray foam due to installer error.

    I'm with Dana, though -- the continuous layer of rigid foam is a better solution in all respects. If you go that route, you should simply extend the rigid foam over the overhangs -- the easiest way to proceed. If you use rigid foam, the Ice & Water Shield is still a good idea.

    For more information, see How to Install Rigid Foam On Top of Roof Sheathing.

    1. Charles_B | | #6

      "Ice & Water Shield over the existing boards will act as an air barrier, which is a good idea. It will provide insurance, preventing possible air leaks between the purlins and the spray foam due to installer error."

      So would this mean having a Ice&water Shield as WRB for roof decking its okay to install closed cell spray foam to the underside of roof decking for a conditioned attic? is there any concerns with the 2 air barriers on either side of roof decking?

      (I'm in zone 2 to 3A) current layers top to bottom (under construction) is Standing Seam Galvalume metal roofing; Ice & Water shield; 5/8 plywood.
      Plan was to add 1-2" of closed cell spray foam then R30 Mineral wool batts. (added hvac vents to attic space to help combat any potential humidity & plan to install sensors just to be safe.)

  5. Expert Member
    Dana Dorsett | | #5

    In zone 5 the IRC prescriptives call out a minimum of R20 out of the R49 code minimum to be on the exterior of the fiber insulation (or roof deck, if the foam is all above the roof deck.) That's a ratio of just over 40%.

    The 2x6 rafters can be insulated to ~ R22-R23 with rock wool batts or high density blown fiberglass, so it would take another R26-R27 above the roof deck to hit code minimum on an R value basis. It can be a bit less to be compliant on a U-factor basis (based on stackup and actual thermal bridging of the framing factor) if the foam is a continuous layer thermally breaking the rafters, and not thermally bridged by purlins. That's do-able with 5" of rigid polyisocyanurate.

    To do it all with a continuous layer of polyiso (no fiber insulation) takes about 7".

    1. Deleted | | #7


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