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Community and Q&A

Insulated sleeper floor joist system

Scott Schmeiser | Posted in General Questions on

With all the great insulation help I’ve received in just a short time, I thought I would ask some questions about another unique insulation situation I have.

An addition we have on our historic home was once a hobby flower shop that was added sometime in the 1940’s.  It was built on a slab directly on-grade.  Assuming no vapor barrier between grade and concrete and about 3.5″ or 4″ thick.  At one point this was turned into a family room and we’ve since gutted and started over from scratch removing absolutely every shred of wood structure down to the brick.

The slab was pitched down to one corner approximately 5″ over the longest distance for water drainage.  To counter this and for the logistics of the living space, we chose to go with a sleeper joist system to level everything out and provide an elevated floor.  Once the slab was stripped, cleaned, small cracks sealed, and prepped, we put down a true 10 mil vapor barrier system (I have forgotten the brand of vapor barrier system we used) followed by the pressure treated floor joists.  Sublooring, before the reclaimed oak flooring goes down will be 3/4″ Advantech glued and screwed.

Before subflooring goes down, we obviously want to put some type of insulation in.  The plan has been to just use a ccSPF product between sleeper joists and right over the vapor barrier.  Sounds like HFO blown ccSPF is a good choice.  The vapor barrier is doing double duty and probably unnecessary, but was put in early during the building phase when we were going a different direction with the insulation.  

The questions:
1 – Is ccSPF the correct solution here?  Minimum depth of the joists are 1.5″.  Max being around 6.5″.  Any level of insulation will help keep the floor warm during the winter.
2- Advantech sheathing seems to offer a built in moisture barrier with it’s engineered composition.  Do we need to add another product between the insulated cavities and the subfloor sheathing, or just glue it, screw it, and forget it?
3- Should we be considering a different insulation option?

Side note about this slab:  It is absolutely a dry slab.  There has never been a moisture issues in the almost decade that it’s been exposed.  I check regularly for any moisture issues.

This floor system has really been an odd construction project and very labor intensive to get it all level and contoured to the slab.  Looking forward to covering it up and never thinking about it again.

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  1. Scott Schmeiser | | #1

    Adding to this...
    Had a foam installer out today to look at the floor system and he said my vapor barrier has to go. Can't lay the ccSPF over top. I kind of has a suspicion that would be the case. No big deal. The system was not terribly expensive and should be easy to pull out.

    Interesting thing is, the other two installers wanted to lay right down over it the vapor barrier. Had I let the others who quote do the work I would have had a catastrophe in short order.

    Going back to the initial question: Should I be putting another vapor retarder of sorts over the insulated cavity before putting down Advantech, or just go ahead and glue and screw the Advantech down and move on?

  2. DCContrarian | | #2

    Not sure why the guy didn't like your vapor barrier. Spray closed cell isn't popular here because of the greenhouse gases it produces. Sheet foam cut into strips would give you equal or better insulation value for less cost, you could do it yourself. If used foam is available where you are it would be substantially cheaper.

    1. Deleted | | #5


    2. Scott Schmeiser | | #6

      Thanks for your input! I will consider that option! Does one just seal around the sheet foam seams?

  3. Expert Member
    Akos | | #3

    I've had spray foam applied over poly. About the only thing is that it doesn't stick to it. This is not an issue in this case as it will still stick to the sleepers. Once cured, its not going anywhere.

    I wouldn't worry about the SPF vapor barrier issue. The two vapor barriers will work together, one to keep the moisture from the concrete coming up, the other preventing moisture from the interior get to the poly and condensing. You don't need anything extra under the subfloor.

    Don't go nuts on the SPF. You don't need all that much, the 1.5" is more than enough for floor insulation in most climates. If you don't have any perimeter insulation, bumping it up to 3" for a couple of feet around the outside will help a bit.

    1. Scott Schmeiser | | #4

      Thanks for your reply, Akos! That pretty much confirms everything. I'll put the Advantech down, glue it and screw it.

      The installer I just had out quoted me for 2" of foam. The shallow end will be 1.5".

      For the spraying over Poly thing. I believe it was just an internal policy within the company so they can properly guarantee the installation. I'm good with it. Not a big deal to cut the poly out. Wasted a couple bucks, but if it creates a risk for the install, it can go. The foam will provide better coverage than the vapor barrier anyway, since it can get into every little floor defect.

  4. Expert Member
    Dana Dorsett | | #7

    In a zone 3 climate anything more than R5 between the slab & subfloor is overkill, and is even counterproductive if applied over the entire field. The IRC doesn't even call out slab-edge insulation for zone 3 in slab-on-grade construction, but it's still "worth it" to install R5 down to 2' below grade or to the footing (which ever is shallower) to keep the slab temp comfortable near the exterior walls. See TABLE N1102.1.2:

    In Charlotte NC the deep subsoil temperatures are about 65F- that's the cool edge of "The Goldilocks Zone".

    The floor temp or a 68-70F room will run about 65F during the heating season, so insulating against 65F dirt does nothing. Insulating the full field of floor will raise the sensible cooling load, and do next to nothing for lowering the heating load.

    Installing 1.5" (the thickness of a 2x4 sleeper) of EPS (R6-ish) on top of the slab for about 2' around the perimeter of the exterior would improve barefoot comfort during the winter without raising the sensible cooling load (much). Spray foam, forget about it. Even cheap Type-I EPS (R5.8) is plenty.

    1. Scott Schmeiser | | #8

      Dana! Thank you for commenting on this!

      You bring up some great points and you may have just saved me $1000, or more!

      If I go with a 1.5" EPS board, do I go ahead and leave the vapor barrier I have down? I've never used EPS or any other foam board in this manner. Do I seal around each panel? Sorry if that's a stupid question.

      Seriously! Thank you for replying!

      1. DCContrarian | | #9

        I would leave the vapor barrier in place and air seal the foam board. Either cut it tight and seal it with tape or cut it loose and seal it with can spray foam.

        1. Scott Schmeiser | | #10

          That's what I'm thinking as well. Basically getting the protection with the VB and sealing everything up with CC foam in a can wherever possible.

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