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Slow rise foam or dense pack cellulose in roof slope?

ehilder | Posted in Webinar Follow-up Q&A on

Cape Cod style house– i.e., under roof is kneewall, slope, and small collar attic above the flat bedroom ceilings

1925 house in Washington DC area with slate roof on front side of house, asphalt shingles in back. Roof deck is boards, not plywood or OSB. Currently there is a small amount of cellulose on “attic” floor and some poorly installed fiberglass batts on some areas of roof deck in kneewalls.

Plan is to use foam on the interior side of the roof deck – open cell in kneewall and attic space and “slow rise” foam (brand name “Tiger foam”) in the enclosed slopes. Doing so would mean AC ducts and AC air handler would be in conditioned space.

I am wondering about the best way to insulate the slopes. Dense pack was suggested initially but it seemed there was a risk of moisture problems.

Also, the installer says best practice when applying foam when there is slate roof is to place a plastic vent shute between roof rafters to prevent foam from expanding between the roof deck boards and disturbing the slate (the foam would push the shute flush with the deck so no air space). I wonder how this might affect moisture issues.

Any advice would be much appreciated


  1. Dana1 | | #1

    Slow rise foams are tricky to use, with a high risk of both blowouts and voids, which would be difficult to correct later. A DIY with a Tiger Foam kit is a risky proposition. While it has a high R/inch, most of that performance advantage is undercut by the thermal bridging of the rafters. It also has a very heavy environmental impact relative to cellulose, blown fiberglass, or rock wool batting.

    A slate roof on skip-sheathing or any board sheathing with at least some spacing between the boards is inherently back-ventilated. Slipping in thin foam barriers that would keep slow-rise insulation out of the board gaps works for cellulose or blown fiberglass as well. If you're concerned about the drying rate of any rain that gets by the slate soaking into the fiber, use an appropriate cavity-fill fiberglass (not open blow wool) such as Optima, Spider, L77, all which have a much faster drying rate than cellulose.

    In the open raftered roof section behind the kneewall you can probably use open cell foam trimmed flush to the rafter edges, with an inch or three of unfaced rigid EPS on the interior side of that, with an intumescent paint fire retarder on the rigid foam. If full-sheet goods are too big to get into those kneewall cubby-attics, installing a layer of MemBrain followed by 2" wide strips of 1.5-2" foil faced polyiso on the underside of the rafter edges and intstalling split R13 batts between the polyiso strips yields a higher performance and higher resilience than a full cavity fill of closed cell foam.

  2. ehilder | | #2

    Many thanks for your reply. I should note that this is definitely not a DIY situation, but instead a professional installer who would use an infrared camera to check for voids or shrinkage. Because we have about 5 feet of plaster slope wall running between the open rafter areas above and below, I assume we are stuck with thermal bridging in that area. But we have been hoping to air seal as much as possible and also not create moisture problems in the roof sheathing.
    two more questions:
    1. Should whatever barrier is placed up against the roof boards be vapor permeable to allow drying to the inside, since this is an unvented roof situation?
    2. Any suggestions on how minimize exterior air infiltration into the slopes, since it seems caulking or taping is not an option given the lack of access to the rafter spec except from above or below?

  3. GBA Editor
    Martin Holladay | | #3

    Yes, a vapor-permeable material is the best choice for the material that you plan to insert under your skip sheathing. For a more thorough discussion of the issues behind this type of material, see Site-Built Ventilation Baffles for Roofs.

    The best way to insulate the sloped section of your ceiling is to add enough rigid foam on the interior to meet the minimum code R-value for your climate zone. For more information on all of your options for this sloped ceiling, see How to Build an Insulated Cathedral Ceiling.

  4. ehilder | | #4

    Can the rigid foam go right up against the skip sheathing or do I need to add some furring strips to create an air gap (this is an unvented roof situation )?

    What type of rigid foam would be appropriate to use as the first layer adjacent to the skip sheathing and should it be faced or unfaced?

    How important is it to cover the rafter faces with foam board in knee walls to prevent thermal bridging? Worth the extra effort/cost?

  5. GBA Editor
    Martin Holladay | | #5

    Q. "Can the rigid foam go right up against the skip sheathing or do I need to add some furring strips to create an air gap (this is an unvented roof situation )?"

    A. If you have skip sheathing and slate, you don't need an air gap under the skip sheathing. This type of roofing is vapor-permeable, and dries readily to the exterior.

    Q. "What type of rigid foam would be appropriate to use as the first layer adjacent to the skip sheathing and should it be faced or unfaced?"

    A. Unfaced EPS probably makes the most sense, both from an environmental perspective and due to the fact that it is vapor-permeable.

    Q. "How important is it to cover the rafter faces with foam board in knee walls to prevent thermal bridging? Worth the extra effort/cost?"

    A. Your question is confusing. At first, I assumed you were talking about rigid foam on the interior side of your rafters, due to the phrase "rafter faces." But the phrase "in knee walls" confused me, since kneewalls don't have rafters -- they have studs.

    Assuming that your question referred to a sloped ceiling with rafters, here is my answer: You are in Climate Zone 4A. In that zone, the 2012 International Residential Code calls for a minimum of R-49 ceiling insulation. I doubt whether you can get R-49 between the rafters of an old house, so that means it makes sense to install additional rigid foam insulation on the interior side of your rafters. As an added bonus, these layers of interior rigid foam will address thermal bridging through the rafters.

  6. Expert Member
    Dana Dorsett | | #6

    If you have 2x6 rafters you get about R20-R24 out of fiber insulation between the rafters, but the rafters themselves are only about R6.5. In the short rafter runs in kneewall attic space the framing fraction is pretty high compared to a full attic, usually over 20%, maybe even 25%. At a 25% framing fraction with R21 cavity fill about half the heat loss is through the R6.5 framing.

    Adding 2" of foil faced foam to the rafter edges and adding another 2" of fluff in the cavity raises the R-value of the framing fraction to about R19- it cuts the framing losses by about 2/3, even though the additional R7-8 in the middle only cuts the heat loss trough the fiber by about 1/4. With fully vented roofing you could just cover the whole thing with foil-faced polyiso, but without the venting you'd need it to be able to dry toward the interior as well, which is why I suggested rafter edge treatment, and not covering the fiber insulation with foil-faced foam.

  7. ehilder | | #7

    Thanks for these helpful replies and yes, Martin, you guessed correctly - I should have said the rafters in the knee wall space.
    One more issue: the gable end walls are brick exterior and the interior is faced with terra cotta tile. How should these be air sealed and insulated? Is open cell foam ok? Are there good alternatives?
    Many thanks.

  8. GBA Editor
    Martin Holladay | | #8

    The best insulation for the brick wall you are describing is closed-cell spray polyurethane foam. For more information on this issue, see Insulating Old Brick Buildings.

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