GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Alternatives to foam under slab or no slab?

Steve Young | Posted in GBA Pro Help on

Are there any alternatives to foam under a slab that might be “cost effective”. I realise that CE is a very loaded term, so don’t get bogged down in that term.
Obviously, I am not interested in VIP’s. But how about the cost and performance of Perlite? Are there other alternitives?
I am looking at a Passive House in a 5, maybe 6, climate zone.
XPS may be available to me at a reduced cost because my wife works for a large company that makes the stuff. Am I wasting my time looking at alternative insulation products if I can get XPS at a discount?

What about not using a slab at all? Would it make more sense to build on pillers and insulate the floor? Has anyone tried that? I don’t think I have seen this approach in any of the PH books out there.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    Alternatives to a slab include a basement, a crawlspace, or a pier foundation. If these ideas are new to you, you should read about them here:

    Foundation Types

    Crawl Spaces


    Pier foundations

    I know of at least one Passivhaus with a crawl space foundation; in that house, the joists bays above the crawl space were insulated with blown-in fiberglass. Read more about the house (the Freas house) here: More Passivhaus Site Visits in Washington State.

  2. Steve Young | | #2

    I appreciate your answer, but I may not have been clear in my question.

    I am trying to keep my material costs down in the construction of the house. In that vein, if I choose to go the route of a slab - presumably cheaper and less detail orientated than a full basement, but maybe more expensive than a crawlspace - I am curious to know if perlite, or something else, would be a less expensive alternative to foam insulation.
    The other part of my post involved asking about building on piers (I used the term "pillers"). It doesn't seem to be that common, except here on the Gulf coast, where flooding is a serious concern (or in permafrost, if I recall correctly). Are there particular issues that have to be addressed? Does it make sense economically?

  3. GBA Editor
    Martin Holladay | | #3

    Pier foundations are common on vacation homes, additions, hillside homes, and homes along the Gulf and Atlantic coasts. They can be affordable, but they have their disadvantages. In colder climates, it's tricky to keep pipes from freezing if the house has a pier foundation; that concern may not apply to you.

    If you are interested in a pier foundation, I suggest that you read the article I linked to.

  4. GBA Editor
    Martin Holladay | | #4

    Information on the use of perlite under slabs:

    GBA links:

    Perlite as insulation

    Trekhaus: A Passivhaus Duplex in Oregon (see comments below article)

    Other links:


    Perlite as underslab insulation

    Leap Frog House

  5. Aj Builder, Upstate NY Zone 6a | | #5

    You should be able to get seconds foam for little to nothing. Perfect use of irregular foam.

    Slab and foam
    build that home.

  6. Expert Member
    Dana Dorsett | | #6

    Reclaimed roofing EPS (it's usually Type-I density, sometimes Type-II) can be sourced from a number of places at ~1/4-1/3 the cost of virgin stock and has sufficient compressive strength for insulating under residential slabs, but you'd need higher-density goods if used under footings. (If you can't find a local source, will ship if you need a truckload of reclaimed roofing EPS.) Sometimes you can get reclaimed XPS through these sources too, but it's a bit rarer.

    At the typical PassiveHouse R30-R50 slab-R XPS is a SERIOUS net-loser on the greenhouse gas front. At those levels the blowing agents used for XPS will exceed (by more than an order of magnitude) the greenhouse gas potential of the fuel it saved over 200 years . Not so for virgin EPS even at R50 under a slab in US zone 5 (~50F subsoil temps). The hydrocarbon gas blowing agents for EPS have about 1/20 the greenhouse potential of the HFCs used for XPS.

    But with reclaimed XPS the environmental hit has already been taken- you're reusing it. Stack it as deep as you like and B-happy- better to bury it under your slab than in a landfill (or worse, burn it in an incinerator.)

    If on the very odd chance you happen to live in a volcanic zone where pumice is being mined, you get something on the order of ~R2/inch out of pumice, and it can pretty cheap if you don't have to ship it very far.

  7. Aj Builder, Upstate NY Zone 6a | | #7

    In fact, foam that whole dang home. Join this site and design a double thick exterior foam insulated home. Above ground seams need to be tight and taped. Read up, use search box above, join site for videos and much more.

    With foam access you have a great advantage. You just need application knowledge.

  8. Steve Young | | #8

    To MH; I did search your site for this information before posting my question, but none of your most-informative links came up.
    Once again, thank you for your attention.

  9. Steve Young | | #9

    To AJ At one time, I had access to free ISO foam - the seconds, that is. Unlike XPS, it is a one way chemical process and there seemed to be a lot of waste. I love the stuff for its insulation value and "fire-proofness". XPS scrap and off spec material is just fed back into the system. It is also like putting gasoline on the outside of your house (if used in the walls, that is).
    When I had access to the ISO foam, I was planning on encasing my structure, but now I need less expensive alternatives than if I had to pay retail.
    To DD; Thank you for your information, in particular the InsulationDepot link.
    I won't be anywhere near any recent (geologically speaking) volcanism, but I will keep the idea in mind. Pumice probably isn't that much different than slag, though - something to look in to.

  10. Aj Builder, Upstate NY Zone 6a | | #10

    Steve, XPS at the right price, use it. You are already via the wife's paycheck. Onward ho my man. Get that home up unless you believe in reincarnation. Then you have lots of time and many second chances for do overs.

  11. Roger Anthony | | #11

    Steve, keeping in mind that heat always moves to cold, and I presume you are interested in keeping the heat from your future home from escaping to the ground?
    Then you should consider placing the XPS sheets above the concrete slab.
    Having the XPS above the slab with a fully floating floor, will stop the heat from your comfort zone from escaping downwards. Your feet will also keep warmer.

  12. Expert Member
    Dana Dorsett | | #12

    A PassiveHouse in US climate zone 5 would typically take 6 to 10 inches of XPS (or 8-14" of EPS) at the slab to meet the thermal spec. Putting it all above the slab would require a taller foundation wall, more concrete, deeper footings, etc, and it would put the thermal mass of the slab outside the thermal boundary, incurring hit in overall performance that has to be made up elsewhere (higher R assemblies or lower-U or smaller windows.) It would also complicate floating the finish floor. With R30-R50 under the slab the slab will usually be less than 1F from the average room temp in a high-R building.

    Putting an inch or two of XPS above the slab and floating the floor can the right way to go in a retrofit though.

  13. Expert Member
    Malcolm Taylor | | #13

    Roger, I am a bit confused. How does the location of the foam affect the R value of the floor?

  14. Aj Builder, Upstate NY Zone 6a | | #14

    Malcolm, Foam inside concrete walls or above slabs would lower interior mass which is nice if someone wants to change interior temperatures quickly such as a weekend home. Other than that refer to what Dana posted. Put your foam below the slab with 6 mil plastic over it against the concrete pour. Details available by joining GBA.

  15. Expert Member
    Malcolm Taylor | | #15

    Thanks AJ I'm familiar with detailing slabs. My question was why Roger thought the location of the foam made any difference in the floor system's propensity to lose heat to the surrounding ground.

  16. Aj Builder, Upstate NY Zone 6a | | #16

    Change in foam location changes mass location. That's it basically. Done. Finito. R is R above or below. The mass location has advantages and disadvantages and or no change. Refer back to my last post and Dana's for what matters location wise.

  17. Roger Anthony | | #17

    The cement industry needs to promote the sale of cement, they have hit on the idea that concrete is
    the perfect heat store.
    It has mass and weight and can absorb a great deal of heat (if its available) from the sun and offers a slow release overnight if you turn the heating down low enough for it to happen.
    The problem is that concrete does absorb a lot of heat, but unlike water and air, the heat doesn't rise
    to the top it travels inwards to the coldest places in every direction.
    The surface of concrete will adjust to follow the ambient temperature, which within a home is over a
    very narrow band, perhaps an air temperature of 20 to 22C. Unfortunately your foot has a temperature of roughly 30C a large difference, meaning that heat is always moving from your foot
    into the concrete. And never from the concrete into your foot, the floor would need to be of higher than foot temperature to be noticed as warm.

    Covering the concrete with wood, helps as it slows the passage of heat. Fitting one inch sheets of
    polystyrene or similar (thicker is better) between the concrete and a floating timber floor slows the
    heat loss even more.

    Placing the insulation below the slab will never slow the loss of heat to the point where it will be noticed by your foot.

  18. Ethan ; Climate Zone 5A ; ~6000HDD | | #18

    Steve - it is probably to late, but you should check out this JLC article: "A Basement Floor Without Concrete"

  19. Expert Member
    Dana Dorsett | | #19

    Probably too late? It's only been five years! :-)

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |