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Old 04-06-2014, 10:38 PM   #1
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Vapor Barrier For Floor Over Crawl Space

Hey people-

I am in a bit of a quandary over whether a particular type of construction in my house requires a vapor barrier. Here's the sitch.

Zillions of years ago (1911), someone built this house that I now live in. From what I can tell, the home was originally built as a two-flat with the upstairs and downstairs floor plans identical. (from what I can tell), Each floor had a porch which was open to the outside, both of which were later enclosed to add additional floor space to the interior of each flat (again, my assumption).

We (I) are remodeling the downstairs and the space (a "bump-out") that used to be this porch will now become our mudroom. The existing floor framing was poorly attached to the walls (this is a balloon-framed house) and installed originally to create a significant pitch toward the front of the porch (about 1 inch in 7 feet). This floor was built over an unvented crawl space/dirt.

I removed all of the existing floor framing and installed 2x10 ledgers and floor joists (plumb and level, something I rarely find in this house). I also installed a 2x4 lattice under the joists and layed down 2.5" thick Owens Corning Foamular insulating board over it. All perimeter and butt joints were beveled to approximately 1/2" and filled with closed cell insulation. The reason for adding this insulation below the new floor joists was due to the fact that there are 2-2" insulated mini-ducts (Unico) which serve this room and I wanted to "lower" the conditioned space (the mini-ducts run above the OC insulation board). In addition to the insulation board, I went ahead and installed fiberglass insulation above it and between the new floor joists.

The big question here is, do I need a vapor barrier between the joists and the new subfloor (I will be laying it down in about a week). My initial thought is that I've tried to create an air-tight, insulated barrier between the unvented crawl space and the new room and thus I don't need one. If I install a vapor barrier above the joists (and the insulation below), I am concerned that I will trap any moisture that might get in and possibly rot the new joists.

I am confused. If anyone might be able to help with this, I'd love to hear your advice!



Well, now, there's what's right and what's right and never the twain shall meet.

Last edited by BigJimmy; 04-06-2014 at 10:41 PM. Reason: Fat-finger spelling
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Old 04-06-2014, 11:53 PM   #2
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put the vapor barrior right on the dirt in the crawl space. 10 mil min.


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Old 04-07-2014, 07:27 AM   #3
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Call Orkin now and then keep their number handy for yearly dirt treatment.
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Old 04-07-2014, 08:00 PM   #4
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It is imperative that the foam board is faced, either poly or foil, fig.7;

Foil-faced, exposed, will decouple the wood from earth temps, poly will not. As you used fiberglass in the cavity, it will settle (a good thing for warm toes) and create an air-gap;

Cover the earth;

Ventilate the crawl to meet code, also.

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