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Community and Q&A

New slab with radiant heat

Torsten Budesheim | Posted in Energy Efficiency and Durability on

As part of our remodel north of San Francisco (ZIP code 94941), we are converting about 700SF of our lower level into living space. The existing slab has been removed we excavated a few additional inches to increase our ceiling height, and we are getting ready to put back a new 5″ slab, in which we will also add radiant heat tied to the rebar.

I just now came across some information, such as Polyethylene Under Concrete Slabs and Polyethylene under slabs – no blotter sand, that are worrisome to me, since they seem contradict the design that our architect has come up with.

What our architect has called for is (from top to bottom):

  1. Conc Slab, SSD
  2. 1″ Rigid Insulation
  3. Min 6Mil WPM
  4. 2″ Sand
  5. 6″ Crushed Rock
  6. 4″Dia Perforated Pipe Foundation Drain

The slab is going to be 5″.

I have also attached an image of the new slab detail.
New Slab Detail

Is the above the right layer structure or do we need to revise?
One thing we do not want to change is the concrete slab and radiant heat in it. So I’m basically interested in what should go below the slab at this point.

We really don’t want to have to worry about ever touching that slab again!


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  1. User avatar GBA Editor
    Martin Holladay | | #1

    You might want to urge your architect to read the same articles you did. You might mention that the American Concrete Institute no longer supports the wrong-headed idea that blotter sand is beneficial. In the April 2001 issue of Concrete International magazine, the ACI advised that for slabs with vapor-sensitive coatings (i.e., virtually any kind of flooring), the vapor retarder should be installed directly under the slab, with no intervening layer of granular fill. In November 2001, the ACI included the updated recommendations in a new edition of ACI standard 302.1R-96.

    So, the sand layer shouldn't be there. And 1 inch of rigid foam isn't enough under a heated slab, even in California. You need at least 2 inches -- more would be better -- of rigid foam under your slab.

  2. User avatar
    Dana Dorsett | | #2

    If they insist on 6 mil poly under the foam, there's no real down side to also putting poly above the foam.

    There are several issues with the detail I don't like, not the least of which is the lack of thermal break between the slab and the grade beam foundation. Not only does there need to be more than an inch of foam (2" EPS would be the minimum for a heated slab, 3" better). Even with an unheated slab you'd want 1" at the slab edge for comfort, and maybe 1" sub-slab on the perimeter, but for a heated slab you'd want 2" (R8.4) under the slab, and at least 1.5" (R6.3) between the slab and the grade beam.

    That "floats" the slab mechanically as well as thermally, which probably makes it safer from a seismic /structural point of view, but consulting local codes on that would be prudent. If the beam & slab must be in contact for some reason, 1.5" of EPS on the exterior would be called for.

  3. Torsten Budesheim | | #3

    Thank you for the feedback. I will be looping back with the contractor and the architect to get at least to the 2" rigid insulation. Could I sacrifice the sand in that case, since I would prefer to not have to sacrifice ceiling height?

    However, in regards to decoupling the new slab from the existing foundation wall, I think that will be difficult as the engineer has called for - what appears to me to be, - some strong connection:

    There is the detail on how to connect to the existing foundation wall on one side.
    There is the detail on how to connect to the new stem wall on the backside of the area, which separates the new space from an upward sloping crawlspace that is covered with the old slab.
    There is the detail on how to connect to the new foundation wall fill-in where previously a door had been.

    I did read somewhere that in my climate, decoupling the slab from the side foundation walls, while helpful, is not as critical as in colder climates. Is that correct?

    And in regards to 2" insulation and install pex tubing with the slab, given the above, I am now considering the Creatherm T45 or S20. Any thoughts or comments on those? Are there other, similar products that would be less expensive?

  4. User avatar
    Dana Dorsett | | #4

    Decoupling the slab from an uninsulated foundation wall isn't as critical in your very temperate US zone 3C climate if it's just a slab, and slab edge is not required in zone 3 by the IRC.

    But when the slab becomes the HEATING RADIATOR it sure matters! If it were a cast-iron radiator it's the thermal equivalent of exposing the ends of the radiators to the outdoors.

    Creatherm looks like it would work OK, but by putting the tubing at the bottom of the slab it as opposed to in the middle the slab is a bit less responsive- there' s a slightly bigger time lag. In practice that probably doesn't matter much. I have no idea how it's priced, but it saves labor on the heating installation end. Sheets of smooth 2" thick Type-II EPS under slabs typically runs 75-85 US cents per square foot, installed, Type X (2lbs density) more like ~$0.90-1/ft^2 . Type-X is preferred by some radiant heating contractors for it's better staple retention, which allows them to simply staple the tubing to the foam (again, at the bottom of the slab) rather than to tie it to the metal reinforcing layer, which is more labor intensive.

  5. User avatar GBA Editor
    Martin Holladay | | #5

    Q. "Could I sacrifice the sand in that case?"

    A. It's not a sacrifice, it's an improvement. Lose the sand, already! The faster, the better.

  6. KEVIN ZORSKI | | #6

    Torsten - The other problem with the sand is: what's to prevent the sand from, over time, filling all the voids in the crushed rock layer, and creating possible cracks in your slab? I agree with Martin - Lose the sand!

  7. Torsten Budesheim | | #7

    I will go without sand, which also gives me literally enough room to do 2" insulation.

    That does, however, raise tow more questions:

    1.) What thickness and type / brand of vapor barrier to use, if I'd use the Corning XPS Foamular 250?

    2.) Where to put the vapor barrier in case I go with e.g.,

    the 2-inch Crete-Heat, which is listed with a Perm Resistance (ASTM E96-00) value of .51 perms, or
    the Creatherme, which is listed with a Water Vapor Permeability ASTM E-96 value of 0.36 Perm-Inches, Max.

  8. Andrew C | | #8

    Vapor retarder above foam?
    The building science answer is to lose the sand and put the vapor retarder directly beneath the concrete. I thought that from a practical standpoint, another reason to put the vapor retarder above the foam was to prevent the possibility/probability of having sections of foam float up into the concrete during the pour. So you'd never put the foam on top of the vapor barrier, right?

  9. David Meiland | | #9

    I'm a bit old-school on heating slabs, and wouldn't be looking at those panel products unless there was a really compelling reason. There are many products that promote ease of installation, but this is already easy. Rip a 45-degree bevel on rigid insulation and install it against the perimeter walls with the bevel at the top. Install crushed rock without fines as your slab base, compacted very flat. Place rigid insulation over the rock and then your VB over the insulation. Install a mat of #3 rebar on 1-1/2" dobies. Tie the tubing to the top of the rebar. Pour a 4-1/2" slab. When you make your 1" deep control cuts, you will not hit the tubing

  10. Richard McGrath | | #10

    Torsten ,

    Sorry to have not gotten back about the radiant design yet , have been very busy .

    As others have stated , Lose the sand , it is of no benefit . Should it eventually get wet it will actually hurt the performance . Water within six feet of a slab in contact with earth must be accounted for in the calculations .

    I have used this product for years and have found through performance and rainfall in slabs that all the claims / tests are accurate . The edge insulation should also not be ignored or omitted , it is missing in the drawing .

    Once again Dana has touched on a very important point . Tubing belongs not in the bottom of the slab . Expansion joints are often the topic when this discussion takes place . First , there are 2 types of slabs , those that are cracked and those that have not cracked yet . Nature of the Beast , if it were not so we would not need expansion joints . Expansion joints are just a place for the slab to crack at instead of wherever it wants due to conditions . These joints should have their location identified and tubing should dip below for 6" on either side or if they are to be saw cut the slice needs not be more than a 1/2 - 3/4 " deep .

  11. Torsten Budesheim | | #11

    Thanks everyone for the comments. Regarding tubing at the bottom vs. in the middle, I like the idea of it being easy to manage and the installation is simplified, there is less risk of damaging the PEX during pouring the concrete as well as down the road. And I would be fine with a delay in response and even be ready to give up a little of the optimal performance.

    That's why I like the concept of Chreatherme or Crete-Heat, but it sounds and looks like that those are not really an option, since there wouldn't be a vapor barrier/retarder between the insulation and the concrete?

    Richard, thank you for suggesting the NOFP materials, if I'd still keep the sand and would not be able to do the 2" insulation, I'd be very likely going with the Barrier X5 now :) But since I'm loosing the sand, I have the space to go with the 2" Foamular 150 or the competing Dow product, whichever one is more readily available here.

    In regards to adding a thermal break between slab and foundation wall, how thick of a break do we need here? Would a 1/2" GreenGuard® Type 4 XPS Insulation Board do the trick?

  12. Torsten Budesheim | | #12

    For the insulation of the inside perimeter foundation and to create the thermal break between slap and perimeter foundation, does there a vapor barrier belong somewhere?
    FYI, there is not proper drainage or insulation on the outside of the perimeter foundation and I'll deal with that down the road a few years from now, just not now...

    I was thinking of using INSUL-DRAIN Drainage Board, with the channels agains the concrete and then a vapor barrier - vapor retarder? - over the INSUL-DRAIN to avoid any moisture problems.

    Does that make sense or is there a better way to avoid moisture problems?
    What, if any, vapor barrier/retarder product is recommended for this perimeter foundation application?

  13. User avatar GBA Editor
    Martin Holladay | | #13

    I'm attaching a slab-on-grade detail to show you one way of installing vertical insulation at the perimeter of the slab. (It's not a great detail, because it is incomplete -- it doesn't show the crushed stone layer or the polyethylene. But it gives you the general idea.)

    I don't think it matters very much how you terminate the polyethylene (not shown) at the slab perimeter. Even if there is no polyethylene between the edge of the slab and the foundation wall, the rigid foam will be an adequate vapor retarder.

    You refer to "moisture problems." If your existing foundation has specific moisture problems, you should describe them. Unless you tell us more about these moisture problems, it's hard to provide advice.


  14. Torsten Budesheim | | #14

    Thanks Martin. There haven't been moisture problems really. The area we are converting into living space was previously used as a workshop area, with the concrete foundation walls being left exposed. There is only one or two very small areas, where the concrete was patched and minor moister residue can be seen, but the foundation has been there since 1953 and I'm guessing that might be within an expected range over such a long period?

    So based on the before mentioned, there would not be the need for polyethylene to be added to where I will install 1" rigid insulation against the existing foundation walls?

    Aside from the existing foundation walls, we will pour a new stem wall agains a sloped crawl space. Should there be any polyethylene added, e.g. on the back of the stem wall where it faces the sloped crawl space? Or will the 1" rigid insulation between slab and new stem wall be all that is needed?

    And is the thickened slab edge going to be a challenge? I'm just wondering how feasible it is going to be to get the rigid insulation laid out that way, meaning under the slab and under/around the thickened slab edge, and then keep it in place during the concrete pour?

    Based on you termination remark for the polyethylene, I will plan on wrapping the polyethylene just a little bit around the thickened slab edge, but not up to the top of the edge.

  15. D Dorsett | | #15

    An inch of XPS is not much of a thermal break. It's less than R5- which better than nothing I suppose, but not exactly great. A 2x6 above grade wall has whole wall performance of R14 or better, so an R5-ish slab edge on the heating RADIATOR is still a very weak point in the thermal design. If you added the 1" of exterior foam on the foundation wall AND the 1" XPS between the slab and footing you'd be at about R10-R11 for that stripe, which is more reasonable.

    Another issue not yet addressed is the higher thermal mass of a 5" slab, which makes it less responsive as a radiator, and more difficult to control to maximum comfort. It's somewhat worse if the tubing is at the bottom of the slab rather than if positioned 2" below the top surface, as previously mentioned. If the slab thickness can be reduced to 4" it will be a bit easier to control with lower magnitude room temperature overshoots / undershoots.

  16. User avatar GBA Editor
    Martin Holladay | | #16

    Dana Dorsett gave you good advice about the need for more R-value at the slab perimeter.

    I hope that there are competent architects and competent concrete contractors in your area to help you with these details. You don't really want professionals who don't understand insulation issues to be working our your house.

    Q. "Is the thickened slab edge going to be a challenge? I'm just wondering how feasible it is going to be to get the rigid insulation laid out that way, meaning under the slab and under/around the thickened slab edge, and then keep it in place during the concrete pour?"

    A. You can do it. I have. You want a crushed stone layer under the rigid foam; you want a reasonable slope where the slab makes its transition from thick to thin; you want to cut the rigid foam carefully; and you want to use a high-quality tape to hold the rigid foam in place if the rigid foam shows any signs of moving or sliding.

    By the way: I don't understand why you need a thickened-edge slab here. After all, you have a stem wall to bear the load of the wall and roof. Why not just install a slab with a uniform thickness (usually, that would be 4 inches)?

  17. User avatar
    Dana Dorsett | | #17

    I had the same question as Martin about the slab design. Slab on grade construction is commonly built with grade-beams (thickened edge) or stem walls on footings, but I've never seen it with both. Supporting the edges of the slab directly by the footings for the stem walls tends to make the slab more prone to cracking than letting it float.

  18. Torsten Budesheim | | #18

    It occurred to me that there may be an issue with the whole design: we are living in an area - a red zone possibly? - with sub-terrain termites and as part of the inspection when we bought the house it was even called out that there were signs of previous activity and treatment.
    We found proof of previous activity upstairs during the demo and even got to "meet" them when we got to demo the upstairs bathroom: sub-terrain termites.

    With disconnecting the slab from the perimeter walls and planning to use XPS, am I just opening the door to the sub-terrain termites invading my house through the perimeter insulation?

  19. Torsten Budesheim | | #19

    Any thoughts on the whole sub-terrain termites issue? My approach will be to have the vertical insulation end 1" below the concrete surface, to prevent them from ever coming through.

    Does the termite issue also require a change in the order of the layers, meaning, should the 6" Mil Polyethylene go below the 2" rigid insulation because of it, to help protect the rigid insulation from the sub-terrain termites?

  20. User avatar GBA Editor
    Martin Holladay | | #20

    I will defer to other GBA readers on termite questions, since I live north of the termite line, and have little experience in termite issues.

    That said, I know the polyethylene is not a termite barrier. If you want a termite barrier, you need a stainless-steel mesh like TermiMesh.

  21. Jonathan Lawrence CZ 4A New Jersey | | #21


    You can buy treated EPS to keep the termites out. I agree with Andrew about foam below the vapor retarder. There is another reason to do that beyond what Andrew mentioned, but it is escaping me right now. Have you thought about Perlite? I spoke with Rich Pedrantii yesterday who designed the Scranton PH and he likes Perlite under slab. Some folks even think you can use it in place of compacted gravel.

  22. User avatar
    Dana Dorsett | | #22

    Compacted clean sand is a fairly effective termite barrier (are we back to that now? :-) )

    Theoretically termites can move through sub-slab EPS, but they don't- the tunneling is usually better elsewhere. Installing sheet copper or stainless mesh between the sub-slab foam and the slab edge foam, wrapping under the slab edge foam is an effective termite barrier too. Copper is toxic to the gut flora that ants, termites, and wood boring wasps and bees need to digest wood. Even minor amounts of copper leaching into the foam or soil will have an impact. But stainless mesh also works as a mechanical barrier.

    Short of a mechanical or chemical barrier, as long as there is an inspectable strip between the slab edge foam and the foundation sill or wood finish floor termites using that path would be easily detected.

    Thinking a the edge of the box (if not completely out) a more expensive solution would be to install a 6-8" wide strip of low density aerated autoclaved concrete as the slab edge insulation, which would also be an effective mechanical barrier against insects.

    Depending on your intended finish floor and kickboard trim selections you may be able to design-in effective mechanical barriers above the slab edge foam by using only dense, hard, non-wooden materials.

  23. Torsten Budesheim | | #23

    Wow, the sand comment brings me right back where I started? Fantastic!

    And, getting the feedback from my the engineer on the project that the thickened slab edge is required and that weighing all the pros and cons, the existing structure, and the location being where it is, he recommended to eliminate the vertical insulation, connecting the slab to the foundation at all locations, and do a monolithic pour.

    In a final attempt to at least reduce the surface amount where thermal bridging will occur, I came up with the attached sketch. But after thinking about it more, I am now question whether the result would even justify the effort, given that the two areas that are truly being impacted are only 203 SqFt (two exterior walls) and 147 SqFt (one exterior wall).
    Add to that the fact that the smaller room is only a guest room and the other room, well, my office/ guest, and the fact the many months out of the year I don't even heat at all, not during the day, a.k.a. office hours, and I am ready to make my peace with the (heat) losses at this point.

  24. User avatar GBA Editor
    Martin Holladay | | #24

    You need to install insulation on the exterior side of your concrete stemwall, down to the bottom of the footing, and to protect the above-grade portion of the insulation with metal flashing or stucco.

    If you have any doubt of the importance of this insulation, read Kohta Ueno's recent comment here:

  25. Torsten Budesheim | | #25

    Quick follow-up on the expansion joints. (Tubing will be tied to the rebar at the middle of a 5" slab.)

    These joints should have their location identified and tubing should dip below for 6" on either side or if they are to be saw cut the slice needs not be more than a 1/2 - 3/4 " deep .

    Do you do/use anything to keep the tubing dipped down and in place during pour?

    Is there any other protection needed to the tubing at control joints? (E.g., Uponor suggests a sleeve or larger diameter tubing for 6” on both sides of the expansion joint if the tubing has to stay in the slab at those points, which is the case here.)

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