GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X 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

Under Slab Vapor Barrier

Jarret | Posted in Expert Exchange Q&A on


I have a somewhat different situation on this one.  We are in Los Angeles. I have a 3 story new build residential construction, which includes a basement.  The basement foundation walls are 12″ thick. The first level exterior walls are 8″ thick board formed concrete. The 3rd floor is stick framed.  Due to the load coming from the concrete walls, the entire footprint of the house (with the exception of two small slab on grade areas) consist of an 18″ thick footing. I do not know why this was not designed as mat footing (which it basically is), but on top of our 18″ footing we will pour a 5″ slab, and on top of that we will pour a 2″ decorative concrete slab (so 25″ thick concrete at the end of the day). The finished floor of the basement is polished concrete.  The soils engineer would not allow a vapor barrier under the footing.    In normal slab on grade conditions, I’d put down a 6mil or thicker poly on top of gravel under structural slab and then maybe a 4mil poly slip sheet between the fully dry structural slab and the 2″ topping slab. I’m wondering how I should adjust my practice given that everything is now being poured on top of 18″ footings. The soil drains very well.  It is still Southern California where temps rarely get lower than 45 degrees, and if we are lucky, we get 6″ of rain a year.  Appreciate any thoughts…

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. Expert Member
    PETER Engle | | #1

    First, I'm not sure why the soils guy is against putting the VB under the footings. It should have no effect on the foundations, other than to keep the footings dryer once the initial moisture slowly creeps up the walls and dries to the outside. Second, why the 5" slab with another 2" of decorative concrete? Do you have expansive soils that require structural slabs? If not, you could probably go with a thinner slab and less concrete. Or no concrete at all, as mentioned in several articles here. Third, why pour the concrete slabs over the footings? With most basements, the foundation walls are built on top of the footings, with the concrete slab "floating" inside the walls. The slab helps to brace the bottom of the walls, but isn't pinned to them.
    If the engineer insists on soil contact with the footing, you could simply run the poly VB across the top of the footing instead of under it. This would keep the upper walls and slab dry. If that's still a no-go, you could paint the top of the footing with any concrete waterproofer: Xypex, Thoroseal, even Dry-lock. These seem more "concretey" than plastic or rubber films and are often an easier sell than one of the membrane products.
    One big caveat here (or two, really): So.Cal. is a serious seismic zone. I don't do seismic design, and there might be some good reasons for what your engineer wants to do. And because of the seismic issues and the generally warm weather, basements are rare in So.Cal. It is possible that your engineer and/or builder are not at all familiar with their construction and so they are being ultra-conservative (at your expense of course).

  2. Expert Member
    Michael Maines | | #2

    I would not forgo a vapor retarder. It sounds like your best option might be to place it between the structural slab and the finished slab, though I'd worry about having a 2" slab without it being bonded to the structural slab. Have you done that before? I'd consider a thicker decorative slab, maybe 3" thick, so it can safely float above a vapor retarder.

  3. Jarret | | #3

    Thanks all. Yes both of your approaches are being considered, just wanted to be sure I was not alone in my assessment. I will have to coordinate with the structural engineer to determine if I can use either poly at top of footing, or a roll on application. If that is a no go, I think I would install a 6mil poly between structural slab and topping slab. A couple answers to your questions:

    1) we do ultra modern custom homes. everything is flush. There are no door thresholds. Door sills are flush with interior and exterior finish floor, curbless showers, etc.
    2) the 5" slab allows me to set in-floor receptacles and run conduit in floor. I cannot run conduit through the footing outside of rebar. It also allows us the room to create floor drops in certain areas of the home where thicker floor build up or shower sloping is needed for large showers. Basically it provides room to play with and allows some tolerance to deliver all these flush conditions. The 2" topping slab provides workability for a fine polished and colored concrete. It also provides cheating and smoothing as you flush out to door sills perfectly with sill pans that have back damns at 1-1/2" in height. You might not be level throughout the whole house and maybe you have to cheat between a 1/2" from east end of house to west end of house to flush out at a different door type of door. Last, 5" slab protects against cracking of the topping slab, something you don't want in multi-million dollar custom homes.
    3) S.E. will not allow vapor barrier under the footing, because two stories of concrete walls are tied directly to the footings and they want friction in the event of seismic activity.
    4) Yes, we typically pour topping slab over 4mil poly. You want it to float and move separately of the structural slab.

    Thanks again for the suggestions.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |