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Community and Q&A

Spray foam the ceiling joists vs. roof sheathing when hvac is in the crawl space?

Bosburn | Posted in General Questions on

Hi, I am putting my hvac in the the crawls space of my new house, but my contractor through me for a loop today when he asked if I wanted to spray foam the roof sheathing or if I wanted to spray foam the ceiling joist of the attic. I have never seen the ceiling joists foamed.

I am still not sure which to do, but I am leaning toward spraying the ceiling joist and using a radiant roof sheathing like tech shield. I feel like this would be the best of both worlds, but I am not sure.

I am in climate zone 3 but a stones throw away from being climate zone 2.

Any advice here would be greatly appreciated.

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Brian.

    You question is a bit confusing because of the talk of the HVAC equipment being in the crawlspace. A crawlspace is below the house. Do you mean that the equipment is in the attic?

    If so, you want to create a conditioned attic by insulating the roof (not the attic floor). I suggest you start with this article: Creating a Conditioned Attic

    1. Bosburn | | #3

      Sorry if it was confusing, I was just trying to show that I do not need a conditioned attic because my HVAC equipment is not going to be in the attic.

      Since I do not need a conditioned attic, would it make more sense to use radiant sheathing and insulate the ceiling joist rather than the rafters?

      1. GBA Editor
        Brian Pontolilo | | #7

        That makes more sense Brian.

        I agree with Dana's comments below. You still have to decide where you want to insulate, at the attic floor or at the roof line and that may depend on how or if you want to use the space. If you go with insulating the floor, I would consider other options for air sealing and insulating that don't require spray foam.

  2. etekberg | | #2

    I'm in zone 3, have a vented roof, 6" 1/2 lb spray foam on the ceiling (somewhat crap job), followed up with about 18" of blown in on top, and have foil faced osb on the roof. Like what you proposed except I have HVAC in the attic like you aren't supposed to do - obviously a mistake, but "everyone" does it.

    Benefits are: Cheaper to do the ceiling (less area). Blown insulation is cheap. Foil faced osb and venting keeps the attic within a couple degrees of outside temp.

    Downside: vented Attic is dusty; I use it for storage. Can't see crap on the ceiling. You need some good detailing on the eaves and the vents which sloppy contractors don't like to do. Also need to pay attention to attic access if it is from within the conditioned space.

    1. Bosburn | | #5

      Thanks for the advice there. I really didn’t think about it being dusty up there, which is not really a deal breaker as long as the dust couldn’t make it into my house and stayed there in the attic.

      HVAC will defiantly be in the crawlspace though

  3. Expert Member
    Dana Dorsett | | #4

    There are cheaper ways to air seal an attic floor than a continuous layer of spray foam. There are cheaper ways to insulate it too. It's usually far cheaper to insulate and air seal at the attic floor, partly due to the ability of using very low cost blown fiber for the insulation rather than expensive spray foam.

    But there are other positive aspects to air sealing insulating at the roof deck that may still make it "worth it", since the attic will stay drier and cleaner, and more suitable for storage. That can be done either with or without spray foam though, vented or unvented (at the roof deck, not the attic space), usually cheaper as a foam/fiber hybrid rather than a foam-only solution.

    How deep are the rafters?

    1. Bosburn | | #6

      Rafter are 6” deep

  4. Expert Member
    Peter Engle | | #8

    With 6" rafters, you can't get a meaningful amount of insulation in the rafters. In zone 3 you need R-38 in the ceilings. Even if you filled the rafters with closed cell foam, it wouldn't get you there on a center-cavity basis and it would be very costly. You could do fluffy insulation in the cavities and rigid foam underneath, but the details start to get pretty fussy.

    By far the best bang-for-the-buck is to air seal the drywall ceilings with caulk, tape, canned foam and whatever else is necessary, then just blow as much cellulose or fiberglass as necessary onto the attic floor. Install a hatch from the exterior and there are no issues with dust in the house or air leakage around the hatch.

    1. Expert Member
      Dana Dorsett | | #9

      >"With 6" rafters, you can't get a meaningful amount of insulation in the rafters."

      With 6" rrafters you COULD hit 40 using all HFO blown 2lbs polyurethane foam. A full-fill of 5.5" into a milled 2x6 cavity of HFO blown foam would be a hair over R38, but that is not that it's what I'd recommend here, since the thermal bridging of the rafters is robbing the fo

      I'm going to assume (unless corrected) that they're actually milled 2x6 rafters, 5.5" deep.

      If soffit-to ridge venting is possible in every rafter bay, installing cut'n'cobbled 1" foil faced polyiso (R6) as a baffle for batts, spaced 1" from the roof deck with a shiny facer pointed at the vent gap would have the correct amount of space for 3.5" R15 batts (fiberglass or rock wool). Then a continuous layer of 2" foil faced polyiso (R12-R13) applied to the underside of the rafters, seams taped with a high quality aluminum HVAC tape delivers a center-cavity R of about R33. Install 1x furring perpendicular to the rafters, through screwed to the rafters with cap screws, mounting the ceiling gypsum to the furring.

      The foil facer pointed at the vent gap would deliver another R1, for R34 @ center cavity, the foil facer pointed at the 3/4" gap provided by the furring adds another R1 bringing it up to R35, not quite the code-minimum R38 if going for compliance on an R-value basis.

      But at a typical 7% framing fraction for roofs, with half-inch ceiling gypsum long-nailed into the rafters or mounted on 1x furring through-screwed to the rafters it comes in a bit under the code maximum U0.030 (= R33 "whole assembly) the IRC calls out for zones 2 & 3.

      If needed, the edges of the continuous foam can be air sealed with a shot of closed cell foam from a small DIY kit (FrothPak, Tiger Foam, etc.)

      If soffit to ridge venting is not possible, a 1" shot of closed cell foam on the underside of the roof deck for dew point control, and 2" foam + 1x furring "Bonfiglioli strips" on the under side of the rafters adds enough depth for 7.25" thick R30 rock wool batts. Even with HFO blown foam it wouldn't quite make it to R38 at center cavity, but with the 2" foam (any type) thermal break on the rafters it would quite easily make it on a U-factor basis.

      A sheet of 2" Huber ZIP-R ripped into 1.5" wide strips might be easier than assembling strips by other methods, or one could laminate a full sheet of 2" foam onto a sheet of 5/8"-3/4" OSB or CDX using foam board construction adhesive and rip it down into 1.5" wide strip if 2" ZIP-R isn't readily available. Be sure to use screws long enough to penetrate the rafters by an inch or more.

  5. Expert Member
    Peter Engle | | #10

    And all of this is quite a bit more work and expense than just insulating the attic floor, and, even if you hit code minimums, you're going to have 30% more (depending on geometry) heat loss than if you insulate the attic floor just because of the increased surface area of the roofs and gables. Unless you need the attic to be conditioned space, go with the insulated floor.

    1. Expert Member
      Dana Dorsett | | #11

      >"And all of this is quite a bit more work and expense than just insulating the attic floor..."

      That's right.

      >"Unless you need the attic to be conditioned space, go with the insulated floor."

      Right again.

      If it's not too late to change the spec on the rafters, making them 2x10s and going with 2" of HFO blown foam on the roof deck (R14) + R30 compressed fiberglass R30s in the remaining 7.25" (~R25 @ ) 7.25" can be pretty cheap and still meets code on an R-value basis. Or go with 2x12 rafters, 2" of closed cell foam, and get the full loft out of the fiberglass R30s for a better-than-code R42-R44.

      Doing 2x12s with 11.25" of open cell foam would also meet/beat code, but has a history of creating ping-pong moisture cycling in the attic in the summer in a zone 3A climate.

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