Inspecting sub-slab ductwork: it’s all about water

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 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.

Definition: 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

It’s important for us home inspectors to carefully inspect sub-slab ductwork for any signs of past water intrusion. 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 their return plenums. No joke. We’ve seen this done many times.

sump pump in furnace return plenum

How to inspect sub-slab ductwork

The best practice for inspecting sub-slab ductwork is to remove every floor register possible and use a mirror and a flashlight to look for signs of water.

Floor register cover removed

I use a long, skinny stick of mirror for this, as shown below.

Skinny Mirror inspecting transite

I mentioned this mirror in my home inspection tool list.

Furnace inspection mirror

If all the visible portions of the ductwork are intact and dry, I don’t have any concerns to report.

My company also offers sub-slab ductwork inspections. We use a sewer inspection camera with a very small head to accomplish this.

What to do

If there are signs of water intrusion with any type of sub-slab ductwork, this is a concern. The next step is to drill down and ask more questions.

If there are simply water stains, was something done to prevent water from coming in again? Exactly what was done? How long ago? Has it stood the test of time?

If there is water in the system, this is a major concern with two potential courses of action: 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. It won’t fix water intrusion, but it should eliminate any air quality concerns, and it’s far less expensive than abandonment.

A few more photos (just for fun)

Here are a few more goodies we’ve found in sub-slab ductwork:

Snake in duct
Dead mice in duct

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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Water in sub-slab ductwork

đź–¨ Print Article 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...

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