So I am building a small cabin (14x28) in Maine and I want it to be able to be moved if I sell it. I will be living in it in the winter so it has to be insulated. The issue is my width limitation is 14' so I am building 2x4 walls. I am thinking bat insulation in the cavity and then 1" foam on the outside of the studs for thermal bridging is about as much insulation I can put into it and stay with a narrow wall. I have two questions. First does this sound reasonable for a small cabin or is there a better way to go in a limited space. Obviously spray foam would be good but I cant afford that. Second, what should I do about vapor barriers? I am thinking framing, taped ISO, Strapping, sheathing, house wrap, siding. Then on the inside batt insulation and then interior wall. Does there need to be another vapor barrier in there somewhere? Any advice is greatly appreciated.

If you think you will move it, how about a shipping container cabin? 8' x 40'.

Jonr has it. It is called a Park Model.

A mere 1" of exterior oam won't cut it from a dew-point control at the sheathing point of view for 2x4 construction in any Maine climate. You need to be looking at R10 for exterior foam in northern ME, and R8 for the rest. The absolute minimum you could get away with is R7.5 if polystyrene, R10 if polyiso (due to derating for low-temp conditions).

http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_7_sec002_par025.htm

You don't need a vapor barrier, you DO want an air-barrier. Air-sealing the OSB/plywood sheathing is a pretty good primary air-barrier, but be sure to caulk under the bottom plate of the studwall, caulk the sheathing to each stud, and between the doubled-up top plates, etc. If you go with foil faced polyiso you're looking at 1.5" min (2" is safer), but sealing the seams with FSK tape is quick & easy. Use can-foam to seal the edges. The fact that the foil facers are also vapor barriers makes it even more important to go with a higher foam-R, since there is zero drying through the foam. Using unfaced 2" EPS would only get you to R8, but it would have substantial drying capacity through the foam.

Then, make the inteiror gypsum air-tight as possible, and don't put anything more vapor retardent than standard latex paint on it. Air sealing around electrical boxes is a bit of a PITA, but can be done.

You'll want at least R30 between the floor joists, with plywood or OSB air barrier underneath to discourage the critters from setting up shop.

Thank you for all the information Dana! How much ISO would I need on the walls and roof if I were to leave out the bat insulation? I love the idea of container buildings but I have all the spruce material I need. If I didn't already have that then I would consider it for sure.

Posted By rmawhinney on 30 Jun 2014 01:43 PM
Thank you for all the information Dana! How much ISO would I need on the walls and roof if I were to leave out the bat insulation? I love the idea of container buildings but I have all the spruce material I need. If I didn't already have that then I would consider it for sure.

If all of the insulation is outside the structural sheathing the sheathing will be at the same temperature and humidity as any other wood in the house, so it doesn't much matter if it's 3/4" of iso or a foot.  But for staying comfy in Maine you'd probably be happier if it were a foot.

To meet code-min performance with only exterior insulation in Maine (climate zones 6 & 7) you need at least 4" of polyiso. You could meet code min LABELED R at only 3.25" (R20), but the mid-winter performance of polyiso would be lower than labeled.  If you only have room for 3", you'll be better off with staggered layers, with 1.5" of polyiso next to the sheathing, and 1.5" EPS exterior to that. While the labeled values would be R9/R6.3 for the iso/EPS respectively, during mid-winter the outer layer of EPS would be about R7, but the inner layer of polyiso would be about R11.   If you put 2 layers of 1.5" iso the outer layer would be only good for about R3-R5. This is due to the funky detrating curves for polyiso, which peaks at about 50F mid-layer temp, but falls off a cliff for layers with a mid-layer temp below 30F. 

EPS keeps gaining performance with lower temp for quite awhile, and doesn't have that steep cut-off at very low temps found with polyiso, which is why the a 50/50  thickness stackup of EPS/iso performs better in a Maine climate than an all-iso solution, despite the labeled values.