Replies

1. GBA Editor
   Martin Holladay | Jun 11, 2012 09:07am | #1

   Bryce,
   I'm a little confused; you say that "The slab is at grade, " but elsewhere you refer to the slab as "this cold basement floor." So which is it -- a below-grade basement slab or a slab on grade?

   In any case, I advise you to go with Option C. This assembly can be improved by including a dimple mat or Schluter Ditra between the concrete slab and the XPS foam.

   Yes, it will work -- as long as your slab doesn't have water-intrusion events.

2. 
   severaltypesofnerd | Jun 13, 2012 12:56am | #2

   Martin: the cold room is slab on grade.
   The other half of the house shares the same grade, but is foundation/cripple wall.

3. GBA Editor
   Martin Holladay | Jun 13, 2012 04:41am | #3

   Bryce,
   Go with option C. But you might consider screwing the plywood through the foam to the concrete below, instead of leaving the plywood to float.

4. 
   user-917907 | Jun 13, 2012 09:28pm | #4

   Would Option C, or a variant, work in a similar situation, except that the room had the possibility of bulk water from above, such as a kitchen, bathroom, or laundry room?

5. 
   allenloreehomes | Jun 14, 2012 12:15am | #5

   I have had good luck with a variant of option c.
   Diamond ground the existing slab to creat a good bonding layer. In your case this may not be possible due to the asbestos tile.
   Used a bostech concrete sealer vapor barier
   Covered the floor with Poly taped at all seams (I prefer the dimpled mat but the client did not)
   Installed 1/4 inch cork then laied a floating wood floor
   Additionally I installed a slab depreasureization system similar to the ones recomended for radon.
   Humidity in the house before the system was 58 to 60% after the system to remains under 48% in most cases.

6. 
   severaltypesofnerd | Jun 14, 2012 01:07am | #6

   What type of floating wood floor did you install? Our contractor is mostly worried about expansion/contraction in the larger room. We're looking at making the subfloor two layers of plywood, overlapped, but screwed only to each other. The contractor is used to tongue and grove ply.

7. 
   severaltypesofnerd | Jun 21, 2012 04:46pm | #7

   What about the edges? What sort of detail should go around the edge of the room at both the foam and plywood layers (for Option C)?

8. 
   severaltypesofnerd | Jul 01, 2012 01:25pm | #8

   Ok, we're down to doing it. Which option do you recommend:

   A) Dimple mat, multiple layers of xps or eps with taped seams, foamed at the edges.
   3/4 T&G plywood, screwed through foam to floor. This is semi-permeable.

   B) Multiple layers of xps or eps with taped seams, foamed at the edges.
   Double ply screwed to itself, and friction fitting onto the foam. This is less permeable, any moisture will rise to the bottom of the foam.

   Our slab is wetter than we thought. If I put plastic down on the nearby dirt crawl (which is at the same elevation) I get water. Drilling into the footings, it gets moist after about 4" down. The slab is always dry to the touch now, and it is dry under the asbestos tiles we have removed.

9. 
   wjrobinson | Jul 01, 2012 02:27pm | #9

   Just my take on finish floor. Carpet would feel warm. Engineered flooring works well in your situation. Open packages, open stack and leave for two weeks. I have 4000sqft stapled down that is flawless.

   Most of your floor insulation ideas should do ok. Here's another. One layer foam taped, one layer PT 3/4" plywood, taped or not, layer of 3/4 t&g plywood glue and screw, then rosen paper, then engineered flooring stapled down. Or carpet......

10. 
    severaltypesofnerd | Jul 03, 2012 02:26am | #10

    Ok, looks like we're landing at:

    Dimple Matt or Schluter Ditra depending on height, taped
    xps taped, with good foam at the edges
    3/4" T&G glued.
    Carpet underlay, carpet
    No screws

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    Anyone else have a story about a floor that did not work? And why?