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Community and Q&A

Metal standing-seam hot roof

Michael Carreiro | Posted in GBA Pro Help on

Hi I’m building a new home for our family and wanted to run our roof design by you as my concern is moisture/ condensation. Essentially we have 2×12 members on a low-sloped 2 5/8″ over 12 large 3500 square foot roof. I’m planning on insulating with 7″ of closed cell (More if recommended) and no vapor barrier because of closed cell not needing it. The roof deck will be covered with Ice and Water HT and then have a standing seam metal roof installed on top of the ice and water. We are located in Massachusetts. One debate is do i use CDX , OSB Standard or advantech. Second is with closed cell it cant dry in would we have condensation or moisture issues on the roof deck? Would this be the recommended installation? I appreciate any advice.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Your plan will work. It's not my favorite approach, but it will work.

    There are four main drawbacks to your plan:

    1. Closed-cell spray foam isn't very environmentally friendly (although there are some new brands of spray foam that address the environmental problems). For more information on this issue, see Next Generation Spray Foams Trickle into the Market.

    2. Closed-cell spray foam is very expensive.

    3. Closed-cell spray foam won't address thermal bridging through the rafters.

    4. Your suggested approach won't allow the roof sheathing to dry to the interior.

    A better approach, in my opinion, is to install one or more layers of rigid foam above the roof sheathing. For more information on this approach, see these three articles:

    Insulating Low-Slope Residential Roofs

    How to Install Rigid Foam On Top of Roof Sheathing

    Combining Exterior Rigid Foam With Fluffy Insulation

    -- Martin Holladay

  2. Michael Carreiro | | #2

    Martin,

    Is the Ice and Water HT not Vapor Impermeable? I'm sure water can still get in possibly through fasteners and condensation of surfaces. That being said would a better approach be to reduce the Closed cell under the deck and add polyiso reflective on top of the roof deck (R20) minimum. Then run an air channel with strapping for air and to fasten the metal roof. Also where would we install the Ice in water if this is a better approach.

    Thanks so much as this is very informative.

  3. GBA Editor
    Martin Holladay | | #3

    Michael,
    1. Yes, Grace Ice & Water Shield is vapor-impermeable. If you want your roof sheathing to be able to dry to the exterior, you could substitute asphalt felt for the Ice & Water Shield, or you could choose a vapor-permeable peel-and-stick product (like Henry Blueskin VP100).

    2. If you decide to install R-20 rigid foam above the roof sheathing, it's best to design the roof assembly to dry inward. You would want to choose a vapor-permeable insulation for use under the roof sheathing in that case -- not closed-cell spray foam. In your climate zone, you will need R-29 of vapor-permeable insulation on the underside of the roof sheathing if you follow through with your plan to install R-20 rigid foam above the roof sheathing. Among the choices are cellulose, mineral wool, fiberglass batts, or open-cell spray foam.

    -- Martin Holladay

  4. Expert Member
    Dana Dorsett | | #4

    The thermal bridging of the ~7% framing fraction severely undercuts the thermal performance of 7" of closed cell foam.

    A cheaper-greener better-better solution would be to put a full 4" of polyiso (which about
    R20, after derating for climate & stackup) above the roof deck, with 8" of open cell foam (R30-ish) below the roof deck.

    If you install more than 8" of open cell foam or fiber under the roof deck, you need to add more above-deck R to maintain a minimum of ~40% of the total R on the exterior. It can be easier & cheaper to fill the 2x12s with fiber insulation (~R38-R45 depending on fiber type & density) and 6" of exterior side polyiso (~R30, derated for climate & stackup.) To further alleviate the wallet pain while going greener-still, there are multiple vendors of reclaimed & factory-seconds foam in MA, the largest of which is Nationwide Foam in Framingham, and Green Insulation Group in Worcester. (But there are others.) Using reclaimed foam takes more than half (often more than 2/3) the material cost out of the foam, and it takes nearly 100% out of the environmental cost, since it adds to the benefit side of the cost-benefit balance, without adding much to the environmental cost (mostly the transportation energy, no new polymer, no new blowing agent.)

    With fiber insulation under the roof deck it's still important to make the ceiling gypsum air tight, even with a 40%+ R-ratio. Painting it with standard latex ceiling paint makes it an Class-III vapor retarder, which limits moisture transfer via vapor diffusion, but air transported moisture from a leaky ceiling can move an order of magnitude more water, especially if convecting through low or mid-density fiberglass.

    The Ice & Water is installed on the roof deck, under the foam (and yes, it's vapor impermeable, but so are foil facers.)

  5. Michael Carreiro | | #5

    Thank you!

    If i go with Fiber or Open cell I'm assuming because of code i have to install a vapor barrier (Poly) over the open cell or fiber. Wouldn't this trap moisture between the Ice and Water and Poly on the inside?

  6. GBA Editor
    Martin Holladay | | #6

    Michael,
    There is no code requirement for a vapor barrier. There is a code requirement for a vapor retarder (a less stringent layer than a vapor barrier), but you already have that code requirement satisfied. (The rigid foam and the Ice & Water Shield are both vapor retarders).

    You don't want to install any interior poly with this type of roof assembly.

    -- Martin Holladay

  7. Expert Member
    Dana Dorsett | | #7

    As long as you have a 40% of the R on the exterior you neither need or want a vapor barrier on the interior side- a Class-III vapor retarder (eg standard latex paint on gypsum board) is all that's needed, and is preferred, since it's higher vapor permenance allows much faster drying. The prescriptives in the IRC chapter 8 in Table R806.5 are for R20 exterior ( for a presumptive R49 total) in US climate zone 5.

    Read all of section R806.5 carefully (near the bottom of the page), particularly sections 5.1.4 and 5.2:

    http://codes.iccsafe.org/app/book/content/2015-I-Codes/2015%20IRC%20HTML/Chapter%208.html

  8. Michael Carreiro | | #8

    Due to cost and complexity I'm leaning towards 3"-4" of closed cell under the deck and open cell for the remainder to get to R49. Additionally going with you recommendation of Henry Blue Skin High Temp (As it's Metal Roof) over 5/8" CDX Sheathing. Do you see any potential problems or additional recommendations with this assembly. Thank you for all your help as this is "VERY" helpful.

  9. GBA Editor
    Martin Holladay | | #9

    Michael,
    The approach you have chosen will work, but it won't perform as well as an assembly that includes a continuous layer of rigid foam above the roof sheathing. (Your approach doesn't address thermal bridging through the rafters.)

    -- Martin Holladay

  10. Expert Member
    Dana Dorsett | | #10

    The thermal bridging of the rafters is a serious performance hit. If it were all continuous foam above the roof deck you could meet code minimum performance on a U-factor basis with 6.5" of polyiso (at the labeled R) or 7" of polyiso (derated for climate), with absolutely NO interior side insulation. Yes, that's less than R40, but it doesn't have to make up for the thermal bridging.

    At 6" roofing foam is very buildable even on high pitched roofs, and dead easy for your low pitch, and probably cheaper than 4" of closed cell foam + 7" of open cell foam between the rafters. And 4" of roofing foam with R30 open cell under the roof deck is easier still.

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