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Community and Q&A

Insulation for slab on grade with elevated stem walls

Josh Klemm | Posted in GBA Pro Help on

Good Evening,

it is very exciting for me to be posting to this board. I’ve been a reader for years, and am looking forward to hearing from the experts on this site regarding my matter.

I own a home that my wife and I are renovating in zone 5A, built in 2012. The first floor of the home was built to be a garage, but we will be converting it into living space. As such, I will be doing some renovations, and have a few questions regarding the best ways to insulate the slab and stem walls.

A few quick notes on the home…

The garage/future first floor living space walls are sheetrocked, finished (painted), and insulated with R21.
The garage/future first floor has HVAC currently with three registers, but will need additional ducting run (the furnace is spec’d for the increased load already).
The garage/future first floor has R30 in the ceilings (between it and the second floor living space
There is no below slab insulation
There is a vapor barrier below slab
There is no thermal break between the stem walls and the slab
There is no exterior insulation on the stem walls, roughly 6-18″ are exposed depending on the side as the home is on a moderate slope
The garage is 1280 square feet, and will become a living room, dining room, and kitchen
There is absolutely no moisture issue with the slab (it is on grade/slightly above grade/a foot or so above grade as the house is on a moderate hill)

Because I cannot insulate below the slab at this time, I will be looking to insulate above it to help make the room more comfortable and energy efficient.

Based on the many readings I’ve come across on this site, it seems as though the preferred method to insulate the floor would be install at least 1″ (2″ preferred) of foil faced EPS or Polyiso with seams taped, followed by a layer of 3/4″ plywood or osb subfloor, or 2 staggered layers of thinner subfloor. This would then be followed by a standard flooring install per the manufacturers reccomendation.

Due to the home having elevated stem walls (above slab on grade by 8″) I want to confirm that the other things I’ve gleaned from this site are the proper way to proceed.

First, install the foam barrier on the slab as describe above. The floor insulation should run wall to wall with no break. Then insulate the stem walls in the same manner on their vertical face. Caulk/tape the seams where the horizontal floor foam meetings the wall foam. From here, I can install the subflooring, and any surface covers for the vertical stem wall interior foam layers.

Is this method appropriate for the interior of the building?

Next question would be that I’ve seen in multiple places on this site that exterior slab insulation to four down is recommended. Can I do this in conjunction with the steps mentioned above? Meaning foam on both the interior and exterior of the stem wall. I know there are additional steps to protect the exterior foam, but am trying to keep this brief as the write ups on that are simple to follow.

Does the above sound, well, sound? If not, what can I improve on? I want to get moving on this, but want to do this in such a manner that is energy efficient and able to prevent any long term issues. My thought is that due to this space being conditioned, and with this insulation approach, moisture below the foam should not be a concern.

I eagerly await your responses, and, again, thank you for the volumes of advice you give to this community. Martin, Joe, and Dana –  I’ve spent countless hours reading your work, and appreciate the pains you go to make it digestible to even non-builders.

I’ve attached two images (roughly the same from different distances) to help if at all possible.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Josh,
    The most important layer of insulation you need to install is vertical exterior insulation on the stemwalls. I advise you to install 2-inch-thick EPS from the top of the stemwalls to a point that is at least 2 feet below grade. Deeper doesn't hurt.

    Once that's done, adding insulation on the other two locations you mentioned -- the interior of the stemwalls and above the slab -- is certainly possible, but not as crucial as the exterior vertical insulation.

    For more information, see these three articles:

    "Insulating a slab on grade"

    "Installing Rigid Foam Above a Concrete Slab"

    "Determining Sub-Slab Rigid Foam Thickness"

  2. Josh Klemm | | #2

    Good Morning Martin,

    Thank you for the prompt response, and additional very informative reads.

    I have one (I believe simple) question regarding the above slab insulation when I get to that point. It appears that non-foil faced EPS/rigid foam insulation is often the product used when laying insulation on top of a slab. Is there a benefit to that over foil-faced, or more so, is there one you'd recommend be used in that scenario?

    Thanks again.

    1. GBA Editor
      Martin Holladay | | #3

      Josh,
      For use as horizontal insulation above an existing slab, your best choice is expanded polystyrene (EPS).

      XPS works fine, but XPS is not generally used by green builders because it is manufactured with a blowing agent that his a high global warming potential.

      Polyiso can absorb moisture, so it's not the best choice in this location.

      For more information, see "Choosing Rigid Foam."

  3. Josh Klemm | | #4

    Martin,

    Thank you, as mentioned previously, the wealth of information your peers and this site provides is beyond value.

  4. Jon A | | #5

    Josh,

    Typically garage floors are pitched as a code requirement and can be as much as 4" from the rear to the front. Is this the case with your floor? If so, did you have a plan to deal with this or are you going to leave the floor of your new living space sloped? Depending on the height difference between your existing garage floor and the finished floor of the exist house there maybe a better way to go about this.

  5. Josh Klemm | | #6

    Hi Jon,

    Using a 6 foot level it does appear to be quite level end to end in both directions. Part of the floor was previously “living space” though not inhabitable due temperatures.

    That said, I am intrigued by your question - is there some better approach towards achieving the desired outcome?

    Also - to confirm - is there a preference for foil faced or non foil faced?

  6. Jon A | | #7

    If the floor is level and in good shape the easiest and cheapest option would be to insulate the stem walls as Martin mentioned and continue on with your project. The floor will still feel cold to the touch and being that it is going to be highly used living space I would insulate above the slab if it were my house.

  7. Josh Klemm | | #8

    Hey Jon,

    Thanks for the quick reply - any thoughts on the foil vs non-foil foam for on top of the slab?

    1. GBA Editor
      Martin Holladay | | #9

      Josh,
      Foil facing is unnecessary but harmless in this location.

  8. Josh Klemm | | #10

    Good Morning,

    Thanks for the latest reply.

    A couple of quick final questions

    1) I’ll be picking up the EPS for the exterior stem wall insulation. Given current temps in NY aren’t above 40, which most adhesives I see need it to be, is there any detriment to using mechanical fasteners? Or, ideally, is there an adhesive that will work on this surface in colder temperatures?

    2) We will be building a wrap around deck come warmer weather, is there any risk in having my PT board directly against the EPS as it will be bolted into the stem wall? I guess my bigger concern is that am I putting too many bolts and fasteners into my stem wall with the EPS and deck.

    3) Back to the interior - do I need to leave space between the OSB/Plywood subfloor that will go over the foam and the walls if I fasten it to the slab? Do I need to leave space between the subfloors and walls if I donthe two later route and don’t fasten it to the slab? Just trying to make sure any expansion doesn’t buck the flooring

    Many thanks in advance, and have a wonderful weekend.

  9. GBA Editor
    Martin Holladay | | #11

    Josh,
    Q. "Is there any detriment to using mechanical fasteners [to attach EPS to concrete]?

    A. No, there is no downside. For more information, see "Fasteners for Concrete and Brick."

    Q. "We will be building a wrap-around deck come warmer weather. is there any risk in having my PT board directly against the EPS as it will be bolted into the stem wall?"

    A. You don't want to have any intervening rigid foam between a structural deck ledger and the concrete. The best solution is to support your deck on independent footings and posts rather than using a deck ledger.

    For more information on this issue, see these three relevant Q&A threads:

    "Install deck ledger with 2″ rigid foam on exterior?"

    "Are there any new strategies for attaching a porch roof and deck to a house?"

    "Porch Ledger Attachment with Exterior Rigid Foam"

    Q. "Do I need to leave space between the OSB or plywood subfloor that will go over the foam and the walls (if I fasten the subfloor to the slab)? Do I need to leave space between the subfloor and walls if I don't fasten it to the slab?"

    A. In either case, a 1/4 inch gap or 1/2 inch gap between the subfloor and the wall is a good idea. Caulk the gap. (The gap will be covered by your baseboard.)

  10. Josh Klemm | | #12

    Martin,

    First, thank you. Second, whatever amount they propose for a raise at your annual review this year, guide them to your helpfulness on this thread and request it be doubled.

    I can't say thank you enough for your guidance on this.

  11. Deleted | | #13

    Deleted

  12. Josh Klemm | | #14

    Okay okay - final question...

    I’ve come across reclaimed insulation for a very affordable price. Is there any reason not to use reclaimed insulation for the on top of slab portion? I found a few other threads on here around reclaimed insulation and things that one should look for, but what I didn’t see is if there is any reason not to use it for interior on top of slab purposes.

    I would think it’s okay, particularly given it will have the subfloor and flooring between it and the inhabitants, but wanted to ensure I wasn’t missing any potential pitfalls in pursuit of savings.

    Thanks.

    1. Andy S | | #15

      Just make sure you're getting a consistent thickness with the reclaimed foam.

  13. Josh Klemm | | #16

    Good Afternoon,

    I am at a point where I will be installing the osb/plywood above the insulation, and had a couple of questions regarding the objective of the subfloor in each of the approaches I read through in the above links.

    It seems that the general approach is either (1) 3/4" sheet fastened to the slab, or (2) 1/2" sheets fastened to one another. I was under the assumption that this was to prevent any cupping in the sheet itself to ensure a level floor, but then read in those same links references to using one sheet of backerboard (when appropriate as the subfloor) due to its weight.

    So my question is, is it a weight goal that is trying to be achieved, a rigidity goal that is trying to be achieved, or both?

    Also, is it a terrible idea to think maybe a product like PL300 could be used with the 3/4" sheet to fasten it to the foam as opposed to using screws into the slab?

    I'm trying to find an approach that provides the simplicity of the two sheets fastened to one another, with the thin-ness of the single 3/4" sheet.

    As always, many thank yous in advance for your help.

  14. Expert Member
    Dana Dorsett | | #17

    >"Also, is it a terrible idea to think maybe a product like PL300 could be used with the 3/4" sheet to fasten it to the foam as opposed to using screws into the slab?"

    With that approach what is going to keep the foam from lifting off the slab when there is curling/cupping forces driven by humidity changes the plywood? A screw will clamp it all in place even under moderately high pressures. Adhesive doesn't. Even if the foam is glued to both the slab and the plywood the likelihood of the adhesion to the slab failing in places is high.

  15. Deleted | | #18

    Deleted

  16. Josh Klemm | | #19

    In the flooring detail, if I were to want to pursue tile flooring I have a question relative to the backer board.

    From other articles on the site, it sounds as though the weight of the backer board itself is enough to hold the floor in place just by laying it over the foam, but if we want to affix tile to the backer board, would our detail look like (from slab up):

    EPS
    1/2" OSB
    Backer board (fastened to OSB)
    Thinset
    Tile

    I say this, as the two sheets of OSB sub-flooring method also referenced in those articles has them affixed to each other and floating over the foam, and with the increased weight and rigidity of the backer-board I would presume it would be the same approach. Additionally, I am presuming we would still need some OSB to fasten the backboard to, and that it wouldn't simply be able to be floated over the foam and then have tile installed above, correct?

    Thanks in advance for any insight you can offer.

  17. Josh Klemm | | #20

    Any thoughts on if the above mentioned flooring detail is adequate, or if the osb would still need to be fastened to the slab as if installing a single 3/4” layer?

    Thanks in advance for any feedback you can offer.

    1. Expert Member
      Michael Maines | | #21

      Josh, I'm usually willing to try unconventional assemblies, and the current Fine Homebuilding magazine shows my detail for a floating wood floor over foam. But I would never come close to considering the assembly you propose. Either run two layers of overlapping plywood or OSB, or fasten the OSB to the concrete. Otherwise your tile installation will fail.

      1. Josh Klemm | | #23

        Michael,

        Thank you again.

        For these purposes are the typical 1/2” sheets when doing multiple staggered layers acceptably substituted for 7/16” I’m having trouble sourcing 1/2” locally. The same would apply to the 3/4” single layer fastened to the slab - is 23/32 safely substituted?

        Thanks.

        1. GBA Editor
          Martin Holladay | | #24

          Josh,
          Yes, 7/16 inch is the current version of 1/2 inch. (Welcome to the world where a one-pound jar of coffee contains 13 oz.)

          Close enough -- and that's the product that builders still call "1/2 inch OSB."

          Ditto for 23/32" instead of 3/4".

  18. Josh Klemm | | #22

    Michael,

    Much appreciated, thank you!

  19. Josh Klemm | | #25

    Good Afternoon GBA Community,

    I first wanted to give feedback on how well the two layer sof OSB feel underfoot over 2" of EPS - I highly recommend this approach, and given that 2 layers of 1/2" is really only about .1" thicker than a single 3/4" layer, I believe the time savings is well worth it the nearly negligible trade off in loss of ceiling height.

    Now for my question, I am at a point where I was going to install flooring, but came across another article on this method of installation that stated all sheets of OSB, on each of the two layers, needed 1/8" of spacing between at the perimeter of each sheet. I did not see this in the GBA articles I've read, and am wondering if I need to go back and uninstall the subfloor, space it out, and proceed. At this time I have 1/2" spacing around the subfloor on all perimeter walls

  20. Josh Klemm | | #26

    Good Afternoon GBA Community,

    I first wanted to give feedback on how well the two layer sof OSB feel underfoot over 2" of EPS - I highly recommend this approach, and given that 2 layers of 1/2" is really only about .1" thicker than a single 3/4" layer, I believe the time savings is well worth it the nearly negligible trade off in loss of ceiling height.

    Now for my question, I am at a point where I was going to install flooring, but came across another article on this method of installation that stated all sheets of OSB, on each of the two layers, needed 1/8" of spacing between at the perimeter of each sheet. I did not see this in the GBA articles I've read, and am wondering if I need to go back and uninstall the subfloor, space it out, and proceed. At this time I have 1/2" spacing around the subfloor on all perimeter walls

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