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Community and Q&A

Wood floor system over new slab-on-grade?

alderwiser | Posted in General Questions on

New house build in zone 4c PNW.
Seeing what folks think of a slab on grade monolithic pour having no radiant heat and a subfloor system installed for wood floors? I have lived on concrete finished floors and def feel the impact on my body as well as my toddler crying every time he falls over! Also don’t see how the radiant system embedded in concrete is resilient when it eventually fails, and have personally had trouble getting the temp even, esp when the sun hits.

10 mil VB
Board Insulation, min R10 code (hoping the County will permit using RockWool CB)
4″ slab, 3000 psi*
2×4 Sleepers (PT?)
3/4″ plywood T&G
3/4″ T&G Wood flooring
*Finished Earthen Cob floor directly on top of concrete for small area abutting wood floors.

Specifically my questions are:

1. Is a second vapor barrier needed between concrete and subfloor? Does ply or sleepers need to be PT?

2. Pros and cons of sleepers under ply vs ply directly on concrete? I believe the floor will be more forgiving with sleepers…

3. Best way to attach sleepers/ply without using adhesive? (tapcons)

4. Thoughts/experience of earthen floors? I’m somewhat surprised to see a lack of natural materials being used/suggested on a Green Building Advisor forum!


Kevin B

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  1. arnoldk | | #1

    Hi Kevin,

    I don't feel I have enough knowledge to comment with confidence on your floor assembly but I can comment on earthen floor.
    I looked into earthen floor a few of years ago when I was planning my house built. The main reason we didn't go with it is because of the higher cost, higher maintenance and local acceptance by trades and/or inspector.


    1. alderwiser | | #3

      Good info. I am a builder and we will be doing the work ourselves. I have built a slip straw clay pumphouse on our land with some well-versed friends so have some experience with cob and plastering. We are also hoping to make this a community event, so the efficiency part is a bit off the table and more for a celebration/hangout. Of course the county planning dept. may have something to say in the matter though!

      I saw you posted a similar question to this regarding the wood flooring, what did you end up doing with your slab-floor situation?

  2. user-6824737 | | #2

    Hi, Kevin. Putting the issue of the earthen floor aside, I'd suggest looking at this Joe Lstiburek article ( regarding insulating slab on-grade foundations, particularly Figure 9 relating to the option of installing insulation and subfloors on top of the slab.

    1. alderwiser | | #4

      This is great info. Much appreciated

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