Compact re-roof with polyiso over spray foam?
Hello everyone!
I’m in the Lehigh Valley, Climate zone 5A (borderline 4A), and I have a roof that needs to be redone and insulated far more than it is now, it was originally a cold cellar back in the day, and is a relatively small area- 17′ x 17′.
Right now It’s true roughcut 2×8″ on 24″ centers, topped with 1″ planking that looks to be original, all in good shape, followed by a vapor barrier, a layer of 2×6″ run parallel to the 2×8″ under them with fiberglass in between, then at most 2″ of rigid on top, though i suspect less, or nothing but sheathing. Roof is something between a 1 and 1.5 pitch.
I plan on removing at least the roof sheathing and roof (old rubber roof) to install metal roofing and new insulation, unvented due to the low pitch, and I’m trying to figure out a cost effective way to improve on the insulation.
I’m height limited outside to roughly 8-9″ due to a roof intersection, and Ideally I want all of the old 2×8 exposed inside, but I’m willing to settle for less if needed. I want to keep them open as much as possible as the ceiling is fairly low as-is, having the open bays helps make the room feel much more open, plus its an 1801 Stone house, with exposed beams and stone throughout, so hiding them altogether would be criminal.
I looked into SIP’s but didn’t like the cost and crew required. I’m fine doing a roof with contractor friends, but renting a crane/pro crew sounds like more than I want to do. I enjoy working slow.
Currently my plan is to use 6-7″ of Polyiso on top of the planking, (either 3 or 3.5 inch-depending on what i can find recycled) then furring strips or plywood, and standing seam. I realize that will only be R30-35, if there is headroom I’d love to go up more unless its unadvisable to just roof with rigid over 6″ deep?
I also had the idea of doing a 1-2″ flash of spray foam on the inside of the planking between the 2×8″ to help boost the R-value and air-seal the living space, but I wasn’t sure if this (having the planking sandwiched between the foil-faced polyiso and the spray foam) was advisable- would it just trap moisture and rot, or would it be fine?
If it IS ok, another option would be flash and batt under the planking, and rigid on the outside, but again I’d hate to hide the joists.
If there are other options I’d love to hear them! Cellulose in the 2×6 bays and rigid on top? Some other combination?
Mike in LV
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Replies
Mike,
First of all, your description of how the 2x6 rafters are oriented with respect to the 2x6 rafters is confusing. I don't understand what you mean: "Roughcut 2x8" on 24" centers, topped with 1" planking that looks to be original, followed by a vapor barrier, a layer of 2x6" run parallel to the 2x8" under them [under what?] with fiberglass in between."
Anyway, I think that most of your questions are answered in these articles:
Insulating Low-Slope Residential Roofs
How to Install Rigid Foam On Top of Roof Sheathing
-- Martin Holladay
Mike,
Q. "I also had the idea of doing a 1-2 inches of flash of spray foam on the inside of the planking between the 2x8s to help boost the R-value and air-seal the living space, but I wasn't sure if this (having the planking sandwiched between the foil-faced polyiso and the spray foam) was advisable. Would it just trap moisture and rot, or would it be fine?"
A. In general, it's not a good idea to sandwich wooden components between two layers of vapor-impermeable foam. If you choose to install the spray foam, make sure that it is open-cell spray foam, since open-cell spray foam is vapor-permeable.
-- Martin Holladay
Mike,
Q. "Another option would be flash and batt under the planking, and rigid on the outside, but again I'd hate to hide the joists."
A. The true flash-and-batt approach requires the use of closed-cell spray foam (which wouldn't be advisable if you have exterior rigid foam). But you really aren't talking about flash-and-batt, because the exterior rigid foam is keeping the roof sheathing above the dew point during the winter, making the "flash" part of the flash-and-batt unnecessary. If your rigid foam is thick enough, you can supplement the exterior rigid foam with batts -- skipping the "flash."
For more information on these topics, see Combining Exterior Rigid Foam With Fluffy Insulation.
-- Martin Holladay
Hey Martin, Thanks for the responses! In regards to your points:
M---"First of all, your description of how the 2x6 rafters are oriented with respect to the 2x6 rafters is confusing."
Sorry for the confusing language! The 2x8 beams are true 2" x 8" measured, instead of the modern 1.5" by 7.25", and the 1" planking was i think the original sub-roof. ---On top of--- that, are 2x6's -running parallel- to the 2x8's, like an over-roof, with the fiberglass in the voids, then the sheathing/roofing on top. It's like someone built a secondary roof structure to house insulation, or reinforce the structure to modern code-I know I'll have to check with an engineer about removing the 2x6 layer completely. I hope that makes sense!
M---"In general, it's not a good idea to sandwich wooden components between two layers of vapor-impermeable foam. If you choose to install the spray foam, make sure that it is open-cell spray foam, since open-cell spray foam is vapor-permeable."
M---"The true flash-and-batt approach requires the use of closed-cell spray foam (which wouldn't be advisable if you have exterior rigid foam). But you really aren't talking about flash-and-batt, because the exterior rigid foam is keeping the roof sheathing above the dew point during the winter, making the "flash" part of the flash-and-batt unnecessary. If your rigid foam is thick enough, you can supplement the exterior rigid foam with batts -- skipping the "flash."
Makes perfect sense, that sandwich with CC foam was exactly what I was worried about, but making sure it all can dry to the inside using OC foam or batts makes sense. Just have to air seal -Above- the planking, right below the rigid then.
Is there such a thing as "too thick" when using rigid foam this way? e.g. if I can fit 9", is it bad for it to be all rigid foam panels, taped and sealed? (besides the fun of getting nice long screws to hit the rafters below, anyway)
Mike,
Q. "Is there such a thing as 'too thick' when using rigid foam this way?"
A. No.
-- Martin Holladay