I'm in the process of replacing the insulation and horrible looking floor tiling in my basement, which, along with the rest of the house, is in Southeast Wisconsin. Walls and floor are all poured concrete from when the house was built in the early 50's. The house is on a rise and there are no moisture issues (taped plastic tested) at this time, though I'm operating under the assumption that they could develop in the future and due to the insulation I wouldn't know about it for a while.
For the walls I'm gluing & nailing 1x2 PT furring strips to the wall (to create a flat gluing surface) and gluing 1" Poly-Iso panels to them with reflective foil facing to the 3/4" air gap created by the strips. I will then be using layered poly-iso (trimmings from the wall panels) to insulate the sill plates with about 6" of R6.5 for a total of R39 at the sill plates. I'll air & vapor seal the perimeter of the whole poly-iso structure with great stuff and foil-faced tape. I've already caulked the sill-plate/concrete joint to ensure a flexible seal that will handle changes in the sill plate.
I'll then build a standard 2x4 wall (all concrete-contacting wood is PT, furring strips, 2x4s, etc.) inside of the poly-iso paneling and may insulate that with fiberglass batting (no vapor barrier) to increase R-value even more. I'll finish with a 4-ft panel of something moisture/mold resistant/proof leading to drywall for the upper part of the wall.
I'm trying to hit the three types of thermal transfer: no convection due to air sealing, little radiation due to foil facing and air gap, and low conduction again due to thermal transfer needing to take place through the furring strips (or multi-layer insulation at the sill). The furring strips also keep the foil facing intact and avoids water transfer to the foam if that currently non-existent problem should develop.
For the floor, I'm using 1x6 PT furring strips glued & nailed to the floor at 16oc, then a radiant barrier (as one might use in an attic in the south) then 1/2" OSB subfloor and a thin plywood underlayment. Part of the basement area is going to be a workshop for me (when my barn is too hot or cold) and the floor will need to support heavy loads (such as a workbench on castors) without deforming, hence the wide 1x6 sleepers.
What brings me to my question is the latest issue of Fine Homebuilding, where they redid the basement in an old cape cod. They put down a layer of XPS directly on the concrete and then the subfloor directly on that. It didn't look like they used solid sleepers at all.
So my question is whether it makes sense to use 1/2" of XPS under my subfloor or leave an air gap? As convective heat rises I don't see a need for an air barrier (and want the floor and basement to dry to the rest of the house), I thought the air gap would minimize conduction (though the 1x6's are wide) and the radiant barrier would limit downward radiation of heat from the floor to the concrete. I thought conduction would be the largest threat for heat loss through a basement floor so putting a whopping R3 of 1/2" XPS down might do more harm than good, especially if I wanted to keep the sleepers for load bearing ability. I'm also not sure about eliminating the airspace below the first water-sensitive part of my build, the OSB. I thought having a 3/4" gap below the OSB would allow for some drying of moisture through the OSB while keeping it out of direct contact with untreated wood.
1. Am I overthinking all this? (Says the guy who just wrote 7 paragraphs about the thermal properties of his design)
2. Should I reconsider putting R3 insulation below my subfloor?
3. Would a full layer of R3 XPS have the same or better load-bearing capacity as 1x6 sleepers at 16"oc?
4. Is having air space (if not circulation) below a permeable subfloor helpful in a situation where moisture could accumulate?
5. Is the use of a radiant barrier layer below the OSB and above the floor airgap/sleepers just silly?
Thanks!
Tom |