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Community and Q&A

Reduce thermal bridging

Steven Dunkel | Posted in Energy Efficiency and Durability on

Hey Everyone,

I live in zone 4 and have open-cell foam on the underside of my rafters which will be an insulated cathedral ceiling. The rafters are 2×6, and I want to address thermal bridging while the drywall is down. I’ve read many other questions by other posters, and although I understand the merits of putting rigid foam on top of the roof sheathing, that’s just not feasible in my current financial state. Additionally, the roof is young and in no need of being replaced. I’ve considered the following scenarios:

1. Offset a another set of rafters and insulate with batts. If this option is available, then should the batts be faced or unfaced?

2. Put a layer of foam on the underside of the rafters.

What do you think?

Thanks!

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Steven,
    If you install additional framing under the rafters, it's usually best if your additional framing is at 90 degrees to the rafters -- assuming, of course, that your 2x6 rafters are structurally adequate.

    In many cases, 2x6 rafters would be considered undersized. In cases like that, it would have been a good idea to consult an engineer before the spray foam was installed. (It's easier to sister 2x8s or 2x10s to your existing 2x6s before any spray foam is installed.)

    To answer your question: It's usually a good idea if a roof assembly can dry inward, so unfaced batts would probably work best in your climate zone. (In Zone 5 or colder locations, the kraft-faced batts would probably be beneficial.)

    If you decide to install a layer of rigid foam on the interior side of the rafters, EPS is probably the best choice, because EPS will allow (a little bit of) inward drying.

  2. Steven Dunkel | | #2

    Thank you Martin.

    Could you explain what you mean when you refer to the roof assembly drying inward vs outward?

    Thanks again.

  3. User avatar GBA Editor
    Martin Holladay | | #3

    Steven,
    If your roof sheathing ever gets damp -- due to a tiny roof leak, or due to moisture absorption that occurs when warm indoor air encounters cold sheathing -- it is useful if the damp sheathing can dry inward. The drying mechanism in this case would be vapor diffusion. Ideal conditions would be sunny weather (to heat up the roofing) when the relative humidity of the indoor air is low. If the insulation and interior finish materials are vapor-permeable, the roof assembly will dry to the interior under these conditions.

    Roof assemblies that can dry outward are more rare, but it's possible to design such a roof. You would need to choose a vapor-permeable roofing for this to work. Options include cedar shingles, concrete tiles, or slate. You would also need to choose a vapor-permeable roofing underlayment or an underlayment that has variable vapor-permeance; asphalt felt would work.

    If you want to install vapor-impermeable roofing like asphalt shingles, you would need to depend on ventilation drying rather than vapor diffusion drying. That can be accomplished by including ventilation channels between the top of the insulation and the underside of the sheathing, or between the sheathing and a second layer of sheathing installed above the ventilation channels.

  4. User avatar
    Dana Dorsett | | #4

    Since the roof is young replacing it with a permeable roofing isn't an option. It's either going to be a vented cathedralized ceiling or an unvented, which requires some low vapor permeance spray foam at the roof deck. The IRC spells out R15 of low-perm insulation directly on the underside of the roof deck, (which presumes R49 total R), but you can safely cheat that a bit. Just 2" (R12-ish) would be PLENTY for a zone 4 climate, even for R49, as long as the fiber insulation is high density and the ceiling is air tight. See:

    http://buildingscience.com/documents/bareports/ba-1001-moisture-safe-unvented-wood-roof-systems

    If you did 2" of ccSPF under the roof deck you could then install unfaced R15s over the foam filling the 2x6s bringing the center-cavity R up to R27-R28. If you also installed a set of 2x6 perpendicular to the rafters and installed R21 fiberglass or R23 rock wool you'd be better than R49 center cavity with low thermal bridging beating code by quite a bit.

    If instead of 2x6s you installed 2x4s 24" o.c. and another set of R15s would bring the center-cavity R up to R42-R43, but you'd probably still meet code min on a U-factor basis (U0.026).

    But key to going unvented is to putting the vapor & air retardent foam on the underside of the roof deck first. At 2" it would run about $2 per square foot for the closed cell foam. It's not very green stuff at high-R, but when used for moisture control in limited quantities to allow an even higher-R assembly there's a case to be made for using it.

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