Carpeting over a cold slab: vapor / xps / double plywood
Background: the house was built in 1938 with slab poured between pre-existing foundation walls in 1954, and topped with vinyl asbestos tile. The slab does seem to stay dry all year, but nearby crawl space dirt is always a bit dark and moist. The slab is at grade. Quake contractors reported wetness at the bottom of each drill hole in the foundation nearby. Termites have been active through the slab. Radon is not a problem.
Climate: San Francisco Bay Area fog zone. It is a heating climate with chilly days possible all year, and only a few hot days ever.
We want to “warm up” this cold basement floor, turning it into a bedroom. We are fully aware there is no really “good” option.
We have about 2.5″ of space to play with:
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Option A:
Poly vapor barrier, wood sleepers screwed into the concrete, plywood above, then carpet underlay and carpet. Rigid board filling the space between the wood sleepers.
Option B:
Epoxy top coat over the existing tile, cork on top of that, then permeable industrial carpet.
Option C:
XPS/EPS board on the slab with taped seams, two staggered layers of plywood floating on top. Then underlay and carpet.
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All the options have some thermal bridging via the foundations, which rise zero to eight inches above the slab. The room is about 10′ x 10′. A second similar room is 8′ x 15′.
We’ve been reading up on the topic, for example:
https://www.greenbuildingadvisor.com/community/forum/gba-pro-help/22675/floating-plywood-floor-rigid-foam-insulation-concrete
https://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/24213/floor-over-insulated-concrete-slab-zone-3
http://www.finehomebuilding.com/how-to/articles/no-mold-finished-basement.aspx?ac=ts&ra=fp
http://www.greenbuildingtalk.com/Forums/tabid/53/aff/21/aft/80185/afv/topic/Default.aspx
http://www.dyplastproducts.com/Customer_Bulletins/CUSTOMER_BULLETIN_0710.pdf (on greenhouse gases and XPS)
But would love to hook up with actual examples of the techniques in action. Do they really work in real life?
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Replies
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.
Martin: the cold room is slab on grade.
The other half of the house shares the same grade, but is foundation/cripple wall.
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.
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?
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.
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.
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)?
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.
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......
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?