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Insulating a vented low-slope roof prior to solar install

nick_vk | Posted in General Questions on

I live in zone 2B (Arizona low desert) and I have a low-slope, vented and uninsulated roof. I’m planning to have solar panels installed early next year and as a result I need to make some decisions about retrofitting the roof.

Based on my reading here, it seems like the most energy-efficient/global-warming-friendly solution for insulating would be to put a deep (6-8 inch) layer of polyiso on top of the existing roof and find a way to seal the current roof venting (the bays between the roof rafters are screened but open to the exterior, but there is no ridge venting).

The roofer is quoting $100 a square for each 2 inches of polyiso installed. The house is 1500 square feet, single story, and with overhangs the roof is even larger, so we are looking at somewhere north of 6K just for foam. I expect that price doesn’t include installing an air barrier. (BTW, the roofer doesn’t want to install more than 4 inches of foam). I would still need to tackle sealing off the “attic”, perhaps by cut and cobble with foam where the rafters meet the exterior wall, or by covering the “vent box” that surrounds the exposed ends and undersides of the rafters with polyiso and then sheathing over it somehow. The attached photo shows the “vent box”.

On to the questions:

1) My goal is to use my limited budget to reduce my long term global warming footprint. Is the roof the place for such a major investment? With a one story house there’s a lot of roof for the amount of living area, and I wonder it would be most sensible just to live with it and put the money elsewhere. Since I’m in Arizona, the “insulate vs generate” question comes up. For 6K I could add another 3 KW of solar to the system being installed, which would generate about 5500 kwh/year. Our current electric usage for cooling is around 4000 kwh from a 20 year old central AC unit, and we haven’t even done the most basic sealing of doors and windows yet. The current heating system is forced air natural gas – 280 therms last winter, the equivalent of 8200 kwh – which I hope to decommission and replace with mini-splits whenever that 20 year old unit gives up the ghost.

2) If I want to go ahead with insulating, how difficult will it be to get a sound, well-insulated roof? My concern is that if this job is not executed very carefully I could end up with a very expensive roof on top of a poorly vented (rather than sealed) attic, so that the benefit of the insulation is lost and the attic traps moisture as described in “Insulating Low-Slope Residential Roofs.”

3) The solar installer wants to the standoffs (the attachment points for the solar racks) through the existing roof to the rafters (so they can get two of four screws into solid wood), then installing the polyiso/sheathing/built-up-roof around it. This doesn’t seem ideal, but I’m not sure how else to create a solid attachment point for the standoffs. The roofer is planning to screw the top sheathing through the foam into the existing sheathing(and not worry about the rafters) but that may not be sufficient for the standoffs. Just wondering if anybody has any designs that would let the new roof be installed first.

Thanks in advance for any guidance!

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  1. nick_vk | | #1

    Forgot to mention that the solar installer and roofer are recommending a built-up roof as the best way to roof around the standoffs.

  2. GBA Editor
    Martin Holladay | | #2

    Installing insulation in your roof is a good idea in your climate. It will lower your heating bills, lower your cooling bills, and increase your comfort.

    You may want to read this article: Insulating Low-Slope Residential Roofs.

    Before we can give you good advice, you need to describe your "attic" better. Do you just have empty rafter bays -- say, 2x10s that are 9 1/4 inches deep? Or do you have a space that is higher than that?

    If you just have empty rafter bays, one option to consider is to fill the rafter bays with blown-in cellulose insulation (that insulation will help reduce air leakage). In your climate zone (Zone 2), you can supplement this R-34 insulation (for example) with 1 inch or 2 inches of exterior rigid foam if you want, for a total R-value of R-40 or R-46. (If you adopt this approach, you would still need to seal your soffit vents.)

    If you decide instead to install only rigid foam, without filling the rafter bays, you may want to hire a home-performance contractor -- someone familiar with air sealing work -- to work with your roofer. It's essential that you get the air sealing details right if you go this route.

    Finally, there are lots of hardware options for mounting solar panels these days, and many of them don't require stand-offs that are bolted to the rafters. Tell you solar installed to look into some of these newer options.

  3. nick_vk | | #3

    Thanks Martin. I just have empty rafter bays. The main house has 2x8 rafters, and the addition has 2x6 rafters. The rafters are very slightly sloped so there's are few extra inches below them at the ridge, but that's it.

    I like the idea of combining blown in-cellulose with foam. I had given up on the cellulose it thinking the area was too inaccessible, but I'd be glad to be wrong. To blow in insulation, would the insulation contractor try to snake a hose up through the exterior ends of the rafter bays, or would they drill holes in the ceiling (or roof)? At this point, without having removed the fascia, I'm not 100% certain how the main roof is framed. Based on the slope I think it's a gable, but the venting on the side perpendicular to the rafters has me a bit uncertain. I guess if the contractor drills enough exit holes to confirm any guesswork we can be certain that all attic cavities are filled.

    Does filling the rafter bays with cellulose and adding 4 inches of polyiso seem reasonable? Rough math tells me I’ll have something like R-44 in the (less-used) addition and R50 in the main house between the bays. Discounting that 10% for framing still keeps me above R-38, even in the addition.

    And I will ask my solar installer to come up with other options, and do some research myself. I will venture to guess that given the dimensions of my rafters, ballasted solutions are out!

  4. user-2310254 | | #4

    As noted here (, you need a minimum of 13% of the total r-value on exterior. That's a minimum, though. You can go thicker.

  5. GBA Editor
    Martin Holladay | | #5

    If the insulation contractor can gain access to the rafter bays from either end, it should be fairly easy to insert a rigid tube (attached to the end of the blowing hose) into each bay. Once the tube is inserted, the installer can move the tube back and forth from one side of the rafter bay to the other, gradually dense-packing the rafter bay as the tube is withdrawn.

    This is one of several possible approaches. It's also possible to perform the work by drilling holes in the ceiling or the roof sheathing. For more information on these issues, see How to Install Cellulose Insulation.

    If for some reason contractors in your area prefer blown-in fiberglass to cellulose, it's possible to substitute blown-in fiberglass. As with cellulose, a dense installation of blown-in fiberglass does a better job of reducing air leakage than an installation that doesn't focus on density.

  6. nick_vk | | #6

    Thank you Steve and Martin. Time to find an insulation contractor! As always, I appreciate the patience and friendliness with which questions are answered here.

  7. nick_vk | | #7

    I’ve gathered a bit more information, which has led to some further questions.

    Blown-in insulation: after talking to an insulation contractor I removed a section of fascia from the main section of the house yesterday to see what kind of access he'd have, and it turns out there is some loose fill insulation, 3-5 inches deep near the exterior wall and getting slightly thicker near the ridge.. I'm not sure what type it is, but I'm guessing it's what was typically installed here in 1950 - rock wool?. There is 3-4 inches of air space above the insulation. In one of the bays I can see some 2x4 bracing about 2/3 of the way up the bay to the ridge that could make it tough to get the hose all the way up to the ridge.

    After reading How to Install Cellulose Insulation and other articles, I realize there are some possible additional complications:

    1) There is a chimney on an exterior wall that we’d have to wrap with mineral batts. We may be able to do this by removing fascia - if not we’ll have to open up the plaster ceiling.

    2) There is also quite possibly original knob and tube wiring (typical for a house built here in 1950). If that’s the case, replacing it will a challenge without an attic. Even determined whether I have it may be a challenge.

    3)Since my contractor didn’t mention these items, I probably want a different contractor.

    Question 1: Should I be concerned about further burying knob and tube insulation? (If it’s there, it’s already covered by the rock wool).
    Question 2: is there any reason to remove the old rock wool? The slight additional R-value of cellulose doesn’t seem to warrant it.

    The roof/polyiso: I got a bid from a roofer, and it’s high enough that I’m thinking about having the solar panels installed first (to meet the deadline required by my existing permit) and then laying the polyiso/new roof myself sometime over the next year. This would allow me to save the labor cost, possibly find some recycled polyiso, and get the satisfaction of doing something other than writing checks to contractors. Once the solar panels on I’m committed to doing the roofing job myself, because while the roofer is willing to work around the standoffs, he’s not willing to install a new roof once the panels are on.

    Question 3: Is there a type of roofing that’s worth using that I can successfully apply myself? It looks like a cold-adhesive modified bitumen is a viable option. I’m curious what, if anything, folks here would recommend for a DIY install over polyiso. And if you think I’m foolish to consider such a thing, please say so!

  8. user-6184358 | | #8

    Hi, If it is the 1950's it is not knob and tube wiring. It is a early version of ROMEX. If it is knob and tube then you rewire the entire house.
    You can extend your permit. Check with the building department for the costs. You would need a new permit to do the roof. You can't do foam and a new roof the way you describe(around the solar mounts).
    To do the roof you could use 2x4 on edge, screw it to the existing rafters and lay in 3.5" of foam between the framing, then cover it with OSB and a new roof then attach the solar panels to the new roof. Talk to the person who will draw your plans and engineer your roof over frame. Your facia would grow by about 4"deep
    You could also look at a peel and stick low slope roofing material.Instead of the glue down.

  9. GBA Editor
    Martin Holladay | | #9

    Don't rush this job.

    1. You need to determine whether you have knob-and-tube wiring. If you do, I recommend that you hire an electrician to rewire. Even though there is some evidence that the fire risk form this type of wiring is quite low, it can still be a problem if you ever have to make an insurance claim or sell your house.

    2. You can't install a solar array on your roof, and then come back later and install rigid foam around the solar brackets, followed by more roofing. That's a recipe for roof leaks. Once the rigid foam, cover board, and new roofing have been installed, you can call up the solar contractor -- not before.

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