Water in sub-slab ductwork

In last week’s blog post on transite heat, I explained that most sub-slab ductwork is not transite. It’s not transite heat, it’s not transite ductwork, and it should not be labeled as such unless the ductwork is actually made from transite. If it is, that means it contains asbestos. The downside to transite heat is that it can’t be cleaned without the risk of releasing asbestos fibers into the air.

The biggest concern with all sub-slab ductwork is the potential for water to enter the ducts, which is what today’s blog post is all about.

Side note: sub-slab ductwork is ductwork installed below the concrete floor.

Water is bad news

Regardless of what type of sub-slab ductwork is present, water is the main concern. If water finds its way into sub-slab ductwork, mold can follow. This leads to poor indoor air quality and health concerns. That stuff at the top of the duct in the picture below sure looks like mold, doesn’t it?

moldy transite ductwork

If you’re buying a home with sub-slab ductwork, make sure that your home inspector checks the sub-slab ductwork for any signs of past water intrusion. Signs of water intrusion are a major concern because it’s usually not a simple repair. The image below shows a modern PVC duct with clear signs of chronic water problems.

water stains on sub-slab ductwork

Here’s another duct with a similar situation, but not as bad.

transite duct with water stains

The rest of these images show sub-slab ductwork with standing water.

water in sub-slab duct

water in sub-slab duct

water in duct

We’ve inspected a handful of homes with such serious ductwork problems that the homeowners have resorted to installing sump pumps inside of their return plenums. No joke. We’ve seen this done many times.

sump pump in furnace return plenum

What to do

There are two basic methods for dealing with water in sub-slab ductwork; abandonment or repair.

If sub-slab ducts are going to be abandoned, they need to be sealed off and new overhead ductwork needs to be installed. This is an expensive option because the new ductwork typically needs to be installed in an already-finished space.

The other option, repair, consists of two steps. First, the source of the water needs to be corrected. This will probably require the services of a basement waterproofing company. Most importantly, the obvious stuff like exterior water management would need to be addressed first. If that has already been addressed but hasn’t solved the problem, the home might need to have an unusually deep drain tile system installed. It must be deeper than the sub-slab ductwork.

The second step requires encapsulation or lining of the ductwork. This is a service that I learned about through my ASHI chapter last year. We had a guest speaker come out to talk to us about duct lining, which is apparently a good option for homes with problematic sub-slab ductwork. It won’t fix water intrusion, but it should eliminate any air quality concerns, and it’s far less expensive than abandonment.

Author: Reuben SaltzmanStructure Tech Home Inspections

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ps – we’ve also found dead mice in sub-slab ductwork, we once found a pipe, and we’ve even seen severely rusted metal ductwork used below the slab more than once.

Written By

Reuben is a second-generation home inspector with a passion for his work. He grew up remodeling homes and learning about carpentry since he was old enough to hold a hammer. Reuben grew up thinking he was going to be a school teacher because he enjoyed teaching others so much. In a sense, that’s a lot of what home inspections are about, so Reuben truly does what he loves. Sharlene has worked with Structure Tech since 2000 and Reuben has been contributing to her blog since 2008.

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