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Basement subfloor assembly details

yacpro13 | Posted in General Questions on


I am about to install a new basement subfloor. My research has led me to select the following subfloor assembly:

-existing slab
-1.5in rigid foam
-5/8 T&G plywood tapcon’d every 24in to slab

No 2-by sleepers in this subfloor.

The old subfloor had a vapor barrier against the slab. This house was built in the 80s; I would think it does not have a vapor barrier below the slab.
Also, as far as I know 1.5in of rigid foam is not considered to be a vapor barrier.

Therefore, in my case, should I install a vapor barrier between the slab and the rigid foam?

Thank you!i

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  1. GBA Editor
    Martin Holladay | | #1

    Yac Pro,
    Q. "In my case, should I install a vapor barrier between the slab and the rigid foam?"

    A. Yes. You can use ordinary 6 mil polyethylene.

  2. yacpro13 | | #2

    Thanks for the quick reply! That's how I will proceed.

    I thought of another question though:
    The more traditional way of building a subfloor is to have 2-by sleepers and only fill the cavities with rigid foam. In many places it is now recommended to lose the sleepers (thermal break plus wood so close to foundation not so good).

    I will have to frame walls on top of the subfloor. Part of me thinks a subfloor with sleepers would be a better base for the walls, but again these walls are obviously non-structural.

    I would tapcon the bottom plate though the plywood and foam to the slab. If done this way, is there anything else I should plan for / consider when framing walls on top of a rigid foam+plywood subfloor without sleepers?

    Thanks again.

  3. GBA Editor
    Martin Holladay | | #3

    Yac Pro,
    If you want to install non-structural partition walls on top of a subfloor that has rigid foam between the subfloor and the concrete, you don't need to use Tapcons to secure the bottom plate of the walls. Just screw the bottom plate to your subfloor.

  4. Expert Member


    Fastening subfloors to concrete through foam with screws is an awful job, and one that yields a fairly uneven surface. One alternative is to use two layers of subfloor glued and fastened together, but otherwise floating on the foam. Framing interior partitions with that method is as simple as nailing the bottom plates into the subfloor.

  5. yacpro13 | | #5

    @Malcolm, uneven surface eh.

    Never gave that a thought as I figured I could get by with the 25psi rating of foam plus tongue and groove and obviously not driving the tapcons so hard as to crush the assembly. Now I am second guessing.

    I thought the 2 layers of subfloor was mostly for when using regular OSB/plywood (ie. not tongue and groove).

    I have to match a total subfloor height of 2-1/8, so going with 2 layers means I'd have to lose a 1/2inch of insulation to accommodate a second layer of plywood.

    If 1 inch of insulation suffices (climate zone 6), I do like the idea of avoiding hundreds of tapcons piercing the vapor barrier, not to mention the easier assembly of just floating the ply on top of the foam....

    1. Expert Member
      MALCOLM TAYLOR | | #6


      Even if you manage to find the hole you have pre-drilled, you then end up either compressing the edge of the plywood, or with the screw heads proud of the surface. Depending on what flooring is going on top, a bit of compression may not matter - and maybe others have had better experiences than I have - but it's not something I'd embark on lightly.

  6. yacpro13 | | #7

    I have been reading a lot of GBA posts as well as FHB and Building Science articles.

    Tapcons are pricey, tricky and time consuming to install, so if they can be avoided without compromising the result, why not.

    Building Science specifically mentions that the plywood can be floated on top of the XPS in one of 2 ways:

    1) Using tongue & groove plywood, and biscuit joining the butt ends
    2)lumber on top of the XPS and plywood fastener to it


    Since I must have a total height of 2-1/8in, instead of using dimensional lumber above the XPS, I am thinking of using 1/2in plywood ripped in 4in strips.
    So the assembly would look like this:

    1in XPS
    1/2 x 4in strips of plywood installed like sleepers, glued to foam.
    tongue & groove 5/8in plywood fastened to plywood strips only

    I've never seen this done, but I think this addresses every single concern while also keeping cost low and installation as simple as possible (no tapcons).


    1. yacpro13 | | #10

      Post project wisdom:
      1) Like Malcolm said, biscuit joining a plywood subfloor is a ridiculous idea.

      2) I can see the XPS-Lumber-plywood option working if you really want to avoid Tapcons and can afford the thicker assembly.

      3) My 'idea' of using ripped plywood instead of lumber would have never worked. I had not realized how curled plywood could be before getting fastened.

  7. Expert Member

    Much as I'm grateful to BSC for their research, there are times when they really should consult to someone who has swung a hammer before offering practical advice. The idea that you could install biscuits on the ends of a T&g plywood subfloor and them somehow install it is just plain silly.

    Your method might work. What would worry me, and maybe it doesn't matter, is the bond between the foam and your strips. It's one thing to glue foam to a wall, it's another to subject it to the intermittent live-loads floors experience. Strips of 1/2" plywood aren't usually very straight. Getting them flat while the glue sets up might be a problem. Maybe it would be easier, and just as good, to glue the strips of plywood to the subfloor sheets and then assemble the whole thing?

  8. yacpro13 | | #9

    I refer to GBA quite often, I thought I would report back on my project now that it is completed, in hopes that this information could be useful for somebody else in the future.

    The reason I was limited to a 2-1/8in total subfloor thickness is because I wanted to match an existing subfloor in another part of the basement. My subfloor construction goes vapour barrier, 1-1/2in XPS, 5/8in t&g plywood. If I had to do it all over again I would very much likely do it the exact same way. Installation went very smoothly, and the resulting subfloor is great.

    Generally speaking I would think most people would not require a specific total thickness for the subfloor, and the goal is more commonly to make the subfloor as thin as possible to maintain a good ceiling height. If that is your case, I reckon some space (and money) could be saved by using thinner insulation. For new construction where I live, code requires R5 insulation which could be met by 1 in XPS only.

    I layed the XPS on the vapor barrier and used foam specific adhesive to glue the shiplap, and tape the seams using tuck tape. I layed the plywood perpendicularly to the XPS, making sure no foam and plywood seam would coincide. I used 15 Tapcons per sheet to fasten to the slab (ie every 2 feet).

    Tapcon installation went quite well. I broke 3 screws (sheared screws) in my first 9, but then completed the installation of about 120 Tapcons without a single breakage. My installation procedure went like this: drill clearance hole though plywood/foam for screw shank, drill hole through assembly in slab, countersink the plywood for screw head, vacuum hole, drive Tapcon in.

    Some tips: I used a proper rotary hammer with SDS chuck, along with a made-in Germany Bosch bit (did not use the one included in the Tapcon box). Drilling was very easy, but you must drill a short distance and move the drill back up to clear the hole. Once I'd reach the desired depth, I would move up and down the finished hole 2-3 times to clear as must dust as possible using the bit. At the beginning I did not do this (I would only drill moving downward into the hole), and even though I'd vacuum every hole I still had a high breakage rate. In my case, the SDS carbide bit lasted about 100 holes.

    Also make sure you use the proper Tapcon length. Tapcon provides a specific engagement length, and the closer your engagement length is to their maximum value, the higher the chance of shearing a screw.

    If I had to do this again I would do it the exact same way. The resulting subfloor is very solid, and less bouncy than older subfloors build on sleepers with empty cavities. Framing of the wall was done afterwards and the bottom plates were drilled into the plywood. I know FHB and other users have been successful installing 2 layers of (thinner?) plywood, but I would personally stick to the construction I used.


    1. Expert Member
      MALCOLM TAYLOR | | #11


      Thanks for coming back and sharing your experience with this - and very glad it turned out so well.

    2. Expert Member
      BILL WICHERS | | #12

      Thanks for the update!

      Let me share my trick for quickly clearing dust out of deep vertical holes drilled in concrete. I have drilled uncountable numbers of holes in slabs for anchor bolts. Besides finding out that concrete contractors have a funny idea of what constitutes consistency in slab thickness, I’ve found the following helps to drill lots of holes quickly and cleanly:

      1 - always use an SDS hammer drill. Regular chucks loosen up quickly on hammer drills and drive you crazy.
      2 - I don’t know what alloy Bosch uses to make their SDS shank bits, but it’s ridiculously durable. Don’t cheap out on bits.
      3 - set the depth stop on the drill. Drill all your holes in one shot. Don’t stop to clear each hole.
      4 - my secret - get a little shop vac and a can of canned air. Go to each drilled hole, hold the shop vac nozzle near the side of the hole, poke the tube from the can of canned air down the hole and give a little blast. The blast of air blows all the dust out of the hole which the vacuum sucks up.

      That canned air trick saves a ton of time and makes sure each hole is really clean so you don’t risk jamming your anchor in a bit of missed dust in the bottom of the hole.


    3. dbaker11 | | #13

      What kind of finish did you put on your subfloor.... I want to do porcelain tile... do you think this method is stiff enough for tile?

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