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Air barrier, uninsulated – raised pier/beam house

huntdupl | Posted in Green Building Techniques on

100 year raised old house in hot humid south louisiana. Following several local publication, my preference would be to install 2” foil faced poly iso to bottom of joist. Cost and time, we can’t swing this..

I’ve restored all the windows and seals where possible, next is door weatherstripping. With constant road construction and pollen season, I’m noticing a tremendous amount more dirty/dust in our house which is a problem with newborn baby.

I’ve order a Froth Pak to spray and partially seal under the house at perimeter wall/beams in crawlspace. I do not plan to insulate the floors at this point in life/renovation.

My Question:

Can I spray an air barrier, something simple like latex paint, to underside of floor decking? I have a HD airless sprayer..

I assume the risk of spraying an air + vapor barrier is condensation without having insulation. My goal is to seal up the solid sawn floor decking (predates plywood).

Note: I refinished all my wood floors with permeable (oil) finish, not film polyurethane which I’ve seen so many problems with.

thanks in advance.

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  1. walta100 | | #1

    Sound to me like you need to work on air sealing this house.

    I think it would be a mistake to cover every visible crack with sprat foam because you are likely to run out of money buying foam long before the last crack has disappeared.

    I think what you need is blower door directed air sealing. They depressurize the house with a large fan locate the leaks and block them. It seems unlikely you will locate this type of service in LA due to the low demand.

    My home brew Idea is to turn off the gas to the home then tape one or more box fans onto windows to depressurize the house. Go around inside the home with a bright flashlight and incense sticks use the smoke to find the leaks and fill them with caulking.


    1. huntdupl | | #2

      Thanks for your reply. I'm familiar with AeroBarrier and others, unfortunately this renovation is done* and we're living here. I can do some leak detection. My main concern is condensations...

      I've seen spray foam wreak havoc on subfloors with polyurethane coating flooring. My floor finish is vapor open, but I'm trying to understand the potential risk spraying underside with paint or spray-applied air barrier with respect to dew point and NO insulation.

  2. Expert Member
    Akos | | #3

    This is the best information out there:

    Look at fig7, about your only DIY option.

    1. huntdupl | | #4

      Thank you. I agree and mentioned this approach - there are several local case studies, data showed this to be most effective. However, this is not cost/time effective at this point in it's 100 year history. Purchased 2019, renovated 18 months > this was our starter home and our family has grown...1450 sf isn't going to last very long

      1. Expert Member
        MALCOLM TAYLOR | | #5


        If there aren't too many impediments beyond the odd pex line I see in the photo, you might consider covering the underside with house-wrap to act as an air-barrier. The downside is it would create a cavity which pests might be attracted to.

        1. huntdupl | | #6

          Thank you. Unfortunately, this is like the clearest, obstacle free location I'd suppose...navigating rigid insulation around all the pipe hangers etc. is mind-bending work with +/-24" of clearance. I could at least fir out the joist and bypass all the face-nailed PEX and Romex. I do have a HD paint sprayer though :) House wrap would certainly be cost effective. I assume being vapor permeable, if any moisture developed in the joist bays (between floor & house wrap), is gravity enough to pull it down and out? Like in fluid applied membrane (permeable), its pressurization of the house that forces vapor outward, correct?

          1. Expert Member
            MALCOLM TAYLOR | | #7


            I suggested it partly because it was a benign solution. Hot-humid climate are tricky, and I don't know enough about them to offer any good advice on what else would work. Hopefully someone with more experience will weigh in.

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