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Subfloor / finish floor suggestions for below grade basement

facesnorth | Posted in General Questions on

Looking to finish a basement below grade in 18301 with concrete block walls and a 2″ slab with poly underneath and very little gravel.

Walls all have 1″ continuous Thermax on the interior then a 2×4 wall with R-15 roxul. That assembly sits on a 1″ layer of EPS against the concrete. Now I’m looking for what to do with the floors.

There’s only 83″ between concrete floor and my joists. I’m about 6’1 and want to plan around my height. Leaning towards a drop ceiling which would need to come down about 2″ or so. The thickest utility I need to cover is a 1 1/4″ ID black iron propane pipe, but just thinking about clearance to remove/install ceiling tiles.

I’ve checked out a few products like Ovrx Barricade, Amdry, etc. I’d like to keep the entire finished floor assembly under 1.5″ if possible, though I could be convinced to stretch to 2″ if needed. I’m concerned having just 6″ above my head won’t feel good in a finished room, though. So wouldn’t mind input from people who have finished basements with low ceilings to offer their thoughts on the clearance.

Also primary reason I’m writing is for suggestions on flooring assembly suggestions.

top, finish layer will vary depending where in the basement (LVT in storage areas, and possibly in the bathroom, laundry area and maybe in the sauna. porcelain tile possibly in bathroom, sauna, laundry area.. storage area would prob have cork or LVT. play room / theater will have either cork, hardwood, or carpet. bedroom either hardwood, cork, or carpet).

my options for underneath:
full sheets of 1/2″ EPS + 1/2″ or 5/8″ OSB
Orvx Baricade
Amdry (there’s diff thicknesses available, prob the R5 one)

If I go with just using 1/2″ EPS + 1/2′ or 5/8″ OSB on top, should I let the foam float, tape between seams, and then install OSB perpendicular to it, and use tapcons through the foam into the concrete?


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  1. GBA Editor
    Martin Holladay | | #1

    I have never installed Orvx Barricade or Amdry.

    The basic principle is that you want R-value. If you are choosing between R-3.2 Barricade, R-5 Amdry, and R-2 for 1/2-inch EPS, the R-5 product wins (assuming, that is, that Amdry isn't exaggerating the product's R-value).

    If you want to install 1/2 inch EPS plus 1/2 inch OSB, you definitely need TapCons.

  2. facesnorth | | #2

    Apples to apples, the 1/2" OSB would also have about .6 R value, so 2.6 vs 3.2, but we're also comparing EPS vs XPS on the ORVX at the same thickness of 1". And with the Amdry, we're gaining R-value but it's a 1.6" product. So before I used that product I'd compare it with, say 1" EPS + 5/8" OSB, so 4.66 of R-value vs 5. The Amdry has an astronomically higher cost.

    I believe the Amdry and Barricade products can be installed as floating (no tapcons). Is there value in that, besides reduced labor? I'd imagine the tapcons would create a cold bridge from the concrete that could be problematic? And maybe also create pathways for radon to seep through?

    Is there any value in the air and moisture "channel" at the bottom of a product like the Amdry? Vs having the EPS foam directly against the concrete floor?

    If I go with EPS, what minimum density should I be looking at?


  3. facesnorth | | #3

    I guess what I'm asking is how much of a difference would it make to go from say, 1/2" of XPS to 1" of XPS (or EPS) on top of my slab, under a 5/8" sheet of OSB with cork flooring or carpet on top.

    Is it just a matter of comfort, or will 1" actually ensure no condensation, whereas 1/2" would be iffy? Or would either one be sufficient, or insufficient?

    Any techniques that I should be concerned with. Do I need to use a foam adhesive between the foamboard and my slab? Do I need to use foam between the foam board seams, and tape all the seams? I assume lay the OSB in a perpendicular orientation to the foam board? Should I be concerned then about the type of underlayment I use over the OSB and under the cork or carpet?

    Should I be using poly anywhere? I know that at least part of my foundation does have poly under it, if not maybe the whole thing.

    I'm also excavating an entryway into the basement from outside, and wanted to know if there was any specific advice or details I should be concerned about to prevent problems with water intrusion or energy loss.


  4. GBA Editor
    Martin Holladay | | #4

    It's hard to give you advice if you won't tell us where you live.

    I'm not sure what you mean by "Looking to finish a basement below grade in 18301."

    Is that a zip code? Are you trying to tell us that you live in East Stroudsburg, Pennsylvania?

  5. facesnorth | | #5



  6. GBA Editor
    Martin Holladay | | #6

    OK. In Pennsylvania, your basement slab can be cold enough during the summer for condensation. My guess is that 1/2 inch of rigid foam would eliminate the likelihood of condensation and mold, but (needless to say), 1 inch would be better if you can afford to lose the ceiling height.

    If you are sandwiching rigid foam between your slab and OSB subflooring, you don't need adhesive. Taping the foam is a good idea. Fasten the OSB to the concrete slab with TapCons to secure both the rigid foam and the OSB.

    If you plan to install flooring underlayment, use whatever underlayment is recommended by the flooring manufacturer.

    Installing a layer of polyethylene between the concrete slab and the rigid foam is always a good idea. Polyethylene is cheap.

    You wrote, "I'm also excavating an entryway into the basement from outside, and wanted to know if there was any specific advice or details I should be concerned about to prevent problems with water intrusion or energy loss." Is this a walk-out basement (direct to grade)? Or are you building an exterior well with a stairway and a steel bulkhead door?

    In either case, there are plenty of ways to screw things up. Provide more details (or a sketch) if you want advice.

  7. Yupster | | #7

    If you already have poly under the slab, poly on top is not necessary and can actually cause some problems if you have a leak someday. (It's a basement, might as well count on it) I'd rather not put it there. If you are worried that your slab might get damp for whatever reason, an epoxy floor covering is a much nicer vapour barrier for your floor.

  8. facesnorth | | #8

    Yeah that's what I was unsure about - I know sandwiching a wall assembly with vapor barriers is bad, but wasn't sure if the same was true for the slab. Plus, like you said, what if there's a spill in the bathroom, the water will leak through and pool on the poly.

    I have no problem spending a little extra money to do things right. Cost/labor of iinstalling poly would not be a concern, just the question of whether or not it's beneficial or hazardous.

    I don't think I would go through the effort of epoxying my slab, only to cover it with foamboard/OSB/finished flooring, though?

    The reason I asked about the underlayments, was because some manufacturers specify a poly underlayment, or an underlayment product that contains poly as part of it - or varying levels of perm in their retarding layer. Like anything, the reason we are drawn to this board as a resource, is because we don't necessarily trust the manufacturer's advice or other advice out there because it doesn't necessarily incorporate a holistic building science view.

  9. Yupster | | #9
    Here is a good article on wood floors over concrete slabs that might help you make a decision. I've used poly with no problems before, but I've migrated over to the epoxy side because it seems more robust to the inevitable water heater leak or rainwater leader pouring water on the side of your foundation wall and sending it inside.

  10. facesnorth | | #10

    Great article, very informative. I'd love to hire him to do my floor in fact. Lots of scenarios, though, and a little hard to discern what totally applies to my own situation. It's too late of course to totally wrap the outside bottom and sides of the concrete with poly to keep it dry. But I did read the suggestion to use an epoxy coating vs poly over top.

    Sadly he could not suggest any manufacturers, but seemed to indicate that many were not suitable due to poor handling of pH levels? He suggested using "foreign sounding, expensive" products. I certainly don't want to waste time and money on a product that wouldn't be suitable. I spent over $1k epoxying my garage last year, with weeks of prep work required, on an area probably under a quarter of what my basement will require. I can't spend thousands of dollars and months of prep work on a vapor barrier that will just be covered over. Can you recommend an affordable product that's unlikely to fail, and doesn't require tons of prep (i.e. perfectly patching every crack, then renting a floor polisher with diamond tip blade to grind down the entire floor surface, etc).

    One other detail is that I've already begun work on my basement - I've begun insulating the walls, and built some utility closets which now have electrical and water pipes mounted on them. The concrete block walls (below grade) have 1" Thermax applied, using vertical beads of Enerfoam adhesive. Under them are 1" strips of EPS, with a bead of Enerfoam underneath and between. I've also got 1" EPS with Enerfoam adhesive under the 2x4 walls in my utility closets. Point being that I wouldn't be able to "completely" apply an epoxy covering (or poly for that matter) on the entire basement floor. There would be a couple of walls, and majority of the perimeter (about a 5" strip) where I would not be able to apply it. Not sure if that will create problems for me? Note also that I wouldn't be able to do much to the majority of the below grade concrete block wall, which is totally untreated, against the 1" Thermax which has already been installed. I know that wasn't mentioned, but the importance of below grade walls were mentioned in that article, and I wasn't sure if it was something that also should be addressed.

    My plan was to use a Foamular 250 tongue and groove XPS, and use a 3" Thermax tape over all seams, and between the existing EPS strips and new XPS. Also note that a few areas - such as the sump pump pit, my massive well pressure tank, I was planning to just "cut around" and not install the XPS, nor OSB nor any flooring, and just leave the bare concrete exposed in those few areas. The bottom of my basement steps too I may just cut around, rather than try and re-do the steps.

    Martin, as for the new entryway we want to install into the basement - my wish is to have a full entry door outside, leading down a stairwell to a door into the basement. I do not wish to install a bilco door, as I want it to be able to used as an entry to a small apartment for my in-laws. I was going to frame out basically a small rectangle from the house, with matching T-111 siding, and a roof, tying the gutters into the existing gutter pipes. And at the bottom of the stairwell another door leading into the basement. I think a double door system like this would help with keeping a stable temperature in the basement. And by enclosing the stairwell completely, it would help keep out water from rain and snow. I assume I would just pour concrete steps in here, not sure if foam insulation underneath would be beneficial in any way. Or if I should pay any particular attention to the insulation in this protrusion vs just putting another exterior door at the bottom entrance to the basement.

    Thank you.

  11. GBA Editor
    Martin Holladay | | #11

    Q. "My wish is to have a full entry door outside, leading down a stairwell to a door into the basement."

    A. Here are the most important pitfalls to avoid:

    1. Make sure that the length of the addition is adequate. At the top of the stairs, you need a landing that is at least 3 ft. by 3 ft. -- bigger is better. At the bottom of the stairs, you also need a landing that is at least 3 ft. by 3 ft. -- and again, bigger is better. The stairs shouldn't be too steep -- aim to follow the 7-11 rule (risers 7 inches or less, treads 11 inches or more).

    2. Tie in the new foundation walls to the existing foundation with rebar dowels epoxied to holes in the existing concrete.

    3. Plan for adequate drainage, with a deep layer of crushed stone under everything, and perforated drain pipes leading from the footing level out to daylight.

    4. Include adequate lighting.

  12. Yupster | | #12

    It's just going to be a vapour barrier, not a finished surface. If you cover 95% of the area, you've got a 95% effective vapour barrier. If you've got any large cracks, caulk them. It doesn't take long to slap on a coat if it doesn't have to look perfect. As far as a specific product goes, 2-part epoxies for concrete floors are pretty common, I'd just go to your local paint supply store or a concrete guy and ask them what they have for a rough application.

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