Cardboard ducts in slab

[Bob Caraher]

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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[Jerry Peck]

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.

[Bob Caraher]

Thank you for your reply - very helpful !!!

[Jim Robinson]

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.

[Markus Keller]

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

[Trent Tarter]

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.

[Joe Klampfer]

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.

[Erby Crofutt]

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.