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wisco
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# Posted: 4 Apr 2018 07:52pm - Edited by: wisco
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Hello,
Great forum! I am planning on building a cabin starting end of May and could really use some help planning my build - only thing I have built in my life is outhouse and woodshed...
Building in Wisconsin. 14x24 cabin with shed roof. I plan to build on two skids, each 1' in from the sides (12' span), and was thinking of using(3) 2x10's per skid.
Then planning on using 2x10's for floor joists 16"OC and using 3/4" T&G for subfloor. Does that seem correct? Hopefully...
I want to do some sort of insulation in the floor when building, but I am not sure what style to do. Ease of installation and getting a good seal is more important than saving money for me...
At this point, I was considering installing 2x2 pieces of wood 4" from top of joists (before I put joists together) and then putting in 2" foam board, using Great Stuff to spray seams, and then dropping in another piece of 2" foam. Is that a good idea? Do I need to worry about critter protection with this? Would I get almost as good with just 2" foam?
Do I need a vapor barrier if using 3/4" T&G as subfloor?
Thanks for all help and ideas!
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rockies
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# Posted: 5 Apr 2018 06:48pm
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Hmm. I was reading an article on the Green Building Advisor website and one of their building experts recommended filling the bay with mineral wool insulation to within 2 inches of the top of the joist (leaving an air gap under the subfloor). he said that by leaving the gap you get a warmer floor than from stuffing the joist bay full right to the top.
A better method than installing rigid foam panels inside the joist bays is to cover the underside of the EXTERIOR sheathing with roxul comfortboard mineral wool panels. These still allow the floor assembly to breath but by adding insulation to the exterior of the floor assembly it raises the temp of the exterior sheathing so that you don't get moisture condensing inside the joist bay. You'll have to cover the exterior mineral wool panels with something so insects or rodents don't get into it..
I would probably add a vapor permeable membrain underneath the subfloor sheathing. Poly is not vapor permeable and could trap moisture inside the floor assembly.
https://www.rockwool.com/products/comfortboard-80/
https://www.certainteed.com/building-insulation/products/membrain/
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SCSJeff
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# Posted: 6 Apr 2018 08:22am
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Rockies,
The comfortboard sounds great. In the interest of protecting the insulation from insects and animals (but, also allowing the space to breath), do you think 1/4" cement board would work?
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creeky
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# Posted: 6 Apr 2018 10:59am
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I would go have a look at BuildingScience too. They have a lot of really good easy to understand (pictures) research there.
I tore out "pink" insulation from an insulated floor with a friend. If you put your vapour barrier anywhere put it right under the t&g. The previous home owner had put poly tacked to the joists. There were lakes under there. It was neat to see.
Mineral wool is the bomb. Critters don't like it (very scratchy). Impervious to most insects.
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rockies
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# Posted: 6 Apr 2018 07:36pm
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Yes, I plan on using cement board panels attached to furring strips attached to the surface of the confortboard. The nice thing about both materials is that they are basically fireproof, so any wildfire that may run under the cabin shouldn't cause any damage.
As to fire protecting your under-floor beams, I would coat the beams with Flamestop 2. It isn't noticeable as a coating but it really helps prevent timbers (and exterior wood siding) from catching fire.
http://www.flamestop.com/cart/fire-retardant-spray-for-wood.html
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KinAlberta
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# Posted: 7 Apr 2018 10:15am - Edited by: KinAlberta
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Quoting: rockies he said that by leaving the gap you get a warmer floor than from stuffing the joist bay full right to the top.
Yeah circulation space evens out temperatures. And little things make a big difference in terms of comfort.
Eg I insulated my city home’s basement ceiling with mineral wool and had the rim joist foamed. However on the main floor, the floor by our living room bow window now has a cold spot that encroaches about 6-7’ into our living room.
In the basement ceiling I left a couple inch gap above the Roxol insulation for circulation and a 1’ gap before the insulated basement wall. Nonetheless it’s amazing how cold the living room floor gets.
I may have to cut some small vents into the basement ceiling below that 1’ gap by the wall (below the bow window) to get warm air in there to combat the cold working it’s way through the foam.
I should have drilled some small holes in the floor joists to let air circulate between these cavities along the length of the outside wall and where ducting is warming a couple cavities. The rim joist foam under the bow window (about 10†deep and extending 2-3’ out) has just allowed the cold to come further into the house. (On either side of the bow window the floor is warm right to the wall.)
So bottom line, interior some air circulation is desirable around all perimeter walls. Better, deeper insulation ironically allows cold to encroach further in towards the living space. (That’s also why foam should be cut away from water lines going to outside taps - even deep frost-proof taps.)
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SCSJeff
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# Posted: 8 Apr 2018 07:52pm
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Quoting: rockies Yes, I plan on using cement board panels attached to furring strips attached to the surface of the confortboard.
Why the furring stips? Can't you just screw them right to the joists through the comfortboard?
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