Subfloor in basement which is correct

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Dana

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2.5-3lb density XPS is pretty vapor-impermeable- sufficiently lo-perm that there's little point to adding 6mil poly from a moisture management point of view.

But if you wanted the continuous sheet of 6 mil poly for air sealing purposes (say, if you have a bunch of cracks in the slab), put it between the Barricade and concrete.
 

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Thanks Dana.I believe I will go Barricade-Quiet walk(taped seams?)-12mm Laminate.Should give me in the neighborhood of R4
 

Dana

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There's a long term financial rationale for up to R10 or more of slab insulation in a WI climate, but R4 would be enough to limit the mold potential under the laminate flooring.

For the cost rationale discussion see chapter 1, particularly Table 2 p.10 of this document.


Most WI is zone 6 (blue), northern WI is zone 7 (purple):


wisconsin.png


Barricade is more expensive per unit R than typical sub-slab 1.5 lb. EPS or 1.5lb. XPS solutions, so maybe R10 wouldn't have as much a rationale, but R6-ish probably still does.
 

blackprohet

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Thanks Dana, I wholeheartedly agree with the wisdom of putting down more insulation and all the benefits that go with it-its not so much the budget that is limited, but headroom 7' and transitions into 2 other areas which will not be getting subfloors.1.75" (barricade 1.125"+.125" for underlayment+.5" for flooring)is about max I will be able to make work.Would love to get to R6 with insulation,subfloor and flooring but not sure I can get there from here.(That is why I was hoping to put laminate directly on top of xps 60psi foam and skip the OSB altogether)
 
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Basement Subfloor - Variant of above

If I may, I have a somewhat different circumstance about which I would like to ask, in particular Dana, 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. Backfill material, where I've seen it, seems acceptable, but landscaping overtop was 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 benefit. :-/

Hurricane Sandy happened. Circumferential winds whipped across Lake Erie, the latter devoid of trees, knocking out power for 8 hours, resulting in 2" of water in basement. Contracted to get basement dried out properly, walls 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 me to drain my footer 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 above, 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? 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? Should the studs be insulated (and thus set off) from the wall?

Many thanks in advance for your thoughts.
 

Dana

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I'm sorry I didn't see this sooner...

Poly sheeting won't help, it would only hurt, since the ground water that wicks up the concrete wouldn't dry to the interior, raising the moisture level in the concrete at the foundation sill.

If rebuilding the wall, re-using the untreated studs is fine, but it's good to have an inch or more of EPS between any wood and the concrete, as both a thermal & capillary break. If you don't want to give up any interior space you can put 2" of continuous EPS against the concrete (tape & mastic-seal the seams), and rotate the studs, mounting the wallboard to the 3.5" face of the studs, and install compressed split fiberglass or rock wool batts in the 1.5" deep cavity between the studs, starting about 2' above the high-tide mark. Alternatively you could install 2.5"-3" of EPS held in place with 1x4 strapping through-screwed to the foundation, skip the fiber. XPS would also be OK (though it has a much heavier environmental footprint than EPS), but don't use polyiso below the high tide mark in a basement with a flooding history.

Greenboard isn't going to have any more flood resilience than standard grades of wallboard, though it's somewhat less mold-prone. A more robust material to use would be exterior-grade fiberglass reinforce fiberglass faced reinforced gypsum board such as Georgia Pacific DensGlass, or cementicious tile-backer board. In fact, it might be better to just use faux-stucco type or other smooth/textured pre-finished fiber-cement siding as a wainscoating for the lower half of the finish wall, since any interior-grade paint on any type of gypsum board would fail in a flood.

Don't bother with a wooden subfloor or water susceptible flooring. If you want to insulate the slab (a good idea in your climate, since it lowers the summertime mold potential of anything that rests on the floor), glue 1-2" of EPS/XPS to the slab (run it all the way to the wall-foam), covered by 6mil polysheeting, then use tile-backer board Tapconned to the slab through the foam, and use ceramic tiles for the finish flooring. Even an inch of foam under the tile keeps the tile much closer to the average room temp in summer, which will be well above the average dew point of the outdoor air, so even a throw-rug or cardboard box on top of the tile would have fairly low risk of developing mold.
 

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Great thread! Like most that posted here, I do have some questions based on my specific situation.

I live in NJ (cold winters and hot/humid summers) and I am looking to finish part of my basement, which will be roughly 300sqft. The non-finished area (furnace, water heater, etc.) is 100 sqft. Currently, my basement has French drains leading to a sump pump. Along the bottom of the foundation, just above the french drain gap are weep holes (just a few.....3-4). The previous owner had dri-lock the foundation walls and used some kind of exterior paint on the slab. In terms of the condition of the foundation and slab, I do not have any cracks. However, on the foundation, particularly on the corners (where drain spouts are located) I do have some seepage when there is a ton of rain or when snow is melting. This seepage has led so some mold development along the corners. I think if the dri-lock wasn't used, the mold may not be present but anyway, I need to clean it up and I am planning to use Ames Blue Max Liquid Rubber about 3-4 ft on the wall, and the rest I would use Dri-Lock. This will hopefully take care of seepage while I still plan to move the drains further away from foundation.

In terms of insulation and finishing the room this is what I plan to do:
1 or 2" XPS foam against the foundation walls with about 1-2" gap at the bottom near the french drain.
Studs with XPS capillary break, batt insulation and dry wall up against the 1-2" XPS foam on the foundation.
For the slab, I was looking at DriCor + which is XPS foam with OSB on top. This way i have insulation and a floor that I can tile and or carpet.

I don't have a water problem unless the sump pump gives out during the storm (looking at options to help prevent this) but I am concerned if the water heater or even washing machine breaks which can lead to a water issue. Finally, the moisture vapor is a concern so I am hoping my plan above will be sufficient to address this

Any thoughts or concerns with the above? I look forward to recommendations.
 
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Dana

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The proposed approach will work, but a few comments:

DriCore works, but it's probably more expensive than 4x8 sheets of foam w/4x8 sheets of Huber AdvanTech subfloor TapConned to the slab through the foam. The higher compressive strength of XPS is irrelevant in this application, since you're not supporting any significant weight, and full subfloor thickness will distribute the dynamic weight of an entire clan of Irish step-dancers having a family beer party without affecting even crummy 10psi Type-I EPS, as long as you stagger the seams of the foam with that of the subfloor.

XPS is blown with a mixture of HFCs, dominated by HFC134a which has a global warming potential (GWP) about 1400x that of CO2. And it slowly leaks out over a few decades, and as it does the R-value shrinks. In 50 years the R-value of XPS will be about the same as EPS of equal density & thickness.

EPS is blown with pentane, with a GWP about 7x CO2. It leaks out very quickly, and does not contributed to the rated R, which is stable over time.

Both XPS & EPS are very tolerant of contact with liquid moisture. EPS is usually 20-25% cheaper per unit-R. Unless you need the lower vapor retardency of XPS, using EPS would have a lower overall environmental impact.

The GWP of XPS will be lower once they start blowing it with the HFO1234xx variants, which may happen in a couple of years. The stuff would still leak out over time, but the GWP of the new blowing agents is sub-4x CO2. SFAIK no US manufacture is using it for XPS yet, but they are replacing HFC134a in other applications (notably automotive air conditioning).

In Europe XPS is blown with CO2, (with a GWP of only 1x CO2- go figure! :) ) but those goods start life at the fully depleted R4.2/inch that the HFC/HFO blown goods will eventually decay to. They have lower vapor permeance than EPS of similar density though. ( The higher vapor permeance of EPS is an artifact of the interstitial spacing between the macroscopic bead structures.)
 

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Thanks Dana. Very informative :)

A couple more questions for you:

I was planning to frame out my sump pump area as well as my main sewer line clean out with an access door. In terms of the sub-floor, should this area be left alone or do you recommend adding the sub-floor as much as possible without interfering my access points

In some previous threads you had mentioned using a 6mil poly vapor barrier. Based on my "fancy" diagram below, should i use the poly between the XPS and OSB on the floor and then under the XPS on the foundation wall (up a few inches)....reason I ask is I am leaving a gap where the foundation wall and slab meet because I have a french drain along with some weep holes. I am worried that moisture from the french drain can lead to mold growth on my wall framing. I have attached two options I am thinking about to address this issue. The first one has my wall framing on the subfloor with a gap near the french drain. My second diagram has my wall framing on a section of XPS which i can have it butt up against the edge of the foundation wall XPS (thus eliminating my gap). Which option would you recommend?
My Basement option.jpg

option2.jpg
 
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Dana

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Since your goal is to protect the OSB from moisture, it's somewhat better to put the poly vapor barrier between the foam & sub floor. If it's below the foam there is a greater chance of liquid water finding it's way along seams of foam and reaching the moisture-susceptible OSB. The foam itself is relatively impervious to water.

If you can bring the poly up 6-12" on the interior side of your wall foam and seal it in place with caulk or mastic so much the better!

Bringing the sub-floor under the bottom plate of the studwall more like your top picture makes more sense than leaving the gap. But don't bring the subfloor any closer to the foundation wall than the studwall plate. (Your picture shows it an inch or so beyond the bottom plate.) Since the studwall isn't structural you don't have to even go the full 3.5". Something like 2-2.5" would be fine, and would give you a bit of slop room for bringing the vapor barrier up the side of the wall foam a bit.

On the sump-pump access closet there's no point to bringing the subfloor any further than under the bottom plate of your partition wall framing.
 

Handyman J

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Thanks Dana.

When using the tapcons for the board through the foam into the slab, should I worry about moisture coming up and contacting the board through tapcons ?

Also, one section of the basement will be tiled and the other would be carpet. For the tile section I'll use tile-backer board but what about the area I will carpet? Plywood?
 
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Dana

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Don't sweat the amount of moisture wicking up from the TapCons, unless the water table sometimes reaches slab level in your location. If you're not sure, squirt a bit of can-foam in the hole before you screw it down, which will seal around the screw, and seal the screw to both the foam and the vapor barrier (mostly).

With or without the can-foam, you'll have to vacuum out the drill-dust before installing the masonry screw.

For the carpeted area go with OSB or plywood. If you have any residual concerns about moisture go with Huber AdvanTech subfloor, which is more moisture-resilient than standard grades of OSB or plywood.
 

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Thanks Dana. Yea I am concerned about the moisture. I may forgo the whole carpet idea and get rubber mats that have the finished wood look. Its soft and mold-resistant and with the EPS foam underneath, the flooring should be insulated enough. The only thing I am not sure of is if I can install these rubber mats over the foam a. I don't see why not but I'll have to do some research.
 

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The foam needs something more rigid than a rubber mat over it to keep from permanent dents/depressions, which is why a wood sub-floor is called for. You could also pour another slab over the foam if you have the head room.
 

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This is an extremely informative thread. Hopefully it is OK to resurrect it after a year's hiatus.

A further question for Dana: Could a the basement subfloor sandwich of EPS/XPS + plywood/OSB described above support an internal structural (load-bearing) wall? Appreciate it will depend on a number of variables (such as weight of structure) but in theory at least could that combination support some load bearing?

The scenario is a wood clad house with a tin roof above. Currently the joists of the basement ceiling (30% of the main house floor is directly above) is supported by a too-thin 60s-installed beam which is starting to bow. My thinking, which I'll be getting an engineer to consult on from a structural angle, is to insert internal load-bearing wall(s) within the basement space to create additional room(s) as well as taking over the load bearing from the old beam.

So really just after some in-principle guidance as to whether the basement subfloor damp-proofing and insulation approach described above could work in principle in a load-bearing scenario or whether typically bearing members need to be connected more directly to the concrete slab?

Cheers
Jake
 

Dana

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With load bearing walls you'd have to do the engineering math- the amount of load matters. That said, there are many building designs in cold/very cold climates with Type-II or Type-X EPS under the bottom of the foundation footings, fully supporting the entire weight of the house, concrete foundation walls included.

In your case the thickness, reinforcement and concrete mix of the slab matters too if using it as a structural footing.

It's probably cheaper/easier to jack up the beam to where it is level and lag-screw a steel plates or channel-iron reinforcements to sides of the beam to give it the necessary load strength, then build the partition wall below without having to do the more complicated analysis on the slab & foam load capacities or the load capacity of the partition wall.

03130196Foto_big.jpg
 

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Very helpful thread! I am planning to finish my basement (approx. 1,000 sq ft) and I was wondering if you see any issues with the following approach. The basement is unfinished (below grade, poured cement walls and cement slab; house built in 2001). The house is located in Rhode Island.

My plan is to have the walls framed 1.5-2" away from the foundation walls and then have 3" thick closed cell spray foam (R21) applied.
The rims joists will have 5" open cell foam (right now they only have fiberglass batts that will be removed). I was quoted 3,000 for the closed cell and 400 for the open cell foam. The sill plates have an (XPS?) pink seal below it so it does not touch the cement foundation wall directly.
I am not planning to add any extra insulation in addition to the spray foam. The ceiling will be drop ceiling tiles (no insulation between basement and 1st floor).

For the floor, I am planning to have 6 mil poly installed followed by 2x4 PT sleepers and 3/4 Advantech OSB (no insulation such as XPS between sleepers). On top of the OSB I will have a carpet pad (with moisture barrier?) and carpet installed. Do you think there will be an issue with this subfloor setup (moisture, mold on carpet etc.).
We had some water during the floods of 2010, but other than than no other serious problems during normal or heavy rain. I used NP1 to seal around the foundation wall/floor seams (had a a small amount of water in the past in a couple of spots but did not see anything after I sealed and we had some heavy rain over the past couple of months). Thank you in advance.

I forgot to add that I do not have a sump pump or french drain installed.
 

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To do 3" of closed cell foam at high quality and low fire risk during curing will take two lifts of no more than 2" per lift, with a cooling period between. It would be better/cheaper / greener to install 1.5- 2" of EPS against the foundation trapped in place with the studwall, and fill the studs with 3.5" of open cell foam. With 2" of EPS + 3.5" of open cell you'd be in the R21-22 center-cavity range, but it would outperform the R21 closed cell foam due to the lower thermal bridging. With the 1-1.5" of stud bridge the closed cell foam you have R1.2-R2 of wood penetrating the R6.5-R7 foam, which cuts the thermal performance of that layer of foam by more than half. With a full fill of open cell you have about R4.2 of wood thermally bridging the R13-ish foam. It's still a hit, but not as severe of a hit, an artifact of the R/inch of the foam relative to the wood being not as large a difference.

You can't put open cell foam against the subgrade foundation wall- it's too vapor open on it's own, but any rigid foam would be fine at 1.5-2". That would be sufficiently low vapor permeance to limit moisture from getting into the studwall, and filling the studwall with open cell foam then becomes mold & moisture safe. The open cell foam would be more than adequate for air sealing as well, but if you taped the seams of the EPS prior to the open cell foam with housewrap tape it would further guarantee air tightness. Open cell foam can be applied in lifts as deep as 5.5" in a single shot without compromising quality or safety.

EPS is blown with pentane, which is a greenhouse gas with a 100 year impact of about 7x C02, but most of the pentane is recaptured at the factory and burned for process heat. Closed cell polyurethane is blown with HFC245fa, which is also a greenhouse gas, but nearly 1000x CO2. At 3.5" of open cell foam would use less polymer than 1" of closed cell too, and 2" of EPS has only 3/4 as much polymer (different type though) as 2" of closed cell foam.

With virgin stock EPS 2" would run you about 75-80 cents per square foot (installed) for R8-8.4 compared to about a buck a square foot for R6.5-7 closed cell foam. If you use reclaimed EPS you can get it down to the 25 cent range. Green Insulation Group in Worcester, MA or Nationwide Foam in Framingham MA both always have reclaimed EPS and XPS in stock at various thicknesses, at pricing low enough that it's worth the drive from RI to pick it up. If using reclaimed goods it's fine to use either EPS or XPS, despite the fact that XPS is blown with HFC134a, a greenhouse gas with a ~1400x CO2 footprint. For walls it's also fine to use polyisocyanurate (also blown with pentane) but keep the bottom edge above the high-tide mark of your flooding history. Unlike polystyrene or polyurethane, polyiso is hygroscopic, and would wick up and retain some moisture in the event of a flood.

Without insulation under the subfloor the rug and subfloor will have mold issues due to summertime outdoor air dew point averages signficantly higher than the subsoil (and thus slab & subfloor) temperature. Skip the sleepers, which are both a thermal and capillary bridge. You'll be a lot better off if you put down a continuous layer of 1.5" of EPS (R6-6.3) and put a 6 mil polyethylene sheet between the foam & subfloor, and TapCon the subfloor to the slab. Run the subfloor and floor foam all the way to the 2" wall foam, and rest the bottom plate of the studwall on the subfloor. Overlap the seams of the subfloor with the seams of the EPS layer by a foot to keep from compressing the foam at the seams with high traffic use.

With R6-ish foam under the subfloor you won't have to keep the basement insanely dry with constantly running dehumidifiers to keep mold from growing in the carpet underlayment in summer, since it's temperature would then track closely with the temperature of the basement rather than the temperature of the slab. If it floods again you'll be scrapping the carpet and underlayment, and the drywall up to a foot or so above the high tide mark, but with Advantech the subfloor would probably survive (unless submerged for days or weeks).
 

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Hello Dana. Thank you very much for your reply. I have some additional questions/clarifications if it is ok with you.

1. Walls: I have decided to use reclaimed EPS or XPS glued with PL3X (stronger than PL300) on the basement foundation walls. I got a quote for $12 per 4x8 sheet 2.5" EPS from the place in Worcester. I am planning to fill the 2x4 paces with R-15 Roxul.

2. Rim Joists: My first message was a little confusing. I was planning to use 5" open cell foam only in the rim joist area, also covering the sill plate (two 2x4's stacked) and up to the top of the concrete wall and the EPS. This part is above ground level. Do you think a 2" closed cell foam would be better in this case? This way I could avoid cutting EPS pieces and spray foam around to cover these areas. Either way, do you think I should add Roxul to increase the insulation R rating (the foam is really a moisture barrier)?

3. Floor: My original plan was to follow the plan that you suggested (rigid foam and no sleepers). The carpeneter who will do the framing mentioned that it would be a lot more expensive to do this (drill holes and tapcon) than nail the PT sleepers on the floor. What if I still use the 2x4 sleepers with 1" or 1.5" inch EPS in-between, then poly, then advantech, carpet pad, and carpet? Would I need the caulk/spray foam the areas where the EPS touches the PT wood?

3.5: Is it generally better to place the poly on top of the sleepers rather than below? Do you think there would be an issue with the OSB being "sandwiched" between the poly and padding with moisture barrier (I have seen some padding with "Breathable moisture barrier" and others with double moisture barrier).
 

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The strength of the construction adhesive hardly matters in this application, but the solvents do. Are you sure PL3X is rated for use on polystyrene? The solvents for polyurethane adhesive will often eat into polystyrene, making it weaker rather than stronger.

Using EPS on the foundation top ledge and foaming in place with the open cell foam when you seal & insulate the band joist works fine. A 2" closed cell shot might be somewhat faster but it's also more expensive, less green, etc.

Sleepers are both a capillary and thermal bridge to be avoided whenever possible- they create more problems than they solve.

If your slab is flat enough it's common to put the sheet poly under the EPS, and just glue the subfloor to the foam, letting it float. To avoid waves or "potato chip curl" this is usually done with 2 layers of half-inch CDX with overlapping seams rather than a single sheet of 23/32" subfloor. I've also seen it done that way with a single layer subfloor, with only a few TapCons in strategic locations, but I've been told there's a higher risk of developing waves in the floor. I don't have enough sample points in my personal experience to assess what that risk level might be. With a rug as the finish floor you might not even notice if a wave develops.

Any rug padding needs to be vapor permeable, so that the subfloor's moisture content will track that of the room air. If the subfloor is under a moisture impermeable layer it may become saturated with moisture over time. While Advantech is pretty resilient to liquid moisture, this would be the akin burying it, probably not a good thing over time. With foam insulation under it the subfloor will be at roughly the room temperature, so as long as the room air is at reasonable humidity levels, a vapor permeable rug pad would keep the subfloor at reasonable humidity levels. Only if the OSB is significantly colder than the room air would moisture drives from interior air humidity be a problem.

At $12/sheet for R10 that's less than 4 cents per R per square foot, considerably cheaper than R15 Roxul. The 2.5" EPS is good for about R10-R10.5, at which point you'd be way over code min even with cheap R11s in the studwall. Unfaced R13 fiberglass is quite a bit cheaper than R15 rock wool at box store pricing and still good enough to blow way past code min performance, if that matters to you.
 
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