Roofing insulation and avoiding moisture build-up — a vexing issue
We live in a 1930s home in the Bay Area, California (climate zone 3C) and need a new roof.
The roof is mostly flat, with two smaller peaked sections. There’s hardly any attic space to speak of (i.e., there’s probably about six inches between the interior ceilings and the bottom of the roof deck). There’s no insulation in that space.
I’m exploring roofing insulation options. Talking to a traditional roofer who wants to put down bitumen, he explained he can’t put down insulation above the roof deck/below the top of the roof without venting the attic space, which would kill any of the roof insulation properties. He said if you don’t insulate or ventilate the attic space, then excessive moisture will build up there.
I also spoke with a SPF roofer. He said the foam layer he’d put down (on top of the current roof) would not lead to any moisture problems in the attic space (even without ventilating or insulating it it) given the depth of the foam.
FYI, insulating the attic space is a no-go at this point in time as we have knob and tube wiring and don’t have the money to replace that and the roof.
Any advice for me? I would really like to be able to insulate the roof for all the reasons you know about. SPF foam seems like the only option, but I don’t want that to lead to moisture problems in the limited attic space.
Thanks in advance.
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Replies
If you are able to put ALL of your insulation on the EXTERIOR of the roof, you shouldn't have any moisture issues in the attic space. If you have SOME insulation on the inside, you MIGHT have issues, depending on the ratio. It is NOT necassary to vent an attic that uses all exterior insulation, and you wouldn't want to, since it would defeat the purpose of the insulation.
Are you able to put ALL the insulation on the exterior?
Bill
Bill, thanks for the reply! Apologies it's taken me so long to check-in. Someone else checked and confirmed that there's no insulation in the attic space. Am I understanding correctly that if we sealed up the vents (basically round holes a few inches wide) and put SPF or rigid foam on the roof deck that moisture issues in the attic space shouldn't be an issue?
je-green,
Yes, but as well as blocking the vents, you also need to air-seal and insulate the edges of the attic, as they represent a bypass for heat to escape.
Thanks, Malcolm. Is there a way to do that without opening up the roof deck? The attic space is too small to crawl into.