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shnnnh
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# Posted: 3 Jul 2012 09:10
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On another forum I came across a description of a roofing insulation method that I want to run by you:
Quote:
Originally Posted by kbs2244
Can you give us a little more info on the roof insulation?
Do I understand that you put it on top of the sheathing and put the sheet metal down? Is there any provision for a vapor barrier?
Yes, The insulation is R-Max rigid foam insulation by Thermasheath. I put 2" tall strips of 2x4 down on the sheeting 24" apart and placed the rigid insulation between them. Then used the strips to staple tar paper down. Then the corrugated roofing went down with screws into the wood strips on top of the tar paper. So the tar paper is the vapor barrier. And the wood strips give the corrugated something hard to be pulled down to.
Jaysin
End quote
If I am understanding this correctly, the benefit to me of this method would be that I could leave my ceiling rafters exposed rather than having to put insulation batts between the rafters and then hang Sheetrock over my head, and I wouldn't have to install vents.
This is for an unheated, occasional-use cabin in western NC.
The guy who posted this method on garage journal used it for a truly awesome fabrication shop (clerestory windows, stained glass, a 10' rolling metal entry door, etc etc) and seems to know what he's doing, but I wanted to run it by y'all before I tried it to make sure that I understand the concepts.
Basically I would put on my 2x8 rafters, cover them with plywood sheathing, put 2x4s on top of that, put rigid foam between the 2x4s, then tar paper over all that, then the metal roof.
Too bad I just finished using all the styrofoam my dad has been squirreling away for years. We stacked up about an 8" thickness of takeout boxes and deli trays to cover the earth floor where the basement crawlspace gives way to the finished basement, then put wood over that to make a platform for the boxes of his other "treasures." The archeologists of the future will really be scratching their heads when they discover that one!
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OwenChristensen
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# Posted: 3 Jul 2012 09:36
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Sounds ok to me.
Owen
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VTweekender
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# Posted: 3 Jul 2012 10:06
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Yes you are reading it correctly.....that is the recommended way to install a metal roof.....any moisture on top will find its way down the grooves in the metal roofing....if the insulation is a full 2" thick I would find a sawmill to get my 2x4 because they saw them a full 2"....so everything would be flush.....just a thought..
You could use something like a T&G pine from the sawmill as roof sheathing instead of plywood or OSB because it will be exposed inside and would look nice..probably not to much more money than buying the plywood.
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Sustainusfarm
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# Posted: 3 Jul 2012 10:13
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You could paint the underside (side facing inside cabin) of the sheathing first...that way you will already have a finished ceiling in the cabin and no need to drywall the inside! I have seen this done and it looks great! I think OSB was used and the painted surface looked like an old plaster finish...
Also 2" of foam board is only R-10 at best...I would maybe think about 2 layers of foam and be sure to used expanding foam to seal the edges nice and tight from air leaks.
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shnnnh
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# Posted: 3 Jul 2012 13:57
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Great advice on the pre-painting--I probably would've only thought of that while up on a ladder with an aching neck!
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Martian
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# Posted: 3 Jul 2012 21:01
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I've done metal roofs using this method. Instead of tar paper, you could use a reflective barrier, but for an "occasional cabin", it should work fine just the way you planned it.
You could use bead board for the sheathing if you like that look. Most of time, I've put T&G 2X6 Pine carsiding as the sheathing. It has the grooves in the face instead of a bead. And yes, you will appreciate having painted it first! For a truely finished look, caulk and paint the joints after the roof is finished. Because I'm a "belt and suspenders" kind of craftsman, I would put a layer of felt over the sheathing, screw down my strips, lay in the foam, put on the radiant barrier, and then screw down the metal. You'll have a roof that will last for years and years and won't heat up your cabin.
Tom
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shnnnh
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# Posted: 4 Jul 2012 10:35
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Have you heard of or worked with Prodex Total? Claims to be a radiant barrier, reflective insulation, and vapor barrier all in one, r-15 and 5mm thick.
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Bevis
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# Posted: 4 Jul 2012 11:46
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The roof on my house was done that way. Attic space is still insulated too.
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Borrego
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# Posted: 4 Jul 2012 16:00
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Yep, I've done this on houses I've built, works great and is ok for flat roofs as well if roofed correctly. You can also use T-111, groove side down to get an easy T&G look to the inside of the ceiling/roof (of course, stain & seal before installing  )
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karl
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# Posted: 17 Aug 2012 13:04
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Hi...I'm replacing my cabin roofing and want to add rigid foam on top of existing plank sheathing then a layer of ply plus new stick on rolled roofing. Just curious...why are u foaming between 2X4 nailers as opposed to strapping over a contiguous layer of foam? I've read that the screws through strapping/foam provide ample shear strength to hold roof secure.
Karl
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TheCabinCalls
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# Posted: 17 Aug 2012 13:17
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I saw in fine homebuilders that a guy that did this noticed after 10 years or so saw the insulation boards shrank. So I guess the lesson learned is to make sure you stagger joints if using multiple layers and to use tape.
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GomerPile
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# Posted: 17 Aug 2012 13:33 - Edited by: GomerPile
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Firs of all BEWARE: There are 3 kinds of foam ISO (foil backed), XPS (pink or blue) and EPS (white). EPS is basically styrofoam and only supports ~2PSI which means it will compact and dent if you walk on it.
The ISO ($$$) or XPS ($$) is what you want to use for a roof. They both typically support 15-25 PSI depending on what you get. There is actually no need to use 2x4s for support (for "insulation bays") in fact, its a bad idea since that diminishes the overall insulation factor of your roof by creating thermal bridging through the wood.
For a shingle roof you can lay down OSB fastened with head lok fasteners (bottom sheating -> FOAM -> top sheating -> F&I -> shingles). For metal roofs you can use strapping fastened down with head lok's (bottom sheating -> FOAM -> strapping -> metal).
BTW. the metal attachment method is the same as you would use for exterior walls
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Vandertree
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# Posted: 21 Jun 2021 10:14
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Hey! I'm doing the metal roofing version for what you're describing. Will this be OK in a Canada winter?
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