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Community and Q&A

Blower door testing on homes with asbestos siding

greenerliving | Posted in General Questions on

Picking up on a comment to a post from 2014 (, I wanted to ask the community about conducting blower door tests in homes with asbestos siding. While this material is clearly “rigid” in nature, it seems that the thousands of nails penetrations could easily have brought some asbestos fibers into the exterior wall cavities.

As stated in the post above (and elsewhere), most of the documented cases of asbestos-caused illnesses originate from people that worked with these materials, or lived near a mine or with a family member that worked in the industry. With that said, there’s a reason it’s banned in 50+ countries and listed as a human carcinogen. Dana Dorsett made the comment on that post saying blower door testing on homes with asbestos “is expressly illegal in Massachusetts”. I have only found documentation to support that the Mass Save program does not allow the contractor to conduct a blower door test if the asbestos material is friable. (Perhaps there is further documentation of the state’s stance on this that someone could share…)

And while asbestos siding would not be considered friable, certainly the bits and pieces of it sitting on top of the bottom plates and batt insulation in the exterior wall cavities could become airborne during either depressurization¬†or¬†pressurization of the home. As a contractor working in a separate utility program (that requires blower door testing unless the suspect material is friable), I always have concerns about running a BD in this scenario. The risk just doesn’t seem worth the reward.

So, am I being too cautious here or are my concerns valid?

Thanks in advance!

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  1. GBA Editor
    Martin Holladay | | #1

    I think your worry is speculative. I can't imagine that there are asbestos fibers inside stud bays that got there when nails were driven into the siding and sheathing. I'm especially doubtful that even if one or two of these fibers existed, that a blower-door test would bring these fibers in contact with a human nose.

    I've never heard of a restriction on performing blower door tests on houses with siding that contains asbestos.

    Now, if your house has vermiculite insulation in the attic -- that's another level of worry entirely...

  2. greenerliving | | #2

    Thanks Martin. I appreciate your feedback as always!

    Could you share any further thoughts on why you don't think the nails would potentially bring fibers into the cavities? And also why those fibers wouldn't come into the home through wall outlets, switch plates, baseboard trim, etc during a blower door test?

    Thanks again!

  3. GBA Editor
    Martin Holladay | | #3

    Cement-asbestos siding is hard and brittle. The asbestos fibers are closely bound to the hardened cement. It's not like the asbestos you see wrapped around old steam pipes.

    I don't think that driving a fastener through asbestos siding creates friable asbestos fibers, because of the way the fibers are bound together. (It's even possible that some brands of cement-asbestos siding came with pre-drilled fastener holes -- the material is famously brittle.)

    Old houses have board siding, and the boards are often a full 1 inch thick. By the time the siding nail penetrates the sheathing, a small fiber is more likely to be pinched between the nail and the sheathing fibers than popping out the end of the fastener hole.

    By now, most older houses have some type of insulation between the studs -- making it highly unlikely that a solitary asbestos fiber would fly through the air to an electrical penetration without getting hung up on some other type of fiber (perhaps a spider web).

  4. greenerliving | | #4

    Thanks Martin.

    Your description is vivid and valid.

    As BPI says "Health and Safety First"!

    We always want to do what's right by the homeowner and their family and this has been an issue we've wrestled with.

  5. mackstann | | #5

    Doesn't the wind sometimes blow just as strongly as a blower door sucks?

    Can't the blower door be reversed to blow into the home instead of suck out?

    1. Expert Member
      Dana Dorsett | | #6

      Blower door testing is usually done in both pressurized and depressurized directions, which often yields a different number.

  6. GBA Editor
    Martin Holladay | | #7

    One final caveat: My judgment is based on the fact that I've been involved with a project where blower-door testing was performed on several homes with cement-asbestos siding, and the experienced blower door operators didn't express any concerns.

    My judgment is not based on and real-world measurements of asbestos levels during such a test. So keep that in mind as you evaluate the usefulness of my comments.

  7. greenerliving | | #8

    The caveat is also valid Martin, especially when dealing with a carcinogenic material.

    Are you aware of any real-world testing that may have been done in this scenario?

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