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steve
Member
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# Posted: 19 Sep 2011 12:47pm
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Hello all, I've finally begun my 12 by 14 cabin! A little miffed about insulating my floor. My joists are 2 by 6. I am going to have PT plywood facing ground, 2 by 4 Roxul batt insulation then a subfloor. My question is do each floor joist have to be vented to prevent moisture? Could I use rafter vents on top of PT ply then lay insulation on top. Either drill holes in each end of floor joist space (16") or drill holes underneath at each end. Of course metal critter proofing. Also would i need a vapour barrier on bottom of subfloor/on top of insulation? Any advice would be greatly appreciated! Oh and the winters are cold!
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trollbridge
Member
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# Posted: 19 Sep 2011 01:06pm
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Hi Steve,
We are in a very cold climate as well and we did not vent or vapor barrier the floor when we insulated. We are up off the ground on post and piers though...not sure what you are planning.
I'm curious what others have done,but I don't think you need to worry about it-venting seems like it would weaken the vermin proofing!
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steve
Member
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# Posted: 19 Sep 2011 05:57pm
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The cabin floor will be approx 1' off of ground level- was wondering if I could use 1/8 inch hardware cloth instead of PT ply as the 2x6 joist will be PT and Roxul can handle the moisture- or should I just sandwich the floor without ventilation? Hard topic to find a definitive answer on the internet!
Thanks for your quick response!
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trollbridge
Member
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# Posted: 20 Sep 2011 10:38am
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Don't know what to tell you. What we did seems to be fine. We are a little higher off the ground, but not much. I know there are other members who have also insulated their floors so someone should have a better answer for you than me. When do you plan on starting your cabin?
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Jerry
Member
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# Posted: 20 Sep 2011 12:06pm
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In general I always try to vent any space that isn't heated assuming that any interior heat reaching that area has the potential to create moisture. But because heat rises I don't think you lose too much interior heat going down and the main reason for the insulation is to keep the cold outside air out, keeping the floor warm. So if you can vent the floor do it - may provide some peace of mind, but keep the moisture barrier toward the heat area and you'll be alright either way. Just my two cents.
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spaceman
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# Posted: 20 Sep 2011 03:50pm
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thanks for the confidence, think i'll vent each joist space, now wondering if i should fill spaces with insulation (2x6) or go 2x4 batts against subfloor and have opn space on bottom from end to end
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toyota_mdt_tech
Member
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# Posted: 20 Sep 2011 10:02pm - Edited by: toyota_mdt_tech
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Steve, I suspect you will be fine. Just a good vapor barrier between the treated lumber and your insultation. Then lay down another vapor barrier right on the ground. The batts themselves are basically a vapor barrier now, so maybe just on the ground is all you will need. They have R21 that can fit between the 2X6. I'd give yourself more room than 12", I think code is minimum 18" in case you have to get under there.
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steve
Member
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# Posted: 29 Sep 2011 07:05am
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thanks toyota, should be easy to raise another 6-8 inches
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TomChum
Member
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# Posted: 2 Oct 2011 11:56pm - Edited by: TomChum
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The vapor barrier should be ontop, at the warmest location or "toward the heated space". Above the insulation, just below the subfloor. Your vapor barrier should be warm on one side and insulated on the other side.
If you have an enclosed foundation, it would be good to put a vapor barrier (plastic) on the ground ALSO. But if you're just on posts, with air blowing under the cabin, there is no point to putting plastic on the ground.
You can use hardware cloth instead of plywood. It will cost less and do the function that you need and won't trap moisture.
Here's how I did mine. http://www.small-cabin.com/forum/6_770_1.html#msg12600 I can push the boards up into the batts, and take them out if I need to get in there. I dids not add vents, but it's not 'tight' either. My cabin's pretty low, at the higher corner - only 1 foot. Like TMT suggests 18" would be better.
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beachman
Member
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# Posted: 9 Oct 2011 08:13pm
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You raise some interesting questions. I have to do the same thing as I was in a rush to get the floor up. My builder said that spray foam between the joists would be fine - but expensive at about $3 per sf. What about painting the plywood with sealer paint before putting in the styrofoam batts? A lot of work but maybe worth it.
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steve
Member
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# Posted: 10 Oct 2011 07:05am
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styrofoam is too expensive for me, spray is def out of the question! The plywood will be pressure treated so it will be ok as is. Id like to use 2 by 8's, vent each joist on either end, use soffit syrofoam vents on top of pt ply then attic insulation on top pressed firmly against vapour barrier which is on underside of subfloor.
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rlsm16118
Member
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# Posted: 10 Oct 2011 09:30pm
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the insulation companies all have told me to think like the floor is an outside wall and finish accordingly, vapor barrier towards the inside, insulation, house wrap like it is and outside wall and then pressure treated ply wood or soffit type ply wood if desired.
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