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