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vacabin
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# Posted: 27 Sep 2014 09:10
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I have a 12x16 with 2x8 floor joist up on skids.Currently I dont have any insulation in floor but i do have a large amount of 2" 4x8 sheets of polyiso rigid foam.My question is would it be okay to install the foam underneath the cabin across the joist in full sheets and taping the seams instead of cutting it to fit between them?
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bldginsp
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# Posted: 27 Sep 2014 10:32
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What that does is to make the empty space between the joists into part of the conditioned space, which is fine, but you have to make sure you seal all the way around so cold doesn't get in there, defeating the purpose.
As well, that open space could be subject to moisture problems, as warm moist air from the cabin encounters cold surfaces in that space and condenses. You can't ventilate it to get rid of the condensation because it is conditioned space.
I'd put fiberglass under there and save the rigid for a vaulted ceiling, where it really helps. But you do have a lot of polyiso. I'd cut and fit it, and seal around the edges.
You only get R12 out of 2" rigid so wherever you use it you may want to double it up, maybe triple it up on the roof if you are where there is regular temps below 30F
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vacabin
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# Posted: 27 Sep 2014 10:59
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bldginsp could it be used across th ceilling joist I have a vaulted ceilling with soffit and roof vents.
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bldginsp
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# Posted: 27 Sep 2014 12:26
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Rigid insulation is most commonly used on top of rafters in a vaulted ceiling. But when you say you want to use them on top of ceiling joists I'm a bit confused- do you want to put them in the attic on the ceiling joists or on top of the roof deck above the rafters and rafter plywood? And I'm confused about your ventilation- you don't want to ventilate below your insulation because you will vent out the heat. I think we are just confusing terminology here- do you have a drawing? Here's a drawing of my vaulted ceiling with rigid above and ventilation above the rigid insulation:
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JDPugh
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# Posted: 27 Sep 2014 15:19
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The area above the insulation might become a nesting place for critters. Would not take much to scratch & claw through that rigid insulation to find a nice warm spot for winter.
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bldginsp
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# Posted: 27 Sep 2014 15:59
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Ridge vent is the only place they can get in, if you design it tight. There are different ridge vent screens/gauzes that keep out the critters, or so they say.
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creeky
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# Posted: 28 Sep 2014 11:38
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JD I did what you are talking about.
I put roxul between my attic joists and then went across the joists/rafters with 1.5 polyiso. gets me around r34. and seals the attic area completely.
It's actually really easy to install. I learned not to get the iso boards to tight. put them up with those plastic cap nails. and then spray foam in between the boards. I use a spray foam gun and not the single use cans.
the trick is to finish them. mine are painted and I've been meaning to put canvas over top. well. next year. I'd be interested if anybody has some other finishing options. Drywall is out for me. Looking for light and decorative. and easy to install. and ...
Now. Nesting critters. Last spring (first spring after install) I had bats. Mice. And ants. You have to seal real well. And I mean real well. After I sealed every gap at the roofline I could find with spray foam. I opened a vent for the bats at just before dusk and closed it at 2 a.m. Took a couple of days. For the mice I trapped. And for the ants. Borax is your friend (ant traps).
Nothing will dig thru the iso for long. they have borax in 'em for just that reason. if they eat it they will die. mice etc didn't seem keen to try. my experience anyway. but they didn't get months to think about it either.
For the floor: I put my insulation on top of the floor. It's a pain to have to remove and lift the doors up, but not that much work. And it works a treat. With the iso I recommend a double layer top with 1/4" el cheapo ply and then click flooring or some'at. to give you a bit more weight spreading. you could go 5/8s or 1/2" and then forget the flooring...
It does eat a bit of ceiling height but honestly I've never noticed.
It's a much easier install than trying to do between the joists or over the floor joists and it's a better install as the floor can freeze etc as one unit. Plus you avoid moisture issues completely.
good luck.
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