basement floor XPS foam

Discussion in 'Remodel Forum & Blog' started by lmei007, Mar 3, 2012.

  1. dlarrivee

    dlarrivee New Member

    Messages:
    1,172
    Location:
    Canada
    Thank you Dana, our friend LLigetfa tends to spew his opinions out as fact.
  2. LLigetfa

    LLigetfa DIYer, not in the trades

    Messages:
    4,185
    Location:
    NW Ontario, Canada
    I skimmed over both docs and didn't find anything compelling but I do agree that condensation could be an issue in high humidity with carpet if there is no dehumidification. The OP stated engineered wood. The approx. R2 of DRIcore + flooring seems to be adequate for many folks that don't have much headroom and the stairs could probably be easily modified to factor for it. The OP may not mind the low ceilings, but come time to resell, others might.

    Around here, we pour our slabs over coarse stone and the air spaces somewhat reduce the conductivity to the soil if kept dry.
  3. Dana

    Dana In the trades

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    3,028
    Location:
    01609
    The OP did specify engineered wood, but the likelihood of throw-rugs/area rugs being used at some point in the future, especially on an uninsulated slab with maritime zone-damp soil with a subsoil temp of ~9C (like this poster's location) is quite high, and would a mold hazard at MA type summertime dew points. He also lives in a 30-50% higher-than-US-average-cost natural-gas & electricity market, which makes the economic rationale on the space heating end more compelling than in some other places.

    Insulated slabs aren't necessarily the best investment for every retrofit, but IIRC code-min for new construction is now R10 subslab in Maine (which is probably warmer than where you live.) If the goal is to lower the energy use profile every time the house is remodeled or upgraded, skipping the foam now makes upgrading it in future cost-prohibitive. The high-R home rule of thumb for this area is R-10/20/40/60 or sub-slab/foundation-wall/above-grade-whole-wall/attic-roof R values. (With air sealing and not-too-many & better grade windows houses built to that prescriptive standard would likely meet the Canadian R-2000 standard in most of Ontario too.) Code minimums are about half that (or less), but building only to code minimum on new housing is crap, IMHO. The comfort factor of code x 1.5 and air sealing to <2ACH/50 is enough of a payback in itself for the people living there, even if there's no margin in it on resale value. (Better to spend the money on the granite counters and the 6 sidespray marble master-bath shower if the intent is to flip it in 5 years, eh? ;-) )

    In a well designed home the cost of going code-min x 1.5 can be relatively cost-neutral using an iterative design process, using best-value materials & techniques to attain those whole-assembly R values, and applying the savings on the reduced cost of the smaller-simpler mechanicals to the R values. Clearly using 5" of 2lb foam cavity fill to raise the whole-wall R of a 2x6 studwall from R14 using fiber insulation to R17 isn't going to cut it in a present-value analysis of the utility savings any energy market. But adding 1-1.5" of 2lb foam to the exterior of a cellulose filled 2x6 wall for a whole wall R in the R20+ range would in most markets, as would taking it to the R25-R30 range with 3-4" of exterior EPS at ~$1/R/square meter in colder climates. The combined annual mortgage + utility payments for tight code-min x 1.5 homes in cold climates is usually lower than a code-min home, if some care was taken on how the whole-assembly R values were achieved. (Code min walls in MA are R20-2x6 or R13 2x4 + R5 exterior foam, either of which comes in at about R15-ish for a whole wall assembly. Foundation walls are required to be at least R15 if continuous foam, or R19 if open-cell in 2x6 studwall, again about R15-ish. Taking that to R20-25 isn't a huge stretch or huge expense.)

    Slabs are required by code in my area to be poured over coarse stone primarily for the capillary break against ground moisture, not the paltry insulation value of the air spaces although it does add about R1-R2 if you go deep enough. But the porosity of the stone and the foundation backfill can in some cases create surprisingly significant air leaks at slab penetrations, notably sumps, which may require air-tight lids for homes to meet even IRC2012 air tightness levels.
  4. mhmmofro

    mhmmofro New Member

    Messages:
    14
    Location:
    staten island
    I have no real comment on payback time etc. . .
    I am diy not a pro.

    I used 1/2 inch xps then a double layer of 1/2 ply over that for comfort and i love it. We use the basement as a tv / play room and are often sitting on the floor. It way more comfortable for both temp and softness compared to every other finished basement i have ever been in.

    I went with 1/2 because i was concerned with height issues. I am rite at 7ft. I think they say 1/2 is r3 so its not a great temperature difference but its a heck of a lot better then sitting on carpet on cement.

    mark
  5. Dana

    Dana In the trades

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    R2.5-R3 is plenty for mold-protection in a Staten Island climate, and is a HUGE improvement in comfort. Your total sub-finish stackup including the R value of the ply is about R4. You'd able to feel the difference between R3foam and R7.5 though.
  6. lmei007

    lmei007 Member

    Messages:
    179
    Location:
    Maryland
    why not use 1" XPS + one layer 1/2" ply which is the same thickness as the 1/2" XPS + two layers of 1/2" ply? I assume the "ply" means plywood.
  7. Dana

    Dana In the trades

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    Location:
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    I suspect they mhmmofro assumed more than 1/2" of plywood was necessary for the sub-floor, which it WOULD be were it spanning joists, but not if fully supported by XPS & slab. As long as 1/2" fastener penetration is sufficient mechanical retention the finish floor (usually is), 1" XPS with half-inch subfloor is fine. Half-inch plywood or OSB on an XPS/slab base flexes less than 3/4" plywood on 16" o.c. joists, so the nail/staple loosening forces are already lower.
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