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I have an old home built on a stone foundation. The stone foundation is solid, as well as the floor joists. I run a dehumidifier in the basement full time, to keep the air moisture low. There is never any water in the basement. The floor is dirt with a covering of gravel. There is currently some fiberglass insulation between the floor joists (installed incorrectly, paper backing facing down), and a foam insulating board nailed to the bottom of the floor joists.

I was advised to flip the fiberglass over, so that the paper backing is facing the living space, instead of facing the basement. If I am going to do that, should I do anything else with the fiberglass at the same time? Should I put plastic sheeting in? Should I wrap each length of fiberglass in plastic, tape it shut, making a bag of insulation in each floor joist void?

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3 Answers

up vote 2 down vote accepted

Buildingscience.com has a decent article that covers a very similar situation with floors over crawlspaces

The two key points with this:

  • One vapour barrier, so as not to trap moisture. Essentially, drying happens on both sides going in the direction away from the vapour barrier, and if you use two barriers you're trapping moisture which leads to mold and rot.
  • Condensation happens when warm meets cold. When you're using batt insulation, you want the vapour barrier on the warm side so you're not getting condensation (and thus water) on the insulation.

The situation that most closely matches what you're talking about recommends this:

enter image description here

The rigid foam on the bottom of the joists is your vapor barrier: it must be impermeable and this means that the connections between sheets and around the edges must be sealed too. Even though the vapour barrier is essentially on the outside of the batt insulation, because the foam has an insulating value (unlike 6mil plastic) you don't have warm meeting cold here and won't get the condensation problem.

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I think a distinction needs to be made between flooring On grade (over a slab) and flooring over a basement. On grade needs the vapor impermeable against the slab. Against the wall or for overhead floor, foam but not impermeable foam(the foil backed is impermeable) should be used . XPS foam has SOME permability and allows drying in both directions –  HerrBag Apr 3 '13 at 0:24
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You do want the insulation backing facing up, or removed if you have a good vapor barrier with the foam board and/or flooring. Do not seal the bottom side of the insulation with a vapor barrier since that will trap moisture which can lead to mold and mildew. What you can do is use a house wrap material (e.g. Tyvek) to reduce drafts and any dust/dirt from getting into the insulation. A house wrap, unlike a vapor barrier, allows moisture vapor to pass through.

The risk with the vapor barrier on the non-heated side is that during the winter, warm moist air from your home could travel through the insulation to the cold side where it condenses.

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The latest advice from Building Sciences is for poly against floor, XPS foam against that and then build up from there.

If you are not having moisture problems, your sandwich meets the best thinking of "allow drying in both directions". I would not do any encasing with poly.

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