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Thermal bridging with spray foam insulation

GBA Editor | Posted in Energy Efficiency and Durability on

If you have 2×12 rafters and are installing approximately 5 inches of foam do you need to encase the rafters in foam to prevent heat loss through the top of the 1 1/2″ of rafter? Or is the heat loss so insignificant that totally encasing the rafter in foam would prove to be unnecessary?

Gary

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Replies

  1. Riversong | | #1

    The higher the R-value of cavity insulation, the more significant become the thermal bridges. It is imperative on a well-insulated house to reduce thermal bridging.

  2. User avatar GBA Editor
    Martin Holladay | | #2

    Gary,
    Here's another way to do it:

    1. Install 5 inches of closed-cell spray foam.

    2. Install a continuous layer of rigid polyisocyanurate foam under your rafters. Hold the polyiso in place with 1x3 or 1x4 strapping.

    3. Poke holes in the foam and install dense-packed cellulose to fill the rafter bays.

    4. Seal the holes with canned foam.

    Thermal bridging problem solved. And your ceiling has a higher R-value.

  3. Armando Cobo | | #3

    Gary,
    A better way to correct Thermal Bridging is to install the full amount of R-value required for your CZ. If R38 > 5” closed cell or 10” open cell + 1”-2” or rigid insulation ON TOP of the roof decking.
    Unfortunately, with the introduction of the “Performance” codes, folks are using less roof insulation where condensation is highly probable and many problems are likely to occur later on. I do specify foam insulation on most homes, but it must be done right, specially in colder climates.

  4. JR | | #4

    Should all ceilings have a polyiso layer under the rafters (or ceiling joists for that matter) to reduce thermal bridging?

  5. User avatar GBA Editor
    Martin Holladay | | #5

    JR,
    If you want to address thermal bridging through framing members, you have three choices:

    1. Continuous exterior foam.

    2. Continuous interior foam.

    3. Redundant staggered framing, with the space between the staggered framing members filled with insulation.

  6. Allan Bullis, CEM LEED AP | | #6

    Thermal bridging is very important to address. For attics, the best way is a fire rated polyisocyanate foil faced insulation with cellulose as mentioned before. The idea of insulation on the outside is not a good idea except when membrane roofing is used.

  7. User avatar
    Michael Maines | | #7

    Martin, in your #2 response aren't you concerned about the double vapor barrier? I realized that with 5" of closed cell foam there shouldn't be much for vapor drive or air leaks through the polyiso, but it doesn't seem like a very forgiving way to insulate. Why not skip the 5" of closed cell?

  8. User avatar GBA Editor
    Martin Holladay | | #8

    Michael,
    I agree with your recommendation. If I were writing the specs, I'd skip the spray foam.

    It was Gary who proposed the spray foam, and I was responding to his suggestion.

  9. User avatar
    Michael Maines | | #9

    Martin,
    Gotcha.

    Gary,
    Starting with your stated case, "If you have 2x12 rafters and are installing approximately 5 inches of foam"

    Your question is, "do you need to encase the rafters in foam to prevent heat loss through the top of the 1 1/2" of rafter?"

    I'm assuming you mean the bottom of the rafter, not the top, as the foam is usually applied to the underside of the roof sheathing.

    There is significant heat loss through that thermal bridge, and there is also the risk of condensation somewhere near the mid-point of the rafter depth eventually causing structural failure. The "picture-framed" system of foam installation helps reduce that risk. Encasing the entire rafter in foam is possible but not usually done because it would interfere with interior finishes.

  10. Riversong | | #10

    Martin,

    I'll take choice number four: create a parallel-chord truss (a reversed Riversong Truss), by dropping a 2x3 or 2x4 below each rafter gusseted 24" oc with 1/2" plywood strips on alternating sides.

    This is not the same as choice #3 - redundant staggered framing - and uses less lumber and offers an excellent thermal break and sufficiently increased insulation space that expensive and hygrothermally problematic spray foam can be replaced with dense-pack cellulose and a ventilation channel for a far more durable roof system. (Alternatively, frame the roof with parallel-chord trusses).

    Option #5 is to insulate at the ceiling level with sufficient depth of blown cellulose to bury the framing and eliminate thermal bridging. This is the least expensive option and offers almost unlimited R-value.

  11. Riversong | | #11

    Gary,

    If you want a quantified answer: assuming your 5" of spray foam is closed cell at R-5.9/inch and your 2x12 rafters are 16" oc, you will have at least a 26% R-value degradation because of thermal bridging through the rafters, and probably more because of 3D heat loss effects. The assembly will have an effective R-value of 21.9, which is woefully insufficient in all US climate zones.

  12. John Brooks | | #12

    Alex,
    I'll take Option #5

  13. John Brooks | | #13

    Robert,
    I think your option #5 is what the intern Architect from this thread should consider:
    https://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/16857/whats-best-thickness-polyiso-roof-insulation
    Avoid Dormers and vaulted ceilings (Keep It Simple,eh)

  14. Riversong | | #14

    Keep It Simple,eh

    Are you Canadian?

  15. John Brooks | | #15

    "Are you Canadian?"

    I am from the Great North ... North Texas
    I still think that folks from all climates can learn much from Canadians, Germans, and youz guys from Vermont
    I think your advantage is that you have a very large number of Degree Days

    As the price of Energy rises.... the rest of us will be in your boat.
    You have been in the boat much longer.

  16. Riversong | | #16

    I think your advantage is that you have a very large number of Degree Days

    We'd be glad to share some with you ;-)

  17. JR | | #17

    Martin

    Based on your earlier response(below), it seems like it would be easier to put foam on the interior wall and not have to deal with putting it up on the exterior & having to deal with flashing around windows, doors...

    JR,
    If you want to address thermal bridging through framing members, you have three choices:

    1. Continuous exterior foam.
    2. Continuous interior foam.
    3. Redundant staggered framing, with the space between the staggered framing members filled with insulation.

    JR

  18. User avatar GBA Editor
    Martin Holladay | | #18

    JR,
    Whether or not it is easier to put rigid foam on the interior side of a wall or the exterior depends on many factors:
    1. Is it new construction or retrofit?
    2. How thick is the foam?
    3. What's the siding?

    Different builders will react differently to the challenges of either approach.

    Here's the bottom line: Exterior foam ALWAYS performs better than interior foam, because exterior foam covers the rim joists.

  19. JR | | #19

    Martin
    New construction..1/2" polyiso..vg red cedar clapboard...2x6 walls..blown celluose insulation between the studs..
    Iif one where to use open cell foam in the basement to insulate the blockers would that be an acceptable method in lieu of exterior foam over the rim joist?

    JR

  20. Dan Kolbert | | #20

    We've built several houses with the method Robert describes in post #10 - option 4. It's not that hard to build and getting rid of spray foam saves a bunch of money (not to mention the GHG potential of the spray foam) We rip long 2x6's in half and make gussets from left over ply scraps, all cut to the same length so you can attach them to the 2x3's and then push them into place, the gussets hitting the roof sheathing. A siding stapler works well for attachment.

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