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Vapor Barrier to Prevent off gassing of CCA treated wood and OSB ?

marion2249 | Posted in General Questions on

I am in the middle of house building for a passive solar designed house in a mild marine Climate in southern Chile ( similar to Portland Oregon) where likely using a vapor barrier would not cause problems from what I understand.

The house frame upper and lower floor was built with OSB which I understand contains formaldehyde and CCA pressure treated wood. These are common construction methods where I live.

In this case might someone recommend a vapor barrier to prevent off-gassing from the CCA wood and formaldehyde in the OSB ?

I know there was a great article recently posted
about vapor barriers but I thought I might ask the forum here also ..

Thanks in advance Im very new to housebuilding

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  1. GBA Editor
    Martin Holladay | | #1

    It looks like you have posted your question in two places. I will copy and paste the same response in both locations.

    While water vapor can diffuse through many common building materials, I don't think that formaldehyde or CCA move through materials by diffusion -- at least not to a significant extent. If any chemist or physicist cares to correct me, I am prepared to be corrected.

    If you want to prevent formaldehyde or CCA fumes from entering your house, all you really need is an air barrier.

    I don't know what you are using as the interior finish material for your wall; perhaps you are using gypsum drywall or plaster. Either of these materials can be installed as an air barrier, as long as you pay attention to air sealing at the perimeter of the materials and penetrations.

    If you are worried about offgassing of materials in your wall assembly, here's another suggestion: it might make more sense to install a supply-only ventilation system (which will slightly pressurize your house) rather than an exhaust-only ventilation system (which will slightly depressurize your house).

    One of your statements is confusing to me. You wrote, "I am in a mild marine climate ... similar to Portland, Oregon, where likely using a vapor barrier would not cause problems." In fact, the installation of an interior vapor barrier in Portland, Oregon is not recommended, because it could cause problems.

  2. marion2249 | | #2

    thanks Martin and anyone else reading . Im not sure what the equitte is but I replied on this thread .. thanks very much Martin ..

  3. davidmeiland | | #3

    Does this house have CCA anywhere but the mudsill and other concrete/soil contact areas?

  4. heinblod | | #4

    There is hardly a barrier which will stop formaldehyde emissions.
    The problem with the timber boards is a very serious one.

    The EU is now acting, the permitted maximum formaldehyde in occupied rooms being reduced by 90%.
    Most timberframe constructions/-builders won't be able to match the new legal limits. Well, not with the cheap methods/boards they used to use.

    Here a short official articel on the issue:

    Look for

    " Formaldehyde emissions from wood-based panels: a need for action for a new test method "

    For existing structures it means that a much higher ventilation rate must be achieved. Meaning anywhere near a PH ventilation rate (0.6 air exchanges/hour) would be illegal.

    Try to avoid these cheap building methods, they'll cost you a fortune in the long term.

    From what I know the higher the humidity of the air the more forrmaldehyde will be released from the timber boards

    Question: what does CCA stand for ? Thanks.

  5. davidmeiland | | #5

    Hein, CCA is copper chromium arsenate, a wood preservative. It was widely used in the U.S. until about 10 years ago, when regulations caused a switch to other formulations. In a typical house, treated wood would be used for mudsills and for deck framing, and not much else. I'm wondering if for some reason the OP is using it to frame part or all of the house.

  6. heinblod | | #6

    Thanks, David Meiland!

    With the REACH research going on and French scientists being asked to investigate into the max. formaldehyde concentrations there are new proposals for France.
    France doesn't seem to want to wait for the rest of the EU so they try to hurry things up in their own terrains.
    The maximum permitted concentration for formaldehyde would be 0.024ppm from 1st January 2015 and 0.008ppm from 1st of January 2023.

    Formaldehyde had been re-classified into the 'worst class' of carcinogenic chemicals recently, see

    It is now a confirmed mutagenic substance as well.

    About 10-20 years ago sheepswool was used in Europe as a remedy against high formaldehyde emissions, curtains and carpets, rugs being installed in rooms with high formaldehyde concentrations.
    The protein in the wool reacts with the gaseous formaldehyde and breaks it down.
    Chile has a mighty wool industry, why not using this wool for thermal insulation and incorporating some of it into the interiors as well?
    Wool from non-dipped sheep of course, otherwise a organophosphate/-chlorine or creosote problem might make things worse.
    If the new limits of formaldehyde concentration can be met by this way with a severe background problem (a new house !) - I doubt it. These new safe limits are simply to low for this I would say.

    There are building consultants specialised on these issues.

  7. Siffe | | #7

    CCA does not offgas, as far as I know. The problem with it is that when wood treated with it gets wet, the CCA leaches, breaks down, and accumulates in the ground. High levels of arsenic were being found in areas where kids play in the dirt, like parks and playgrounds. Once that was discovered, most manufacturers agreed to stop using it.

  8. marion2249 | | #8

    thanks everyone for you contributions and advice .. I more clearly understand the matter .. Im not sure how much formaldehyde the OSB used here has but I will be looking into it ..

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