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jimmer
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# Posted: 2 May 2012 16:37
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I'm planning on building an insulated floor as follows:
- 2x8 frame (8'x12') and joists, 16"OC covered with 5/8" PT plywood
- water resistant insulation stuffed between the joists
- vapor barrier stapled to the bottom of the joists and frame
- wire mesh (hardware cloth) stapled to the vapor barrier
- all this sitting on top of concrete blocks
I'm looking at some R14 Roxul insulation to stuff between the joists, it costs $35/bag designed for 2x4 walls.
Because my frame is 2x8, not 2x4, I figure I could double-up the insulation , but I don't really think temperatures will dictate that where I'm building (on the ocean on the west coast of Vancouver Island, which rarely dips below freezing).
If I only use 2x4 insulation, there will be a 4" air gap between the insulation and the plywood. Is that too much of an air gap? Will the R-value be significantly decreased? Do I need to fill that gap with another layer??
thanks
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Anonymous
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# Posted: 2 May 2012 17:38
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Jimmer,I also used 2 x 8 on my floor. I did use the Roxul but used the 8". It works nicely and it is pretty waterproof. The only thing I did diferent is use CDX in 1/2" under the joist to keep out pest but also to stop the wind. Pretty easy to do. Just lay the cdx down before you put the joist on the concretee blocks. After the rim joist is attached then go under and screw the plywood to them. Insulate from the top.
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tbjohn
Member
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# Posted: 2 May 2012 17:40
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By the way Jimmer, I had forgot to sign in.That is my post above.
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jimmer
Member
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# Posted: 2 May 2012 19:06
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Thanks tbjohn...
I didn't see any 8" Roxul at my building shop. I'll look around town, but it looks like I'll be using the 4" stuff... which still makes me wonder if that air gap is bad if I only use one layer?
What is CDX? A type of ply? Is it pressure-treated? Are you concerned with water damage?
Originally I was going to cover the bottom of the floor frame with plywood to keep out critters, but for the money I can't bring myself to do it... Instead I'm going with the wire mesh on top of the vapor barrier. Hopefully that's sufficient!
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trollbridge
Member
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# Posted: 2 May 2012 19:19
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Quoting: jimmer Originally I was going to cover the bottom of the floor frame with plywood to keep out critters
That is what we did and then used the "pour and rake" type insulation which was pretty cheap and easy to use. The bottom of the floor to our cabin sits up about 30 inches from the ground.
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tbjohn
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# Posted: 2 May 2012 19:48
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I used the 8" because of the water resistance. We are on a stream and it flooded 2 years ago. jacked it up about 30" but not taking a chance. We got the 8" Roxul at Lowe's. Someone orded it in and did not take it wo we offered them $30 per bundle and got it. Cdx is an exterior plywood but i would not leave it out to the rain for long. It is good for shiething.
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MtnDon
Member
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# Posted: 2 May 2012 21:41
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If you leave an air space in the floor the air space should be on the bottom. The insulation should be faced up against the underside of the floor. If the air gap is above the insulation that will set up convection currents in the air space. I've never used Roxul do don't know if it can be held up with the same spring steel wires, sort of like bike spokes, as are used to hold fiberglass in place. They are jammed in between the joists after the batts are pushed up into place.
Hardware cloth that is made from wire large enough to discourage larger rodents than mice can be pricey. When I checked 3/8 CDX plywood was a lot cheaper.
CDX has a class C face on one side and a class D on the other. Class D will show voids. X is for exterior, made with waterproof glue. It's not supposed to delaminate. Mostly it does work as expected.
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