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Community and Q&A

Insulation of PWF sleep floor

duhmojo | Posted in Green Building Techniques on

Hi all.

I’m having a really hard time sourcing some advice on insulation for a PWF sleeper floor. I have a very old cottage that I am renovating and I just finished pouring the new concrete exterior footing and walls (I had to jack up 1/2 the 2nd floor and but hand, dig up the old rock+mortar foundation, then lazer level new forms and used a mixer to make concrete). I’ll be framing this coming weekend but the whole time I was planning an insulated concrete slab for the inside. It’ll be basically at grade, not below the ground. However after all the concrete work, hauling material down to the work site, etc… it’s clear that the 4 man team I’ll have to schedule this Fall with impending colder temps, might be a recipe for disaster.

My neighbor who’s been lending me a hand and advice (he’s been a master carpenter for decades, and built his own home solo, maybe out of spite) suggested thinking about a wood floor instead of cement. He suggested a packed gravel base (I was going to do that anyways) with a PT 2×6 frame sitting on it over a vapor barrier. I’ll be able to source the material fairly easy and build it in stages (e.g frame a 9.5×4 frame at a time) then cover with plywood.

This lead me to PWFs and sleeper floors. Specifically page 5 of this PDF:

I downloaded the PWF spec manual as well. None of the material I’ve read covers basement floor insulation.

EPS/XPS insulation could be cut and inserted between each floor joist cavity with spray foam to seal, but I believe the PWF sleeper approach requires (it’s not clear) the air cavity above the gravel/vapor barrier/sleepers. So sitting insulation inside would negate that.

I “think” I’ll get creative and construct the 9.5×4 frames upside down, with the plywood installed, and I’ll insert and spray foam some EPS (there’s a lot of room for insulation under there, no need for XPS), then flip it on top of the sleepers so I’ll have a air space in the cavity, and insulation right under the plywood.

With this approach I could actually use regular ply wood as the insulation and foam would act as a vapor barrier and be raised up. 

Any advice? Thanks!

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  1. duhmojo | | #1

    I just realized I'll have a hell of a time securing each 4' wide frame to each other if the ply is pre-installed and insulated. I wonder if securing them to the sleepers is enough. e.g. the sleepers will be the footing for the floor, and each 4' wide section is secured to it, not to each other. I could do this similar to how those hidden wood deck plank brackets work where you clip the back of the board and screw the front.

  2. duhmojo | | #2

    I suspect it'll be fine if I assemble a 4' wide frame, screw the ply on top, flip it, insulate the underside of the ply, flip it back over on the sleepers with construction adhesive, then toe nail into the sleepers. (maybe I can find a bracket that'll do a better job)

    Each subsequent frame would be glued on the frame side against the previous frame, and toe nailed into the sleeper.

    Along the top of the ply wood seams between the frames I can screw in a metal strip to keep the sections from widening apart.

    1. Expert Member
      MALCOLM TAYLOR | | #4


      As Jon said, there is isn't much point in including framing for a floor on grade. Here is how frequent contributor Michael Maines did it:

      1. mikeolder | | #14

        Are there any completed examples of this method Malcolm? I could see a PT timber framed perimeter set over rail road rock and with good drainage and ride out the winter heave. Then floor joists set on top for a small house.

  3. Jon_R | | #3

    What would happen if you didn't use 2x sleepers at all? Ie, gravel, plastic, EPS foam, plywood glued to the foam (with seams staggered). The entire floor would be similar to a single piece of nailbase panel.

    If the plywood isn't fully supported (ie an air gap), then you need to worry about span/stiffness.

    If you want to be truly independent of the ground, then a raised floor on suspended joists would be appropriate.

  4. duhmojo | | #5

    I'm not so keen about laying it all directly flat on the earth+gravel, but there's something to what you're suggesting of course. A concrete floor would be directly poured on foam insulation, over vapor barrier, over the gravel. So why not just put a floor on the foam?

    To me I like the unified solid firmness of a joined wood floor where as the foam will just become hilly if there's heaving (Canadian winter). I have a feeling it'll play out over time like my parent's first attempt at laminate floating floor over a hilly concrete basement (hilly and floating, not solid and flat).

    However this approach in general (PWF instead of concrete) is new to me and I've only been thinking/researching it for a couple of days.

    I've already dug the basement floor 9-10" below grade on average (via the work I had to do with the foundation), and I'm on the verge of leveling the earth out for the gravel.

    If I frame a floor with 2x6 with ply and sleepers it'll be 7.5" tall. I'll have (right now) roughly 3.5" of gravel to pack down to about a 3" base. That'll bring me to about the height needed.

    If I go with a ply on EPS approach, I'll have say 1/2" ply on top of 3" EPS, over vapor barrier, with 7-7.5" of gravel base to build up. There won't be a lot of weight on the floor, so if the floor heaves a bit, the gravel will adjust and hills will appear. There won't be much informed weight to settle back flat with just the foam and ply.

    Having to build up the gravel is not a bad thing, but do I really need that much? The effort is not free and time consuming (I have to ATV a 52"x72" trailer, which can reasonably carry 1/2 a cubic yard at a time because I can't drive my truck close enough). The 2x6 frame will provide a place for moisture to occur normally during the hot/cold months.

    Thanks for the advice guys. I'm all ears.

    1. Expert Member
      MALCOLM TAYLOR | | #10

      I'm not sure I understand the concern with heaving. If your new foundation is protected from freezing, why should there be any w0rries about the area inside?

      1. duhmojo | | #11

        It’s a seasonal cottage. I’m not heating it sufficiently during winter (as low as -30C). It won’t be as cold inside but if the ground goes below 0C it’ll freeze (heaving occurs when ground water freezes)

  5. Expert Member
    Akos | | #6

    Before going too far down the road with PWF, I would look at how you would keep critters out. This benefit of even a thin rat slab is hard to get with wood construction.

    1. duhmojo | | #7

      Hi Akos. My foundation is 100% sealed. One of the motivators for rebuilding the old foundation was to close up the old, porous rock and mortar. Maybe a mole could dig deep enough but the exterior footings and walls are likely deep enough. When I talk about PWF it's only for the on/near grade flooring inside, not the foundation walls as well. Mice would have to find their way down there from inside, which is possible but I intend to foam and seal the perimeter and baseboard.

  6. thrifttrust | | #8

    I'm not quite clear on your plan but it seems complicated. After leveling the gravel (typically 4") I'd put down a layer of EPS then 2X4 sleepers on 4' centers. then poly. It is important not to seal or tape the poly. Any water must find it's way to the gravel. The poly is only a vapor barrier. The earth and your foundation are the air barrier. 2X4 floor joists are secured to the sleepers then a plywood subfloor. Since the subfloor is within a foot of the ground, in spite of the vapor barrier, It must be pressure treated. Nothing over the subfloor need be treated. You must use wood rated for wood foundations. It is treated with chromated copper arsenate (CCA). Use stainless steel fasteners. The floor should have air vents to the building interior. Drainage is the key to any PWF. Typically a 2' X 2' X 2' treated plywood open bottom box called a sump crock is set into the ground and is emptied with a pump. A 4" layer of gravel should surround the crock sides and bottom.

    1. duhmojo | | #9

      This is basically my strategy but I was going to construct the joists from 2x6 PT instead of 2x4. There will be drainage but no venting. Like a sealed crawl space. I proposed insulating the underside of the plywood.

  7. thrifttrust | | #12

    A crawl space must vent to the outside or the inside. Only a concrete slab need not be vented. By putting all the insulation below the vapor barrier there will be adequate air flow to keep the floor system dry. With sleepers ≤ 4' OC 2 X 6 joists are overkill. Save the 2" and the money to put more EPS under the poly.

  8. duhmojo | | #13

    Ok! I think I’m settling on a PWF approach:
    1 4” packed and leveled gravel
    2 Minimum 2” EPS board
    3 2x4 PT sleepers
    4 Vapor barrier
    5 2x4 PT frame
    6 plywood

    I’ll glue and screw the joist frame and ply together. The perimeter of the floor frame will not be sealed. I will have some base boards, but internally they can vent above the vapor barrier if needed. Just no real air flow.

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