Does your home have a tuck-under garage? While these garages can help make the most of your space, they usually present a challenge when mitigating a home. Let’s take a look at how we mitigate a home in this 1976 split-level home with a tuck-under garage, a finished basement, no drain tile, and a radon level between 4 pCi/L and 5 pCi/L.
SEALING OPENINGS TO THE SOIL
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Radon mitigation works by sucking the radon-laden air out from underneath the house before it has the chance to enter. Therefore, the key to getting radon levels as low as possible when we mitigate a home is to create a vacuum under every area of the home.
Sealing openings to the soil helps us create a more effective and efficient system. Gaps and cracks limit our ability to create suction under the home. They also draw conditioned air, air that the owner is paying to heat and cool, from the house. You can read more about the importance of sealing here.
There was a hole open to the soil under this customer’s furnace. We think that they built the house in the winter, and when they put in the furnace or a temporary furnace, they set it on a brick. When they poured the floors, they couldn’t get all the way underneath.
There were a few different options to seal the hole under the furnace. We could have used spray foam, but we’d have had to turn off the natural draft water heater because spray foam fumes can ignite. So instead, we cleaned out the area under the furnace and put concrete underneath.
The hole beneath the furnace was not the only gap we had to worry about sealing. Because the customer had a finished basement, we weren’t able to seal the block tops or floor-to-wall joint behind the drywall. However, we were able to seal the area under the stairs by the entryway.
PRESSURE FIELD EXTENSION
To measure the amount of suction we were creating under every area of the home, we performed pressure field extension testing. First, we drilled test holes—half-inch holes through the concrete slab—throughout the basement. Then, we sealed tubes into the holes and ran the tubes back to micromanometers—devices that measured the pressure in each test hole. In the winter, we aim for at least -3 pascals of pressure in each area of the home under max depressurization (by turning on all of the devices that will suck the air out of the house).
In the photo below, the purple tube leads to a test hole just a few feet out of frame. The blue tube leads to the mechanical room on the other side of the basement. You can see that we were not creating any negative pressure in the mechanical room. It was equal at 0 Pa. Then, in the purple test hole under the stairs, we had positive pressure, which meant the house was sucking air (and radon) inside.
Next, we used our shop vac to apply suction to a third test hole. In the purple test hole under the stairs, just a few feet away, we achieved -10 Pa. However, we weren’t reaching the other side of the basement or the mechanical room.
IDENTIFYING ADDITIONAL SEALING
Using smoke as a visual indicator, we could see that gaps and cracks were drawing in conditioned air.
Using Great Stuff Pro foam, we sealed the floor-to-wall joint, the block tops, and around the sill plate.
Once the foam had a chance to cure a bit, we reran the test. Our suction jumped from -10 Pa to -50 Pa in the closest test hole.
INSTALLING THE SYSTEM
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Identify Suction Points
The next step was to figure out where to put our suction point. This house did not have drain tile, which meant that we wanted to place our suction point next to the footing and plumbing to take advantage of any settling. In this case, we had plumbing at the back of the house, but there was also a makeshift water drainage system in that area. We weren’t quite sure what that was, so we didn’t want to seal it and risk flooding the basement. When there are air leaks that you can’t seal, place your suction point away from them.
The garage floor was new, so there wasn’t a footing running through. There was also an old vs. new slab, which often isn’t air-tight.
In the laundry room, we had a center footing about 20 inches wide and 8 inches deep. It sat on top of clay and impeded airflow. The center footing ended and transferred to a beam in the garage and at the opposite side of the laundry room. The first suction point was placed by the wall opposite the garage, near the load-bearing wall.
Next, we ran a flex duct from the suction point to a radon fan to test our pressure field extension. We found that this single suction point was not enough to create suction under the entire house. There were still some areas that we barely reached, and the test hole under the stairs read positive. We ran a temporary system and saw radon levels drop some but not as much as we would have liked.
To create better suction under the house, we wanted to find a way to get next to plumbing. We dug a few feet in either direction from our suction point, but we still couldn’t find it. We even consulted with a plumber who agreed that the plumbing should be in the area we thought because it was unlikely to run under the center footing.
Finally, using a sewer camera with a transmitter in the head allowed us to trace the plumbing depth and location.
We ran the sewer camera down the cleanout for the floor drain and found that the plumbing did, in fact, run under the footing to a cleanout in the living room.
We dug a 20 to 25-gallon suction point next to the plumbing and footing. You can see the footing and cast iron plumbing in the picture below. The water line is on the other side of the plumbing, but it’s not visible in this image.
From there, we dug over and tied into our original suction point, which was just a few feet away. Before we dug this second suction point, the radon fan applied 4.9 inches of suction and moved about 1 CFM. Afterward, the fan applied 4.3 inches and moved between 20 and 30 CFM.
We ran the exhaust pipe from that second suction point next to the plumbing.
How to Plug a Suction Hole
We then plugged the hole for our first suction point. From the bottom of the suction pit, we measured to about an inch from the slab. Then we cut a piece of 2-inch pipe to that length to use it as a support for the plug. After that, we got our brush wet and moistened the concrete to clean it and prevent it from pulling the moisture out of the new concrete as quickly.
We cut a Styrofoam plug with a hole saw and set the pipe we cut earlier down in the hole to support the plug.
Putting the plug in place, we then foamed around it to create a seal. We wanted to ensure that it stayed air-tight even if the concrete shrank as it cured.
Next, we drilled a 4-inch Tapcon into the side of the stitching hole to support the concrete. Lastly, once the foam dried, we added our concrete.
After all that, we still did not have great suction under the stairs. We added a 15-gallon suction point in that area and included a valve to control airflow.
From under the stairs, the pipe went up, over the hallway, into the laundry room, and tied into the primary suction point. Next, we added the manometer, radon system alarm, and MDH system tag.
The pipe went up along the B Vent for the water heater. It’s important to note that you need an inch of clearance next to a double-wall B vent.
We added an access panel in the kitchen where the fridge would hide it. You can see the B vent on the right and the radon pipe on the left.
From there, the pipe ran up to the attic, where we installed the radon fan and vented out the roof.
Before mitigation, we saw radon levels between 4 pCi/L and 5 pCi/L. Our post-mitigation radon levels were between 0.5 pCi/L and 0.8 pCi/L