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Attic insulation – floor vs underside sheathing

user-5482467 | Posted in Energy Efficiency and Durability on

We’re building a new home in zone-5, Colorado Springs area. We have radiant heat and an HRV (distributed with an air handler). Our original plan for attic insulation was on the ‘floor’ of the attic using 2in. closed cell with additional blown fiberglass to bring it to R63. However, on the walk through with the insulation contractor yesterday, he expressed concerns about accessing the hip corners, mainly due to the 5/12 roof pitch. As an alternative, he recommended R38 open cell installed under the roof decking/sheathing. He said he’s been doing a lot of houses that way with good results. He’s had personal experience with poor cellulose installations and settling in his own house so strongly recommends against that approach.

I’ve been frantically re-reading the articles here about using spray foam on the in this location, especially the potential for damp sheathing. (

Is the additional thickness of spray foam enough to compensate for the decreased R-value? Is an easier installation a better approach than a more difficult, and therefore potentially inadequate, but better option?

It sounds like I should also be asking about closed cell and/or a vapor retarder.

What other options or questions should I be considering?

Thanks for your help,

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  1. GBA Editor
    Martin Holladay | | #1

    If you aren't using an air handler to deliver heat, it makes more sense to use the HRV fans to distribute ventilation air than to use an air handler fan. The air handler fan will use unnecessary energy.

    You situation sounds like a variation on the raised-heel truss problem. It sounds like the trusses used to frame your hip roof don't provide enough room for adequate insulation at the perimeter of your attic. That problem could have been easily solved at the design stage, when you were ordering your roof trusses. (If there are any GBA readers who want to avoid these problems, make sure that your roof trusses are high enough above the top plates of your exterior walls so that you avoid the headaches that Alex is having.)

    I don't usually recommend the use of open-cell spray foam on the underside of roof sheathing, because of all of the reports of moisture problems in this type of roof.

    One alternative to consider is to use the minimum amount of closed-cell spray foam against the underside of the roof sheathing needed for your climate zone (41% of the roof assembly's total R-value -- meaning that if you want a total of R-63, you'll need R-26 of closed-cell spray foam), and to make up the rest of your R-63 goal with fluffy insulation like fiberglass or cellulose (installed under and in direct contact with the cured spray foam).

  2. user-5482467 | | #2

    Thank you for the prompt reply, Martin. We do have 10in raised heel trusses, but it still gets pretty tight for someone to pull the spray foam hoses to the edges. I'll look into the combined closed cell/netted fiberglass approach, too.

    I had to settle for the compromise on the air handler with the HRV, especially after the HVAC manager made the comment that "houses are built too tight these days..."

    Thanks again, Alex

  3. GBA Editor
    Martin Holladay | | #3

    If I understand your original plan correctly, it was:
    2 inches closed-cell spray foam
    16 or 17 inches of loose-fill fiberglass (about R-50)
    1 inch (minimum) for a vent channel

    That sounds like at least 19 inches -- more than can be accommodated by 10-inch raised heel trusses.

  4. iLikeDirt | | #4

    If you don't already have the roof on, an easier, cheaper, more environmentally-friendly approach is to put rigid foam above the roof decking, no spray foam required.

  5. user-5482467 | | #5

    Hi Martin, yes, you are correct. That was also an issue that was realized too late for which we were planning to compensate with extra closed-cell around the perimeter until there was enough clearance for the fiberglass. It was mainly an issue of some parts of the design process moving along before other parts, i.e. insulation details, had been fully decided.

  6. user-5482467 | | #6

    Nate, Thank you for the suggestion. We're doing a standing seam metal roof, have you had any experience doing that with rigid foam? We would probably still have to do something underneath in order to get adequate R-value and a better thermal barrier.

  7. GBA Editor
    Martin Holladay | | #7

    You can certainly install a standing-seam metal roof over rigid foam (with or without an intervening ventilation channel). However, the metal has to be installed on OSB or plywood sheathing.

    This can be achieved with nailbase panels (which are available with integrated ventilation channels if you want them), or by installing a layer of OSB or plywood on the exterior side of rigid foam panels that you install above the existing roof sheathing.

  8. Dana1 | | #8

    It's common to use layers of rigid foam with a top layer of nail-base foam under standing seam roofing. The nailbase gets per the manufacturers' instructions (typically a through-screw pattern to the structural roof deck using specified fasteners. ). In zone 5 you need at minimum of 40% of the total R to be above the roof deck for dew point control.

    A code-min installation would be R20 above the roof deck with R29-R30 below. If you have 10" of energy heel that's enough for about ~R38 of open cell foam (which would have to be installed in two lifts of no more than 5.5-6" each for a quality and fire-safe-while-curing installation.) If you're installing R38 open cell foam on the underside, you'll need a minimum of R25+ above the roof deck. Derated for climate that would be 5" of polyiso, which could be a 2.5" nailbase panel layer on top of a 3" layer of fiber-faced roofing polyiso. The labeled R would be R28-R31, but mid-winter performance would be R24-R26 depending on the actual outdoor temps. It would be fine with R38 open cell below the roof deckdespite under performing spec during the very coldest weeks- the higher performance during the less-cold weeks would offer adequate drying time for the moisture accumulated during the coldest hours. Average seasonal performance would be R27-R28.

  9. user-5482467 | | #9

    Thank you both for the additional details. Now to see what our builder thinks about the options...

  10. iLikeDirt | | #10

    Of course with rigid foam above the roof decking, you don't need to use spray foam under it, which will save you a ton of money. I like mineral wool batts for this kind of application.

  11. Chaubenee | | #11

    There is another option to consider if you have limited space. You can attach polyiso foam to the bottom of your trusses and tape the seams, and caulk the edges well, and fir them out to attach the sheet rock. Above that you can spray open cell foam to ten inches that would dry to the outside with a fully vented soffits/ridge vent system. You should get an R50 or so scenarios easily. Plus super tight.

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