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  1. #1
    Join Date
    Feb 2019
    Location
    PA
    Posts
    2

    Default Cardboard ducts in slab

    Hello - I am selling my home and during a home inspection, the inspector noted damaged cardboard like material lining in slab ducts. Can anyone advise as to what this is, why it was used and how to remediate the problem?

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  2. #2
    Join Date
    Mar 2007
    Location
    Ormond Beach, Florida
    Posts
    26,693

    Default Re: Cardboard ducts in slab

    It's hard to tell from the photos, even zooming in to 500%, but that looks too thick to be cardboard - looks more like some type of insulation which was wrapped in a paper covering?

    If so, the ducts may have been intended to act as a form for making ducts in concrete (concrete surrounding the ducts, with the insulated ducts acting as both a form and insulation for the end result concrete ducts?

    I haven't seen anything like that before, then again, I haven't been in an area which use underground ducts either - but the code does allow them under certain conditions. I would think that the frost depth in PA would be such that concrete slabs on grade would not be used, thus having ducts in the ground would not be found - but that's just a presumption on my part based on what I suspect the frost depth in PA would be.

    Maybe someone from your area has seen the practice done before.

    Jerry Peck, Construction / Litigation Consultant
    Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )
    www.AskCodeMan.com

  3. #3
    Join Date
    Feb 2019
    Location
    PA
    Posts
    2

    Thumbs up Re: Cardboard ducts in slab

    Thank you for your reply - very helpful !!!


  4. #4
    Join Date
    Mar 2007
    Location
    New Mexico
    Posts
    1,281

    Default Re: Cardboard ducts in slab

    There is a neighborhood in my area full of that type of duct under the slab. It has been problematic for that area. Most of the houses were built in 1968 - 1973. In that area, it is usually a sonotube like product, with a mylar liner. I assume the mylar was to allow for easier air flow. Yours looks a little different, but the end result is about the same.
    Why it was used is probably because it cost less than metal ducts. We usually have a camera scan of the ductwork in the houses that have them. It is just like a sewer pipe scan, but in the ducts. A flexible camera pushes through and sees if any have collapsed. If there is damage, there are some options. You can ignore it if you are getting some heat through. Most of our houses do not have AC. You can abandon the system and install ducts in the attic space if there is an attic. The other option is to repair the damage by coating it with a spray on material. They use companies like this one: https://www.ductarmor.com/the-product/

    It's not designed for use with cardboard, but it seems to work so far. There aren't any real easy solutions that we have found.

    Jim Robinson
    New Mexico, USA

  5. #5
    Join Date
    Feb 2008
    Location
    Chicago IL
    Posts
    2,002

    Default Re: Cardboard ducts in slab

    I can't tell what it is either but it looks bad.
    We have a lot of slab ranch houses with in slab ducts around here from post WWII era. Asbestos based material is common.
    Around here there are two common issues with these in slab ducts. First off they act as great rat highways. Secondly they tend to collect lots of slab water contaminating the air stream.
    At this point, around here at least, the prudent option is to close them up, replace the furnace with an up flow and install ducts in the roof cavity.
    You are selling your house so you probably don't want to spend any money. Difficult position to be in. If its feasible for you, giving the buyer some sort of credit for a new up flow system to get out from under it all might not be the worst option.
    good luck

    www.aic-chicago.com
    773/844-4AIC
    "The Code is not a ceiling to reach but a floor to work up from"

  6. #6
    Join Date
    Mar 2007
    Location
    Washington State
    Posts
    588

    Default Re: Cardboard ducts in slab

    Here's a few of the potential concerns with older homes that have ducts routed below slabs. An increased chance of possible radon gas entering home, water/moisture entering ducts, mold, pest intrusion, poor indoor air quality etc. I would look at removing any loose material that's accessible from register vents, make sure it does not contain asbestos, then have ducts professionally cleaned.


  7. #7
    Join Date
    Mar 2007
    Location
    Vancouver - Canada
    Posts
    222

    Default Re: Cardboard ducts in slab

    I've seen it before on a '50's or '60's built home, it is very much like a sonotube material. If it's at the point of breakdown or deterioration, that's a huge problem as mentioned earlier. Any ground water or moisture in those runs can leave a very unhealthy atmosphere (pooling water, mold, insects, radon, etc) which can now contaminate conditioned air blowing through the home.

    I think the only fix is to have new duct work rerouted & installed, or use those existing voids to install a new boiler and run the lines out for hot water baseboard heating. At any rate, those floor openings should all be sealed to avoid contaminating the living space with toxic air.

    Joe Klampfer RHI
    www.myinspection.ca
    Pacific Home Inspections

  8. #8
    Join Date
    Mar 2007
    Location
    Georgetown, KY
    Posts
    538

    Default Re: Cardboard ducts in slab

    Under slab ductwork just flat sucks. I'd throw a bit of money at the buyer to get out from under and let them deal with it.

    Erby Crofutt, Georgetown, KY - Read my Blog here: Erby the Central Kentucky Home Inspector B4 U Close Home Inspections www.b4uclose.com www.kentuckyradon.com
    Find on Facebook at: https://www.facebook.com/B4UCloseInspections

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