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Community and Q&A

Epoxy Vapor Retarder over Existing Slab

dan_saa | Posted in General Questions on

I’m looking for recommendations for epoxy floor coating to serve as vapor retarder on existing concrete slab. I am an architect in Los Angeles designing a conversion of existing garage to an Accessory Dwelling Unit (ADU). The garage was built in 1922 slab on grade, and no underslab vapor barrier is assumed. Plan check requires a code modification to specify an epoxy coating equivalent to 6 mil polyethylene sheet per residential code R506.2.3. The modification requires an approved listed product by ICC-ES or IAPMO-UES, also floor coatings in California have VOC limit of 100 grams per Liter.

So far I’ve found products by Laticrete, Rustoleum, and Maxxon, but no ICC or IAPMO listings. Other products that have listings are full fledged waterproofing or deck coatings, which is overkill.

I had intended to just install a 10 mil vapor retarder under wood floor on sleepers, but the city will only accept if we install underfloor ventilation which is pretty difficult with only 1 1/2″ clear.

Appreciate any recommendations.

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  1. Expert Member
    KOHTA UENO | | #1

    "Plan check requires a code modification to specify an epoxy coating equivalent to 6 mil polyethylene sheet per residential code R506.2.3. The modification requires an approved listed product by ICC-ES or IAPMO-UES, also floor coatings in California have VOC limit of 100 grams per Liter."

    Two epoxy concrete coatings that I we have used include BASF Chemrex® mV-Block and KOESTER VAP 1® 2000. No idea whether they meet CA VOC limits.

    Also, I assume that "equivalent to 6 mil polyethylene" is either a Class I vapor barrier (0.1 perm or less), or directly equivalent to mil polyethylene (0.06 perm).

    1. severaltypesofnerd | | #17

      That's our AJH interprets it: "directly equivalent to 6 mil polyethylene (0.06 perm)"

      See also:

      Never mind that poly is often put down poorly, or abused afterward.

  2. rags_8 | | #2

    Daniel - I am dealing with the exact same modification for an ADU. I did some research on the post from Kohta but could not find the ICC or IAPMO listings. Did you have any luck solving this issue?

  3. dan_saa | | #3


    After much time googling, I couldn't find any listings either and decided to punt. The LADBS plan checker had given me a pre-filled out Modification form for an approved epoxy coating as an alternate method to under-slab vapor barrier. This form had an ICC number already filled in, #1413. However this for Custom Building Product Redgard fluid applied waterproofing, not an epoxy coating, and the ICC report doesn't even mention vapor barrier, just waterproofing. Rather than point out this contradiction I just went with Redgard since we had planned on a sleeper system above anyway. Probably cheaper to install than epoxy. But I have no ICC approved solution for exposed floor coating.

    Let us know if you find a solution. Thanks,


  4. rags_8 | | #4

    I've been at it for a few days as well, and cannot find any resolution. I assume all is complete at this point for your project since you were dealing with this back in October of last year? Was the LADBS modification the same default modification you mentioned in the language from your initial post? Any chance you can send me that pre-approved mod also?

  5. dan_saa | | #5


    Yes we got the permit, not yet under construction so I don't know the resolution with actual site conditions and building inspector. Here is a link to PDF as filled out by LADBS plan checker, minus specific project info:

    It does show the specific wet thickness that Redgard uses, so there was confirmation that the ICC was a match for the other info filled out, just not an epoxy. This was obviously dealt with by LADBS plan check before as he just pulled up the form and filled out without my asking. This was after his supervisor rejected my proposed non-ICC above slab vapor barrier solution.
    I added my firm link to profile, feel free to email direct if you have other questions.


  6. Expert Member
    BILL WICHERS | | #6

    You might try contacting PPG. They make a lot of floor coatings and might have something that can work for you. I’ve used their stuff in commercial mechanical rooms but for oil/solvent resistance so I haven’t actually looked for a vapor barrier before.


  7. FlipGreat | | #7

    As a guy who has done extensive research and completed one in the city of LA, everybody is opting for Redguard. However I got thrown a curve ball by a smart planner this week. Apparently regard is only approved for tile over it. I know there are tons of people using this and putting laminate, engineered etc. over it and I dont think this information is well known as I had 4 different inspectors over the life for the project an none said anything about my engineered floor over the redguard

    1. arquirobby | | #10

      The blurb on regard reads: "RedGard® creates a continuous waterproof membrane barrier with outstanding adhesion and reduces crack transmission in tile and stone floors. It bonds directly to clean metal drains, PVC, stainless steel and ABS drain assemblies and can be used as a slab-on-grade moisture vapor barrier UNDER ALL TYPES OF FLOOR COVERINGS."

  8. FlipGreat | | #8

    Has anyone gotten anything approved by LADBS planning other than Regard?

  9. Sessma | | #9

    Please let us know if you get anything approved. We intend to leave the historic slab exposed. If it is not impermeably sealed, it would breathe (no mold could develop because no moisture would be trapped under finish flooring). If there were an approved clear finish for use, that would be viable; however, the approved products I've seen ruin the original finish with their thickness and coloration.

  10. ianmci | | #11

    BASF Masterseal AWB 660, ICC ESR-3310 Class I vapor retarder

  11. Corey_Z | | #12

    user-6931947, were you able to get the BASF Masterseal to pass? That is what I had on my plans for a garage conversion into an ADU, and the city kicked it back saying it was flashing for walls.

    Also for anyone else, has the Redguard continued to work for floating floors, or have the city planners and inspectors caught on that it's only to be used for tile?

  12. MrH1325 | | #13

    After much research, I'm going with RedGard. My local building department mentioned a vapour barrier on the garage conversion and I said I'd seal it up, there was no further discussion after he saw the RedGard under the tub and I mention it had a great perm rating. I'll use it under click together LVP and carpet. It has the perm rating I want, ease of use, reasonable price, and I've tested it on the concrete floor under my stairs and tub before closing them in and I'm satisfied with the durability and adhesion to the concrete. I would not use it where any hydrostatic pressure is present, there are better products and real solutions for that. All that being said, I was going to confidently go with the BASF as it's abrasion resistance also appears to be suitable for use under a floating floor ( but it wasn't as readily available as RedGard and I suspect the smell might be harder to dissipate and that's a concern as we're occupying the upstairs during construction. At the end of the day, it's my understanding that a professionally installed epoxy coating appears to be the only real approved solution for concrete floor prep prior to flooring install.
    Edit: Just spoke with a local epoxy floor coating company and he immediately suggested RedGard as an appropriate coating if vapour control under a floating floor was desired. He said his proprietary epoxy product was vapour permeable to avoid blister/peel and that LVP flooring would also breath enough to let vapour pass.

  13. dan_saa | | #14

    Following up, found that LA County has nice guidelines for ADU, and they list four options:

    1. ICC ESR#1413 – RedGard Waterproofing and Crack Prevention Membrane, CCure
    Pro-Red Waterproofing Membrane 963, CBP 232 Waterproofing and AntiFracture
    Membrane, and Jambo Waterproofing Membrane
    2. ICC ESR #2417 – LATICRETE Hydro Ban
    3. ICC ESR #2785 – Polycoat-Aquatight and Flexideck
    4. ICC ESR#3474 – Mapelastic AquaDefense Waterproofing Membrane

    1. dan_saa | | #19

      LA County now lists an epoxy product:

  14. severaltypesofnerd | | #15

    An option here is 6 mill poly, then DriCore R+ (, and the finish floor of your choice. That gives you just about everything in under an inch, and R3 insulation to boot.

    1. Jon_R | | #16

      > An option here is 6 mill poly
      Unless this is put under the slab, I don't see any indication that this is an allowed option.

      Also read what Joe L says about non-fully adhered vapor barriers above slabs.

      > 10 mil vapor retarder ... city will only accept if we install underfloor ventilation
      I'm curious - what kind of ventilation? Do they say why?

      > DriCore R+
      Does anyone have data about the OSB moisture level when this is covered with low perm flooring? It's slightly ventilated, but warm/humid interior air circulating next to a cool slab can *add* moisture. And distribute mold odor and radon.

  15. severaltypesofnerd | | #18

    What about using, literally, adhered sheet vinyl as the vapor barrier, then float a hardwood floor on top of it? The liquid applied products are trouble to install and if epoxy based won't bridge future cracks.

    1. Expert Member
      BILL WICHERS | | #20

      Moisture coming up from the slab can sometimes cause the adhesives used with flooring to seperate, which can cause blistering and lift in the floor. I've never seen anything like that happen with epoxy coatings.


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