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Zone 4 unvented cathedral retrofit challenge – 2″ closed-cell spray foam + open-cell fill?

bradesp | Posted in Energy Efficiency and Durability on

Hi all, particularly Dana and Martin 🙂

I’m looking to confirm my roof assembly plan based on reading and re-reading all the Q&A threads from the last couple of years related to challenging retro-fits and the technical and economic constraints of an un-vented roof.

Roof / Rafter / Insulation Retrofit Details
– Zone:  4 – in Raleigh, NC
– Current Rafters:  2×6 @ 16″ O.C.
– Roof Deck: 1/2″ Plywood
– Roof Covering:  GAG 50 Year Architectural Shingles
– Structural Reinforcement Requirements:  We are being forced to double / sister ALL rafters with 2×8’s, reducing the rafter cavity to just 14.5″

Assembly Plan
After we install the new 2×8 “sistered” rafters, we will have significant rafter “mass” and just 14.5″ of rafter space to fill.  Given the shingles are brand new, we are not looking to tear the roof off and apply nail base to the top side of the roof.  Instead, our design goal is to do everything “under roof”.

So.. My current plan is to do the following:

1-Spray 2″ of HFO (LAPOLLA 4G closed-cell spray foam) on the underside of the roof deck to seal and still allow for modest drying to the interior.

2-Tack/Glue 1″ of Poly to the bottom edge of the new 2×8’s to reduce thermal bridging.

3-Fill the remaining rafter cavity space with Open Cell Foam, being careful to fill the cavity as close to the outer edge of the poly  strips that are attached to the bottom side of the 2×8 rafters.. Also making sure that the Open Cell foam covers the bottom of the existing 2×6 rafters to reduce bridging and “seal” the space between the two sandwiched rafters.

OK guys, hoping to hear back on whether I’m on the right track, or if there are adjustments you might suggest.


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  1. Expert Member
    Dana Dorsett | | #1

    By "...being forced to double / sister ALL rafters with 2×8’s..." is that for structural reasons? Even with some polyiso edge strips for mitigation doubling the framing fraction is going to take a thermal hit. (BTW: That reduces the cavity width to 13" , not 14.5".)

    In stead of sistered on 2x8s, can you make a truss out of it? Use a 2x2 chord and half-inch 6 " x 6" plywood gussets 24" on center glued and nailed to the chord. Install 3/4" edge strips on the 2x6s (or chord) to fill the space between the 2x2 chord and the 2x6, then glue & side-nail the gussets to the 2x6. That will be the same 7.25"total depth as a 2x8, and almost as strong. If you were going to take it to 8.25% with 1" edge strips, use 2" polyiso instead of 3/4".

  2. bradesp | | #2

    Unfortunately we're converting a stick built truss to a true cathedral and we're using steel tie-rods to hold the two walls together once we remove the bottom chord. So, yes, the 2x8 sistered Rafters are unfortunately a requirement from a structural POV.

  3. Expert Member
    Dana Dorsett | | #3

    With R14 of closed cell foam you're good for about R28 of fiber or open cell foam insulation from a dew point perspective. If you're at 8.25" total depth (2x8 + 1" polyiso) that leaves 6.25" of space to fill.

    R28/6.25= R4.5/inch.

    Half pound foam is only R3.7/inch, so at 6.25" would deliver ~R23 for R37 total. Open cell 0.7 lb foam is about R4/inch, or about R25. for R39 total, which is better.

    But with polyiso on the rafter edges you won't be able to trim the open cell foam flush using the normal tools without scarring up the polyiso.

    In this instance sculpting an R30 rock wool batt designed for 2x8s but compressed to 6.25" from it's 7.25"manufactured loft will deliver about R27, which is still fine from a dew point control perspective, an you'd be at R41 total.

    Damp sprayed 1.8lb JM Spider would deliver about R26, for R40 total. This is probably your best bet, since it can be rolled flat without crushing the polyiso, and would deliver a complete fill. Other 1.8lb fiberglass (Optima, L77 et al) would have to be blown in netting adding an extra step to the process.

    To stick with the open cell concept, using 1x furring over the edge strip polyiso (Bonfiglioli strip style) would add about R3 to the open cell numbers, and would stand up to the abuse of foam trimming. At 7.0" total depth 0.7lb foam would deliver R28, which is still OK from a moisture accumulation risk point of view- it won't get into the humidity ping pong situation that sometimes occurs with all open-cell solutions.

  4. bradesp | | #4

    Dana, this is amazing and gives me several options to explore... Most importantly, it sounds like my plan for 2" inches of HFO closed cell foam plus the right compliment to fill the cavities keeps me out of harms way from a moisture pov.

  5. Deleted | | #5


  6. bradesp | | #6

    I found your furring design reference! For others, this style of furring for insulation foam can be found here:

    1. Expert Member
      Dana Dorsett | | #7

      I apologize for not linking to that useful detail document- I usually do when mentioning Bonfiglioli by name. Glad you managed to find it without me!

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