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This is in NW Washington, climate zone 4-C. R-49 required for roof insulation.

I have a 100-year-old, partially-finished attic that I plan to gut and re-finish in the coming years into proper living space. I've gone back and forth on strategies to insulate this space both to code and for longevity.

My current plan is as follows.

This gives me 1” x 5 + ½” x 0.5 + 6¼” x 7 = R-49.25

The current roof system is standard 2x6 on a 12/12 pitch. To achieve this insulation level, I'd need: 1” air gap + 1” foam board + 6¼” closed-cell foam = 8¼” rafter depth. Meaning I need to add 2¾ to the rafter depth.

An alternative thought I had was to slightly reduce the spray foam depth and add an additional 1" of foam board on top of the rafters before hanging drywall, which would reduce thermal bridging from the lumber.

Am I on the right track? Are there better/alternative solutions I could be considering? My main concern is maintaining as much interior space as possible.

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    R49 seems an unreasonable goal for a rafter roof. While that's a modern standard, it usually involves roof systems designed with capacity for it in mind. If you can get to R-30 or a bit more you're in a good place, considering your relatively mild climate. Of much greater concern are air leaks elsewhere in the home.
    – isherwood
    Commented Jan 5 at 18:59
  • Yes, R-30 is reasonable, but R-49 is required by the municipality.
    – Sam Morgan
    Commented Jan 5 at 23:15
  • Required when? Have they demanded that every century home be immediately upgraded? Modern codes usually don't apply to existing homes.
    – isherwood
    Commented Jan 7 at 15:20
  • Modern codes do apply to new work on existing homes.
    – Sam Morgan
    Commented Jan 8 at 1:40

2 Answers 2

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In part depending on the current shape of the roof, and in part depending on how much saving interior space is a priority, putting the insulation and ventilation space on top of the current roof (and re-roofing over that) allows for the most interior space and as much insulation as you'd care to add.

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  • I had considered this, however the current roof is less than 5 years old and I wouldn't want to add the additional cost of an entire roofing system if I don't have to. The current interior space is quite large. I could lose a few inches, but a foot would be an unreasonable reduction.
    – Sam Morgan
    Commented Jan 5 at 23:16
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I think you are on the right track. Like you said you will need to add some depth to the rafters. I would tweak it slightly by changing the air gap to 1.5" and adding the second layer of rigid panels on top like you suggested. You will end up using 4" of ceiling space however. I would put 2x2 furring strips perpendicular to the rafters which you can fill with 1.5" rigid foam panels. Here's a diagram:

enter image description here

So you R-value will be:

1" rigid foam R5, 5.5" spray foam R38.5, 1 1/2" rigid foam R5.5, 1/2" Drywall R0.5 for a total of 49.5

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