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Under cabin insulation

Hi All,

I am planning my cabin build and I want to insulate the floors. The supporting beams will sit 12" off the ground, resting on Metal Techno posts, with 2x8 floor joists over the beams (16 OC). Not only do I need to insulate correctly but I most make sure that it is done in a way that insures no rodents can get in there.

I had this in mind: a 6mil plastic sheet that will be between the joists and exposed to the soil under the cabin basically. Over this, I will place 1/2" OSB - hanging between the joists, while insulation batts are resting on top of it=between the joists. This will crate an air tight environment between the joists.

I was told that this is a bad idea as the plastic sheet will trap moisture between the joists and create a mold issue.

Any thoughts or recommendations?

Asked by cabinseeker asgood
Posted Apr 27, 2012 4:00 PM ET

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3 Answers

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1.

Cabinseeker,
The only place to put polyethylene in this situation is on the dirt, at least 12 inches away from your joists. You don't want any polyethylene in your floor assembly.

Leave out the poly, but play close attention to air sealing. More details here: How to Insulate a Cold Floor.

Answered by Martin Holladay, GBA Advisor
Posted Apr 27, 2012 7:22 PM ET
Edited Apr 27, 2012 7:23 PM ET.

2.

Thanks martin.

A few issues though -
1. If I only leave a ridgid foam sheet under the joists it will be an easy target for rodents to go in. no go
2. If I place plywood under there - unless it's pressure treated, it will rot in a few years (it will hang about 12" over the ground). I am on a low budget for this cabin and .5" PT plywood is 27 a sheet which for me will be about 450$ with tax.

I was thinking about using some chicken wire or steel mesh under a sheet of rigid foam with batts on top.

Answered by cabinseeker asgood
Posted Apr 28, 2012 9:42 AM ET

3.

Cabinseeker,
As I noted in my article -- the one I linked to -- "It’s always a good idea to install a continuous layer of rigid foam on the underside of the floor joists, especially if the joist bays are insulated with fiberglass batts or cellulose. ... The rigid foam should be protected by a layer of plywood or OSB."

Cutting corners on details will not save you money in the long run. If your grade is to high, it's time to get a shovel and start removing soil.

Answered by Martin Holladay, GBA Advisor
Posted Apr 28, 2012 11:46 AM ET

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