Finishing basement: Floor and Walls

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JF.

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Hello, everyone!

I am about to purchase a home in northwest Indiana and will be finishing ~90% of the basement. This particular basement does not have a sump pump installed, but apparently has very little issues with water due to the property elevation. (There was a large flood about a decade ago and this home did not suffer any water damage) There is, however, a bit of efflorescence on the walls, likely caused by air infiltration or slight water seepage over the last few decades.

The house is a brick exterior home built in the early 1960's. The current owners installed a finished bathroom in the basement utilizing non resistant wood for framing. The remainder of the basement to be finished is approximately 1100 square feet, concrete floor.

My plan is to install steel studs for the perimeter walls and in two sections, one to partition off a bedroom area and one to separate the utility room, which will remain unfinished.

My main question (finally) is how to go about creating a subfloor? After reading up on many, many methodologies I think I've settled on placing a 6 mil polyethalene vapor barrier on the concrete, followed by an inch of XPS foam. On top of the foam (I believe) it is standard practice to install a half inch layer of OSB. I will finally finish the project with the installation of an engineered hardwood floor. I am unsure as to where any of these layers should be anchored to the floor (or if they should all be left floating).

My larger concern in this project that I have found no solid information on are that I wish to use a layer of thin cork (maybe 0.25 inch?) to muffle sounds and provide more heat retention. If I were to add this cork in, should it be located below the hardwood, or potentially between the XPS foam and the OSB? (My thoughts were that since cork resists moisture and temperature changes that installation should probably be between the XPS and OSB to provide a solid temperature barrier preventing water condensation from causing issues with either the OSB or the engineered hardwood, but this would likely not muffle as much sound) Or is utilizing cork in the subfloor just the pipe dreams of a madman obsessed with providing the best quality final product?

Also, will not sealing underneath this already finished and tiled bathroom cause any issues with water infiltration at that point? (If water becomes trapped under the 6 mil poly barrier and can only escape back into the concrete or by coming up through or adjacent to the bathroom floor. I do not know if there was a barrier installed at that location.)

And finally, since I am obviously insanely concerned (/ focused (/ obsessive)) with water intrusion would it be a complete waste of time to seal the OSB panels before installation or would that potentially help? (Or would I need to worry about the sealant reacting with the XPS?)

Thank you all for your time in reading this short novel. Thank you more for helping me out!
 

Dana

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For the record, efflorescence is never from air- infiltration, only from water seepage. It is the re-deposition of dissolved minerals in the water.

It's slightly better to put the polyethylene sheeting between the OSB and foam rather than the concrete & foam. This keeps any spilled water on the floor that manages to get in there from pooling under the foam, which would then take about 2x as long to dry, which has mold & cosmetic appearance repercussions for the finish floor.

Using EPS (blown with pentane) is much more more environmentally benign than using XPS (blown with HFC134a). Both are extremely moisture tolerant. In 50 years when the XPS has given up most of it's damaging blowing agent it's R value will be the same as EPS of equivalent density anyway.

Put the cork above the foam & poly, which will keep the more susceptible OSB warner & drier. Use t & g OSB, and Tapcon it to the slab at least 24" o.c. or tighter. Make sure that the OSB seams overlap those of the foam by a foot or so.

Engineered hardwood is fine- just be sure that anything above the OSB layer is at least semi vapor-permeable, to avoid trapping moisture in the OSB.

Running the floor polyethylene 8-12 up the foundation wall and sealing it to the foundation with duct-mastic is a good idea.

Steel studs are fine for the partition wall. Insulating the foundation walls & exterior walls is important though, and that should NOT be merely a studwall w/batts, and definitely NOT STEEL studs for any insulated studwall, due to the severe thermal bridging of the studs. In any IL location 1.5" of EPS trapped against the foundation with a non-structural wood studwall w/ R13-R15 UNFACED batts has sufficient vapor retardency to control groundwater migration into the studwall cavites, and sufficient foam-R for wintertime dew-point control at the foam/fiber interface. In Southern IL you could get away with 1" EPS.

Don't sweat the details of the already finished bathroom- just make sure that the elevation of the poly sheeting on the adjacent new floor is above the bathroom floor (do the best you can) to keep any leakage in the bathroom from chronically wetting the edge of the subfloor or finish floor.
 

M Bej

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Basement Subfloor Question -- With existing finished walls

If I may, I have a somewhat different circumstance about which I would like to ask, Dana in particular, if willing to advise.

The background:
Location - northern Ohio ... soil is ~3' of clay, then ~2-3' sandy clay, then sandstone (based on geothermal well data).
River 120' behind house, about 30' below basement floor; floodplains are 1/2 mile downriver. No realistic chance of flood.
French drain + drain tile, courtesy of yours truly, around 2 sides of the house, thinking of extending this to 2 1/2 sides of the house.

Previous owner finished the basement. Outside walls were dug out and waterproofed, Rub-R-Wall was the brand/company/contractor, if memory serves. Insulation of approx 3/4" on the outside, though without flashing (have to get that put in). Backfill material, where I've seen it, seems acceptable, but landscaping overtop was done with rather poor, very clay-y "topsoil".

Previous owner then contracted with Big Orange Boxstore to finish the basement. Carpet on pad directly on concrete; walls of untreated 2x4s, standard gypsum board, no greenboard, Drywall ceilings as an added feature. :-/

Hurricane Sandy happened. Circumferential winds whipped across Lake Erie, the latter as in previous years still devoid of trees, knocked out power for 8 hours, resulting in 2" of water in basement. Contracted to get basement dried out properly, walls anti-mold treated, etc. Carpet and pad now gone. Sump pump removed, sump crock filled with concrete ... because I had the sump drain excavated out, given that I have the aforementioned drop that allows my footer drain to relieve itself by gravity without relying on electricity. Basement floor has been entirely dry since.

Planning on putting in a floor according to the recommendations already well discussed in several posts, on 1" XPS etc. etc. etc.

FINALLY we can get to my question - what, if anything, should I do about the untreated 2x4s and regular gypsum board? Leave it alone? Cut out the lower 4' of gypsum board and replace with greenboard? What about the studs and sill resting on the concrete floor? Leave as is? Cut out the existing studs (one at a time) and replace the lower 4' or so with a "knee wall" built from treated lumber or steel studs? If I should really replace the wall in some way, then how should I run the 6 mil plastic - up the wall for the 4' distance, or less, or not at all (I gather not, given that the wall is insulated on the outside)? Should the studs be insulated (and thus set off) from the wall? What if the current ones are not? (Ideally, of course, I'd rip out the walls, too, and just start over ... but that is a bit impractical.)

Many thanks in advance for your thoughts.
 

Dana

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Ideally you'd be able to put the 1" foam and subfloor under the bottom plate of the 2x4 studwall. If the exterior is well drained and the wall is waterproofed you won't be getting much moisture into the studs, but the bottom plate is still susceptible to ground moisture wicking up through the footing & slab.

With only 3/4" of foam on the exterior (I assume that was XPS, at about R3.8) you'll have some wintertime condensation potential at the fiber/concrete interface at the wall if you insulate the studwall to more than R11 in your location, but it's worth insulating it nonetheless. For new construction R15 continuous insulation or R19 between studs would be IRC 2012 code-min for basement walls in your US zone 5 climate. With R11s between studs the studwall's performance comes in at about R8.5 after factoring in the thermal bridging of the framing (not counting the R-value of the wallboard or concrete), to which you add the R3.8 continuous foam and you're at R12+. If you installed R13s that would be just about R13.

If you want to pop the studwall out and re-use the lumber you could put a cheap inch of EPS on the walls (R4 -ish) and install R15 in the stud cavities. In some ways rebuilding the studwall is the right thing to do, since you can install the wall foam then run the floor-foam & subfloor under the bottom plate of the studwall. You can also put the studs 24" o.c. for a lower framing fraction.

If you left the studs uninsulated the thermal performance would be very poor, and you run a slight risk of mold inside the wall cavities unless you dehumidify to some low relative humidity. With an inch of foam (any type) between the stud edge and concrete even the cold edge of the studs remains above the summertime dew point temp. If space is at a premium, 1/2" polyiso instead of 1" foam would give you another R3, which in combination with the exterior foam would be sufficient for wintertime dew point control even with R15 cavity fill (rock wool R15s recommended), and you'd be at current code-min performance or slightly better.
 
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