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Vapor Barrier Sanity Check


I'm looking for confirmation or advice about where a vapor barrier belongs.

I have a concrete slab floor. Underneath the slab is a 5' crawl space, open to the air. I'm in the northeast, so it's cold in winter and hot in summer.

I want to build a room on top. The floor will be 2x6 framing to bring the final floor level even with the rest of the house.
I'll insulate between the 2x6s, probably just batt insulation.
On top of the framing, plywood subfloor, electric heat mat buried in thin set, engineered floor pad and snap together flooring on top.

From what I understand, the vapor barrier should be on the warm side of the floor- so would the vapor barrier on the engineered floor pad be enough?

Or should I use padding without a vapor barrier and install a plastic sheet barrier between the subfloor and framing?

Again, the order of materials top to bottom is:

indoor warm air
engineered flooring
1/8 under flooring pad
thinset with embedded electric heat
3/4" AB plywood
2x6 framing with batt insulation between joists
10" concrete slab
outside air via crawlspace (northeast US climate)

Where is the vapor barrier best installed?

Thanks for any advice or suggestions!

Re: Vapor Barrier Sanity Check

If there is exposed soil in the crawlspace, that is where the vapor barrier should be installed -- on top of the soil. The crawlspace should be ventilated to the outside, and your floor should be insulated.

A fairly new concept is the "conditioned crawlspace." In this design, a vapor barrier is placed on the soil/ground of the crawlspace and a thin concrete slab (about 2") is poured on top of the vapor barrier. The foundation is sealed (no ventilation) and no insulation is placed in the floor. The crawlspace is provided with furnace supply and return vents to proved air circulation to prevent moisture buildup.

Re: Vapor Barrier Sanity Check

The "Conditioned crawlspace" is essentially just creating another conditioned "room" under the structure. I'm not convinced it is a worthwhile idea unless you have humidity or radon issues involved. Standard floor insulation techniques are quite effective at thermal isolation which is what insulation is for. In this homeowners case, I would place a vapor barrier over the concrete and under everything above that. Adding a layer of thin closed-cell foam atop the vapor barrier will go a long way to break the thermal path between the joists and the slab which will help too. So long as the perimeter of the new room is sealed there's not going to be much benefit from insulating between the joists, and if you do this anyway you change where the vapor barrier belongs (between the conditioned space and the insulation) which will call for not isolating anything from the slab underneath. You also create a lack of any air movement between the joists, which coupled with direct contact with the concrete may result in wet rot issues down the road. Concrete holds moisture even if just the relative air humidity, so it needs to be where the isolation (vapor barrier) goes. Thus you then have to treat the areas on the conditioned side of it accordingly, either by not insulating or by using a closed-cell foam which cannot hold moisture.


Re: Vapor Barrier Sanity Check

Ah- I see. So vapor barrier over the concrete and framing over that. And I can lay rigid foamboard in between the joists for insulation. I was thinking of using rocksil- that sounds like definitely not the way to go. Ideal is probably spray foam, but that's expensive and not DIY friendly. I can take the time to cut rigid boards to fit in between the joists.

Please let me know if I have this right- I really don't want to mess this up and I think I understand you (but maybe I missed something).


P.S. The space underneath doesn't have special radon or moisture issues, but I store my mower and snowblower there... it's kind of a shed space. I don't really want to do anything in there if I don't need to...

Re: Vapor Barrier Sanity Check

You've got it right. Concrete needs to be isolated from all non-treated wood, and even with treated, concrete will wick moisture so direct contact is not ideal. The vapor barrier solves this problem.


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