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

What is the best way to insulate concrete basement floor and keep it waterproof?

CamillaGN | Posted in General Questions on

I am remodeling in North Georgia a lake cabin by adding a second level using a steelframe prefab that has posts in a few locations already bolted down into foundation.
Existing slab is not insulated and has floor drains in kitchen, mud room, and bathroom that is functional and will be reused.

I want to insulate the top of concrete floor with 15ml plastic + 1″ of structural foam+??? something+ tiles in kitchen, mud room and bathroom  and other water proof option for living space.
I have seen the plywood options for the wood floors, but
1: what is the option for the subfloors when floors would be constantly wet?
the slab gets very cold, and
condensation in summer as doors are opening all the time.
2: What can I transition  wood to tile with in an open floor plan on top of the foam board?
3: Do I build the weight bearing walls on top of the structural foam or directly to the concrete? I am  using 2×6 for internal weight bearing. Counter intuitive for me.

Your experience and feedback is greatly appreciated.

Thank you!

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. user-2310254 | | #1

    I am replying to your post to give it a bump.

    1. CamillaGN | | #3

      Thank you!

  2. GBA Editor
    Brian Pontolilo | | #2

    Hi Camilla,

    If you are planning to lay down a poly vapor barrier and rigid foam before the subflooring, condensation will be minimized and if it does occur will happen below the foam, so your subfloor should not get wet. If your basement is leaking rain or groundwater, that's another issue. This article should be helpful:

    Tile floors can be installed in many ways, all intended to either create a rigid substrate (usually concrete board) or to isolate the tile from the moving floor system below (an uncoupling membrane). If you install a floating subfloor, tile is a risky proposition, because the floor will be somewhat spongy. If you install sleepers, you can approach the tile like any other tiled floor.

    Load bearing walls need to be installed above a footing. They can't be placed anywhere on the basement floor slab. I have always seen them installed to the concrete, but they may be able to be installed above certain foam. That's a question for an engineer.

    1. CamillaGN | | #4

      This is very helpful. It answered my doubts perfectly. Btw, The footings are in place and the weight bearing posts are bolted in already. So the walls I mentioned are incorporating the posts. There are no leaks or water problems.
      Thank you!!!

  3. severaltypesofnerd | | #5

    Dricore R is a quick way to get a subfloor on slab, which still gets some ventilation below. You get R3. But it requires a pretty flat slab, your floor drains (and the likely slope involved) may frustrate the ability to build on top of the slab with any reasonable floor, without a lot of shims.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |