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soupy8728
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# Posted: 28 Aug 2024 21:46
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Hello all,
I'm very new to this and I would like to gather as much information as possible. I purchased some land up in Northern Maine near Weston and would like to build a small cabin. I think I'm good with everything EXCEPT the insulation part. My plan is a 16x20 cabin with a loft, 12x12 pitch cathedral style roof with metal roofing. I'm confused about the ceiling insulation and insulating the floor as well. As I read, the R-value for that zone seems to be R-38. If that's the case, do I need 2x12 rafters? Thanks for any advice. (I'm not a construction person so please keep that in mind...LOL). Thank you.
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ICC
Member
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# Posted: 28 Aug 2024 22:01
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Owens Corning makes a special high-density fiberglass batt insulation specifically for use with 2x12 rafters. It is 10-1/4" thick, so leaves an air space. There may be rockwool available that can work in 2x12 rafters.
Foam sheet insulation can be used externally on top of the roof sheathing. A second sheathing is installed on top and then the roofing.
Sprayed in foam is another method. Sprayed in eteeen the rafters on the inside against the underside of the roof sheathing. It can be s DIY thing, but it is messy. Full hazmat suit should ne worn.
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soupy8728
Member
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# Posted: 28 Aug 2024 22:16
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Thanks! Not interested in any type of spray foam. I’ll keep researching for now. Thanks!
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Brettny
Member
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# Posted: 28 Aug 2024 22:24
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Only R38? You sure it's not r48-60? There are Rvalue charts available.
You can do a combo of foam and batts. I would never use just batts with a cathedral ceiling and metal roof. Just to many chances for air leaks and air leaks cause mold.
We are in central NY. We have 2x10 rafters with 4in polyiso foam then R21 fiberglass, equals about R42. With 2x6 walls this means we only had to use two types of insulation.
I can say putting the foam in between the rafters took for ever and alot of cans of foam. I would have done 2in above the rafters and fill in the rest with batts. No air gap because I dont believe its needed if you have proper insulation and air sealing. Snow stays on our metal roof with the wood stove gearing for days.
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soupy8728
Member
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# Posted: 28 Aug 2024 22:30
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I think you’re right…after reading some more it seems new requirements went into effect for Maine to have R49 in the ceiling. Thanks for the great info!
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Nate R
Member
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# Posted: 29 Aug 2024 19:18
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Some codes allow a trade-off. Like if you use more insulation in the walls, you can use less in the ceiling.
In my area, I was able to use the federal software program "RESCheck" to show my cabin would meet the required code, as a whole. I had to put in window, wall, ceiling, foundation and door R-values, sizes, etc. I was able to play with different values then and figure out what would work.
In my case, I ended up putting R38 batts in the ceiling between trusses, and R4 (3/4") foam sheets on the underside of the trusses, between the drywall and truss.
-Nate R
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Nate R
Member
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# Posted: 29 Aug 2024 19:19
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Ahh, looks like Maine allows REScheck to show compliance: https://www.energycodes.gov/states-can-use-rescheck-show-compliance
-Nate R
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ICC
Member
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# Posted: 29 Aug 2024 20:52 - Edited by: ICC
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Quoting: Nate R In my area, I was able to use the federal software program "RESCheck" to show my cabin would meet the required code, as a whole. I
Good point about REScheck Nate R. REScheck is a very good tool. It has been around since 2010 or earlier. I used the trade-offs in both my cabin and my home. I juggled window sizes and U-factor as well as, wall and ceiling R-values. I have seen a number of DIY'ers balk at having to work at all the details, but I find it very nice to trade off higher R-values in a wall or floor to gain some window size.
Several years ago they announced that the downloadable version would not be updated past the 2015IRC as they are concentrating on the web-only interface. I think that is too bad, but all software seems to be headed to online only.
https://www.energycodes.gov/rescheck
In NM many jurisdictions mandate the use of REScheck when submitting applications for new construction.
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