I’m considering using rigid insulation under a metal roof with open rafters at the interior? Can I reach an R-30 value?
sfwjg
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In a mixed climate area in Northern California (Lake County) where the summer temps can often reach over 100 and the winter temps fall below 32 at times, can I effectively reach the required R-30 insulation requirements with a metal roof over rigid insulation and an open rafter interior situation?
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Replies
Sure, but you probably want to use a polyisocyanurate foam core nailbase or curtainwall (with drywall) panel for maximum R-value at minimum thickness. If you're going to use anything but a very light-colored roofing, I would recommend venting the roof between the insulation and the metal (there are some venting SIPs panels on the market).
William,
Your choices: 5 inches of polyiso, 6 inches of XPS, or about 8 inches of EPS.
You can order one layer of thick foam or multiple layers of thinner foam (which should be installed with staggered seams).
As Robert suggested, you could also choose nailbase or SIPs.
William,
Reaching R-30 above the rafters/trusses is not difficult but be mindful of the following details:
- make sure the metal roofing has the appropriate substrate.
- make sure the contractor has a good solution for fastening the roofing through insulation back to the roof structure
- Building up roof insulation above the rafters/trusses requires a trim enclosure around the sides of the insulation around the perimeter of the roof. I would ask your builder/designer to show you how that is resolved on paper instead of letting someone figure that out as they built in the field.
Also a simple roof shape makes good execution much more likely.
Don't forget your air barrier. Some builders assume that SIPs and rigid foam create good air barriers, but the fact is they still require careful detailing.
See the BSC link for a great article on roofs and rigid insulation. I think this will help you negotiate some of the issues with this technique. It will also give you a context (and pictures) to better understand the previous suggestions.
http://www.buildingscience.com/documents/insights/bsi-036-complex-three-dimensional-air-flow-networks/files/bsi-036_complex_3d_airflow_networks.pdf
I see many are concerned about screwing into the rigid insulation, which makes sense. Could you use hat channel style metal as furring to mount the roofing panels on? And then run the rigid or semi rigid insulation between the horizontal hat channels?
By hat channel, I mean something like this: https://www.clarkdietrich.com/products/drywall-framing/furring-channel-hat-channel
That one only comes in up to 1 1/2 inch deep, so that wouldn't get to the R-30 level, but I'm wondering if it would help.
Kim,
This discussion is almost nine years old. Since then techniques of fastening strapping through rigid insulation have evolved, and the size and spacing of fasteners has been codified, and is readily available.
it is a lot more desirable to fasten the strapping through the rigid foam than to provide backing and insulate in-between, as this creates large uninsulated thermal-bridges at regular intervals.