Insulating a vented low-slope roof prior to solar install
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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