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bobbotron
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# Posted: 11 Oct 2012 11:01 - Edited by: bobbotron
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So we're thinking of insulating our floor soon, I was wondering what you guys and gals thought of my plan.
The subfloor is 2x12 deep 16" on centre, so I don't think I need to aim for filling that up with insulation ($$$$). The cabin is on piers, about 1' to 1.5' off the ground. I think we're either going to use Roxul rock/mineral wool or XPS foam. Both seem like pretty neat types of insulation, and well suited for this purpose - critters don't like either, and it handles some moisture better than pink insulation does. Either way, 1/4" hardware cloth will be stapled to the bottom at the end to keep stuff out.
The subfloor is 3/4" plywood, with a good coat of oil based stain on it. Thinking of vapour barriers, I'm thinking we may not need one, as the cabin is quite open underneath, the joists are all PT, and between the plywood and stain, there isn't going to be a lot of vapour movement... I'm quite curious about this, there seems to be quite a bit of debate for floor vapour barriers on the net. If we DO do a vapour barrier, that will be the first thing to line the joists, followed by insulation.
If we do XPS, I'm planning on cutting it to fit in the joist bays, and then seal it up with tape and hold it from falling down with wood blocks? I once saw a video where someone sealed the corners with great stuff. Seems like an interesting way to really seal up the seams, but I'm not sure it's legit.
If we do rock wool, I believe it'll just be a matter of putting it into the joist bays, and then stapling the hardware cloth underneath.
We have some left over tar paper and stain. I'm considering using one or the other on the underside of the floor, but it sure would be effort to do either unless I rented a paint sprayer to blast the underside with stain! 
Edit: forgot to mention, this will be mostly for a three season cabin (with some winter trips), in southern Quebec, Canada. (Ie: going to see a real range of temperatures.)
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TheCabinCalls
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# Posted: 11 Oct 2012 12:22
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For your region your vapor barrier would be on the warm side in the winter, and your insulation would dry to the outside. If you use at least 2" of the xps you will have a vapor barrier built in.
I assume the goal to prevent heat loss in the winter. If so:
- kill any air leaks
- kill any thermal birdging
The hardest - 2x12 will serve as a thermal bridge to allow heat to transfer out of the building in the winter. You could put a 1/2" of polyiso on the bottom of your joists, tape seams and then put your hardware cloth on that. This would seal it up nice.
So you could put 4" of XPS board in the bottom of the bay and put a 1/2" polyiso with foil as your last layer before hrd cloth. Seal these up as one unit and you'd be great. You might run the xps up the sides/ends of the joists if there is a thermal bridge there.
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bobbotron
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# Posted: 11 Oct 2012 13:35
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Yeah, I hear you about thermal bridging. The whole goal of this is to make the floor somewhat warmer in the winter. I'm ok with bridging though, I don't really care that much if it's not the warmest floor, I just want to take some of the cold floor shock factor away. I was thinking of using that roxul "sheating" to cover over the joists, if I did go with roxul.
Polyiso is neat stuff, although would it on the bottom be a vapour barrier on the wrong side?
I'm sure there'll be a thermal bridge all around the outside of the building at the rim joist and bottom plates - no real plans to deal with that, if I started adding foam on the outside I'd have to start thinking of ways to deal with that siding wise. Ugh, it could get away from you, the cabin doesn't need to be toooo insulated (could be mistaken?)
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TheCabinCalls
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# Posted: 11 Oct 2012 14:00 - Edited by: TheCabinCalls
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If all of the foam is together then it doens't matter which side the vapor barrier is on. It would all be one piece so it wouldn't matter. You just want to avoid having a cavity where moisture could be trapped.
How big is the area for this floor space?
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bobbotron
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# Posted: 11 Oct 2012 14:13
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16'x22', 352 square feet.
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rayyy
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# Posted: 12 Oct 2012 17:32
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All I know is these first few cool nights of october have been just fine.9 inches of blown in fiberglass from home depot have really paid off in my floor joists.Temps at night have been dropping into the high 30,s and I wake up just toasty warm,unlike last winter with no insullation,,,Bigggg difference.It's money well spent!
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MtnDon
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# Posted: 12 Oct 2012 18:17
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The joists as thermal conductors won't be noticed by your feet. Unless you are using expensive fossil fuels you also will not notice any extra costs. The floor is usually recommended to be R25 in the colder areas. We did that and with a tile floor after one days heating in winter it is comfy in socks.
I'd do the Roxul and hardware cloth. The plywood subfloor glue acts as something of a vapor barrier. You should be fine with that and the Roxul stuffed up against it. One advantage to sheathing the underside like we did is that will keep air movement through the insulation (when the winds blow) to a minimum. The hardware cloth is good, when heavy enough and a fine enough weave, to keep rodents out, but it does nothing to stop air movement. We used 3/8 plywood as bottom sheathing and spring wire to hold the batts up in place. Most places that sell batt insulation should carry the spring wires. You need a bunch but they are cheaper than hardware cloth. Our floor bottom was covered in 2008 with the 3/9 ply and to this day there is not a trace of any rodent activity.
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bobbotron
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# Posted: 26 Dec 2012 22:01
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Hey MtnDon,
How about putting a vapour barrier between the subfloor and flooring? Then we could integrate it right with the VB from the wall.
I do like the idea of just letting the plywood be the barrier, but fear regretting my decision down the road... hrm.
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MtnDon
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# Posted: 26 Dec 2012 22:58
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VB and floors is something that does not have much documentation available. Mostly ignored unless building slab on grade. So with a raised wood floor, subfloor of plywood, Advantech or OSB you probably don't need an specific VB. If in a climate with a predominance of heating days (not the deep south) a floor membrane should be fine if you want. Not sure I'd do one in a hot 'n' humid climate though.
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DungeonX
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# Posted: 27 Dec 2012 08:17
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I have a 16x 24 camp that is built 2 feet off the ground on cribbing, I plan to sandwich some 4x8 foam boards between the actual floor and either another sub floor or some laminate flooring. saves on cost and saves the trouble of trying to work from underneathe with batting. And you can choose 1-3 inches of foam
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bobbotron
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# Posted: 27 Dec 2012 10:06
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Hey MtnDon. I'm in Canada, I wouldn't say it's predominantly heating days here. The subfloor already has a (thick) layer of stain on it, I'm guessing it's pretty vapour impermeable. As always, I should talk to the inspector here, see if he has any req's/ideas.
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