Sub-slab depressurization is the workhorse of radon mitigation. It is the system most York basements end up with, and for good reason: it stops soil gas before it reaches the house. This guide explains how it works, how the suction point and pipe route are chosen, and how to tell the system is doing its job.
Official source: EPA radon resources and guides
How does sub-slab radon mitigation work in a basement?
Sub-slab radon mitigation works by creating low pressure under the basement floor. A suction pit is formed under the slab, a sealed PVC pipe connects it to a fan outside the living space, and the fan vents soil gas above the roof. Because the area under the slab is at lower pressure than the basement, radon flows into the pipe instead of the house. A gauge on the pipe shows the suction.
Pressure Is the Key
Radon enters a home because the house is usually at slightly lower pressure than the soil around it. Warm air rising inside, exhaust fans, and the wind all contribute. That pressure difference pulls soil gas through cracks, joints, and openings in the basement floor.
Sub-slab depressurization flips that relationship. By pulling air from under the slab, the fan makes the soil gas area lower in pressure than the basement. Now air flows from the basement toward the soil, not the other way around, and radon is drawn into the pipe and vented outside.
The fan does not need to move a huge amount of air. It needs to create steady, gentle suction across the slab. That is why the gravel layer under the floor, and the sealing of big openings, matter so much.
Choosing a Suction Point
The suction point is where the pipe meets the soil under the slab. Contractors look for a spot that balances performance and convenience: an unfinished area, close to where the pipe can exit or rise, and well connected to the rest of the slab.
A quick suction test helps. The contractor drills a small hole, applies a vacuum, and checks how far the effect reaches at other test holes. If the suction spreads widely, one point may cover the whole slab. If not, a second point might be needed.
Homes with additions, split levels, or more than one slab often need extra suction points, since the soil under each section may not connect. The contractor will explain this in the estimate.
Routes, Fans, and Gauges
Once the suction point is chosen, the pipe needs a path. Interior routes run through a closet or chase to a fan in the attic. Exterior routes run out through the rim joist and up the side of the house, with the fan on the outside wall. Either way, the fan stays outside the living space and the pipe vents above the roofline.
The gauge is your window into the system. A U tube manometer mounted on the pipe shows suction as uneven liquid levels. The installer usually marks the starting level so you can spot changes over time.
Check the gauge every month or so, retest every couple of years, and call if the level drifts toward even or the fan sounds different.
The Standard Fix, Done Well
For most York basements, sub-slab depressurization is the right answer. The details that make it work well are the suction point, the sealing, and the route. Call us and we will connect you with a local contractor who can plan those details for your home in a written quote.
Frequently Asked Questions
Will a sub-slab system make my basement colder?
A sub-slab system draws mostly from under the slab, not from the basement air, so the effect on room temperature is usually small. Sealing large openings helps keep conditioned air from being pulled into the system. Ask the contractor if you notice drafts near the suction point.
How much electricity does the fan use?
Radon fans are small and use about as much electricity as a light bulb or two running continuously. The exact amount depends on the fan model. The contractor can tell you the wattage of the fan they recommend. The contractor can confirm the details for your home during the free on-site visit.
Can a sub-slab system be added to a basement with a sump?
Yes. The sump is usually sealed with an airtight lid, and in some homes it becomes the suction point itself. Sealing the sump keeps basement air from leaking under the slab and helps the fan build suction. Your written estimate will note how this applies to your home.