It’s no surprise that some homes are more challenging to mitigate than others. There are a variety of factors at play: house layout, the presence of drain tile, and the number of crawl spaces, to name a few. However, one factor that people don’t often consider is the existence of sub-slab ductwork. While it isn’t common, sub-slab ductwork can significantly affect how we proceed with mitigation. This article will discuss what sub-slab ductwork is, how it impacts radon mitigation, and what to do about it.
WHAT IS SUB-SLAB DUCTWORK? HOW CAN I TELL IF IT’S IN MY HOME?
Sub-slab ductwork refers to HVAC ducts that run beneath the concrete floor in the basement or lowest level of your home. If you have sub-slab ductwork, you will have registers coming out from the floor in the basement, as shown in the images below.
HOW DOES RADON MITIGATION WORK?
Before we get into the complications with sub-slab ductwork, we’ll want to quickly cover the basics of radon and radon mitigation. Radon is a radioactive gas that’s formed when uranium in rocks and soil decays. It can rise from the ground and collect in your home. Radon is the second leading cause of lung cancer in the United States. Unfortunately, the only way to measure the radon in your home is by testing.
If your radon test results come back high, your best option is to mitigate. Radon mitigation works by sucking the radon-laden air out from under the house, preventing it from collecting inside. However, for a radon mitigation system to be most effective, it must create suction under every area of the home. That’s where sub-slab ductwork can create a bit of a problem. Sub-slab ductwork has air leaks, making it incredibly difficult to form the necessary vacuum under the house.
WHAT CAN I DO IF I HAVE SUB-SLAB DUCTWORK?
In order to get radon levels as low as possible, we often recommend working with an HVAC contractor to abandon sub-slab ductwork. Abandoning ductwork is a big project; however, we’ve found it is the best way to ensure success in homes where ductwork complicates mitigation.
HOW DO WE MITIGATE HOMES WITH SUB-SLAB DUCTWORK?
Let’s take a look at how we mitigated a house in Minnetonka, MN, with sub-slab ductwork. The trunk line from the furnace used to go down into the ground. As a result, the supply went through the floor, providing the basement with conditioned air. This customer abandoned their ductwork, which meant they had an HVAC contractor remove the old trunk line that fed the sub-slab ducts and add new ducts above the basement ceiling. This process can be very costly in a home with a finished basement as they need to take the ceiling down, add ducts, frame soffits, add drywall, tape, and paint. Below you can see this customer’s new ductwork (left) and old ductwork (right).
USING SUB-SLAB DUCTWORK FOR RADON MITIGATION
When the sub-slab ductwork is abandoned, we can use it to draw suction with our radon system. Several registers like the one shown below came up through the floor throughout the basement.
We sealed them with concrete and placed duct tape over the top to slow the curing process.
When installing our vent pipe, we added a tee to draw suction from two different areas. The pipe on the front left drew air from the sub-slab ductwork, reaching the areas of the home with ducts. The pipe on the far right led to a suction point that we created where the plenum used to be. That suction point helped us reach the areas of the home without ducts.
Further up the vent pipe, we added a U-tube gauge and a system alarm to alert the customer if their system stops working. Should a problem with the system arise, the fluid in the U-tube gauge will read level at zero, and the alarm will sound.
We also added a tee with a cap on it in case we needed to come back later to encapsulate and depressurize their crawl space. However, since we weren’t sure that mitigating the crawl space would be necessary, we decided to install our base system, test, and then come back if needed.
From there, the vent pipe ran up to the abandoned B vent, which is where the old water heater and furnace used to exhaust.
Then it ran up to the attic, where the radon fan generated the suction needed to create negative pressure under the entire basement floor. Finally, the vent pipe terminated out the roof and exhausted the radon-laden air outside.
After we installed the system, we re-tested the radon levels. When we started this project, the customer’s radon levels were 8.4 pCi/L in the basement and 7.4 pCi/L in the room above the crawl space. After mitigation, their levels were 0.3 pCi/L.