Subslab Depressurization vs. Sub-Membrane Suction: Which Do You Need?

Two systems dominate residential radon mitigation. Active sub-slab depressurization (SSD) and sub-membrane depressurization (SMD). Both pull radon from under your home and vent it above the roofline. The difference? What's between the soil and your living space.

The short answer? If you've got a concrete slab or basement floor, you need SSD. If you've got a dirt-floor crawlspace, you need SMD. Got both? You might need both.

How Sub-Slab Depressurization Works

SSD is the bread and butter of radon mitigation. A contractor cores a 4 to 5 inch hole through your concrete slab, digs out a small pit underneath for suction, and runs PVC pipe from that point up through the roof. A fan in the pipe creates negative pressure under the slab -- basically a vacuum that pulls soil gas up and out before it seeps into your home.

Sounds simple because it kind of is. And that's why it works so well.

Your slab might look solid, but it's full of entry points for radon. Construction joints, shrinkage cracks, pipe penetrations, the gap where the slab meets the foundation wall. Warm air rising inside your home (the stack effect) naturally pulls soil gas in through all of those openings. SSD reverses that pressure gradient.

Most homes need one suction point. Larger homes, additions with separate pours, or homes with very tight soil might need two or three. If you're in a city like Philadelphia or Albany where housing stock varies wildly, the number of points really depends on the specific home.

How Sub-Membrane Depressurization Works

No slab? No problem -- but it's more work.

SMD covers the exposed dirt in your crawlspace with a heavy-duty poly membrane (6-mil minimum). The membrane gets sealed to the walls, around support piers, around pipes -- every penetration. Then a suction point goes under the membrane, connected to the same kind of fan-and-pipe setup as SSD.

Real talk: the install is harder. Crawlspaces are tight, dirty, and awkward to work in. Cutting and fitting membrane around obstacles takes time. And if the sealing isn't thorough, the vacuum under the membrane weakens and radon bypasses the system. That's why SMD costs more -- you're paying for careful labor in uncomfortable conditions.

What If You've Got Both?

Split-level homes and houses with a partial basement plus a crawlspace are common in the Northeast and Midwest. For these, you typically need SSD under the slab portion and SMD in the crawlspace. Two suction points, potentially two separate pipe runs, sometimes two fans.

It's more expensive but there's no shortcut. You can't just treat half the foundation and hope for the best.

So Which One Actually Works Better?

Neither. They're both excellent when installed properly. The EPA rates active soil depressurization as the most reliable technique for reducing indoor radon, capable of cutting levels by 80% to 99% in most homes.

The key word there is "properly." A sloppy SSD install with poor sealing will underperform a meticulous SMD job, and vice versa. What matters isn't the method -- it's the contractor's attention to suction point placement, crack sealing, fan sizing, and vent routing.

After installation, a post-mitigation test confirms whether levels dropped below 4.0 pCi/L (the EPA action level). Ideally you want below 2.0. If levels stay elevated, the contractor should evaluate whether they need more suction points, better sealing, or a stronger fan. We cover what to do with elevated results in our high radon test guide.

Choosing the Right Contractor for Either System

Look for current NRPP or NRSB certification -- that applies regardless of system type. And ask specifically about their crawlspace experience if you need SMD. Not every radon contractor loves crawlspace work, and you want someone who does it regularly. Check our 8 questions to ask before signing anything.