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Community and Q&A

Flat roof with no gases or fumes

knstyleinc | Posted in Green Building Techniques on


I am renovating a home in Laguna Beach CA and redoing the roof as well as most of the house. I am highly sensitive to chemicals and the engineer is calling for the following:

1. Ply
2. 4” High Density Rigid Insulation Foam
3. CDX sheeting
4. Mule Hide roof system

I am wondering if you have any suggestions about how to create a gas/vapor barrier so that when the roof heats, I am not exposed to fumes/gasses from the materials. The inside will be finished with 1/2” drywall board.


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  1. jinmtvt | | #1

    Hi Kimberly,

    please define : "Mule Hide roof system "

    Why not go with a fully adhered membrane that could reside on polyiso fiber glass faced panels,
    which are quite inhert, and water based adhesive( reduces unwanted chemicals when heating up )

    I am no expert on the subject, but a poly sheet usually blocks alot of diff. chemicals and gazes.

  2. GBA Editor
    Martin Holladay | | #2

    Every roof needs an air barrier. If your roof is properly constructed, you won't have to worry about vapors flowing downward.

    You mention that the lower layer of roof sheathing is "ply" -- which I assume means plywood -- while the upper layer of roof sheathing is "CDX" -- which also means plywood. Are these two different layers both plywood?

    If you want your lower layer of plywood to be airtight -- and that's a good strategy -- you should tape the seams of the plywood with a high quality tape like Siga Wigluv.

    If airtightness is very important to you -- and it sounds like it is -- you can install a roofing underlayment with taped seams (for example, Solitex Mento, a product that can be purchased from 475 High Performance Building Supply) directly above the lower layer of taped plywood.

  3. jinmtvt | | #3

    A quick second idea might be to built the roof with a "vented" space in between the insulation,
    other more knowledgeable on that might chime in ...or your engineer/situation might not agree to that added complexity.

  4. GBA Editor
    Martin Holladay | | #4

    I'm not sure where you intend to include the vented space. You suggest that the vented space be located "between the insulation." I'm not sure what that means.

    One thing is for sure: you don't want a vented space between the insulation and the interior of the building, or the insulation becomes worthless.

  5. Expert Member
    Dana Dorsett | | #5

    I'm not sure how 4" of any rigid foam meets Title 24 R-value requirements, even in temperate Laguna Beach? A high density polyurethane might come in at R27-ish, whereas IRC 2012 code min for US climate zone 3 is R38. Even though the R27 is continuous, not thermally bridged by rafters, it would need to be R30-ish to hit the same performance level as R38 between joists.

    If instead you skipped the rigid foam and installed R30 rock wool between 24" o.c. 2x8 rafters, and put 1" (R6) of foil faced polyiso underneath the Mule Hide, it would hit code min, and you would have at least one foil facer between you and any of the synthetic polymeric materials to limit gas diffusion rates by several orders of magnitude. Then it's a matter of how susceptible you are to the outgassing of the wood products. Rock wool has effectively nothing to outgas, since it is manufactured at extremely high temperatures, which drives off/burns any volatile components.

  6. jinmtvt | | #6

    ahhaha i missed a word

    i meant in between the insulation and the roof finish of course ...

  7. Richard Beyer | | #7

    In my opinion, Dana is giving excellent advice using mineral wool.

    From the vast research I performed over the last 3 years regarding foam insulation products, chemically sensitive people should not use foam, period.

  8. jinmtvt | | #8

    Richard: which foam ?

    Foam could be many things... probably all have very different behaviors ...
    please Richard share your research info with us!!

    Dana : Rockwool use binders at mid/low temp in a certain % to hold the fibers togehter .

    ANyone would care to explain the " mule-hide" roofing ?

    all i get is a company with the same name selling many different types of roofing materials...

  9. Expert Member
    Dana Dorsett | | #9

    Mule Hide is indeed just a trade name for a company that makes EPDM , TPO and PVC membrane roofing systems (as well as some other types of roofing.) It's not clear which version Kimberly's architect specified, but if I had to guess it's probably TPO (thermoplastic olefin), since she is in southern CA, where "cool roof" materials are required by code. Untreated unballasted TPO is white, with a naturally high solar reflective index that would meet code with margin, but it could be PVC. With EPDM you'd have to specify a solar-reflective coating to meet code.

    The binders used for rock wool batt goods is minimal and not very volatile- far less volatile than the oils they use to keep blown rock wool from clumping. If the batts ever get hot enough to cook out binders into gases at a significant rate it means the roof is fully engulfed in flames. You would then have more immediate & acute issues to be concerned about than the binders.

    You might be able to use 1.5" rigid rock wool rather than polyiso above the roof deck in the 2x8 raftered scenario but I'm not sure how "walkable" that stuff would be under a membrane roof. It would have the advantage of having far less outgassing in that layer, but would not have foil facers to keep the membrane roof's emissions at bay.

  10. Richard Beyer | | #10

    Dana's the wizard! Great advice!

  11. jinmtvt | | #11

    I still don't get what would be wrong with using regular iso or EPS under the membranes?

    Last time i checked, EPS boards under a HD recover ( 0.5" polyiso between 2 FG coatings )
    under a black EPDM does not even get warm ...why would it emit any gases if used with a white membrane ??

    Wouldn't a simple poly block off gases?

    Please enlighten me, as i do not see how the roofing boards could emit anything toward the interior space...

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