Sub-membrane depressurization (SMD) is the standard radon mitigation method for homes built over a crawl space rather than a poured concrete slab. A heavy-duty polyethylene liner is sealed across the crawl space floor, an inline radon fan pulls soil gas from beneath that liner, and a PVC pipe exhausts the gas safely outside the living area. Most installations in the Atlanta area are completed in a single day.
What is sub-membrane depressurization for a crawl space?
Sub-membrane depressurization creates negative pressure beneath a sealed ground cover so that radon-laden soil gas gets drawn away before it can enter your home. A contractor lays a thick vapor barrier across the crawl space floor and seals it against the foundation walls, then routes a suction point under the liner to a dedicated radon fan and exhaust pipe.
This differs from active sub-slab depressurization (ASD), which applies to homes with concrete slabs. If your home sits on a crawl space, SMD is the correct method. Using the wrong technique for your foundation type produces poor results and failed post-mitigation tests, so identifying the foundation accurately before any work begins matters.
Is radon actually a problem in Atlanta and the surrounding counties?
Yes. Radon is a problem across metro Atlanta and especially in the counties to the north and northeast, where the underlying geology is granite and gneiss bedrock. These rock types contain naturally occurring uranium, which decays into radium and then into radon gas. That gas migrates upward through soil and into homes.
Georgia’s EPA radon zone designations reflect this. Fulton, Cobb, DeKalb, Forsyth, and Cherokee counties carry Zone 1 or Zone 2 classifications, meaning predicted average indoor radon levels are elevated enough to warrant routine testing. Hall, Gordon, Pickens, Newton, and Clayton counties, all within the service area, also see meaningful radon activity given the geology of the region.
Warm climate does not reduce radon risk. Radon comes from bedrock and soil, not from cold weather. Atlanta homes are at risk year-round, and crawl space construction, which is common across the metro area’s older housing stock, gives soil gas a direct path into the living space through gaps, vents, and unsealed penetrations.
If you want to understand the specific radon level in your home before deciding on a course of action,
residential radon testing in Atlanta is the right starting point.
What radon level means you need mitigation in Georgia?
The EPA action level is 4.0 pCi/L (picocuries per liter). Georgia follows this guidance: homes testing at or above 4.0 pCi/L should be mitigated. The World Health Organization reference level is 2.7 pCi/L, and many certified radon professionals recommend considering mitigation between 2.7 and 4.0 pCi/L, particularly in homes where young children or immunocompromised family members live.
If you are selling a home in Georgia, the Georgia Seller’s Property Disclosure Act (O.C.G.A. § 44-1-16) creates obligations around known material defects. Elevated radon that has been tested and not addressed is a disclosure issue.
A
radon level assessment gives you the number you need to make an informed decision about whether SMD or another approach applies to your property.
How sub-membrane depressurization works: the process from start to finish
Here is what actually happens during an SMD installation.
Step 1: crawl space inspection
Before any material goes in, the crawl space is inspected for standing water, severe moisture damage, or structural issues that would interfere with a proper liner seal. The size of the space, the number of suction points needed, and the best fan location are determined at this stage.
Step 2: liner installation
A reinforced polyethylene vapor barrier, typically 6-mil or thicker, is laid across the entire crawl space floor. Seams are overlapped and sealed with compatible tape. The liner is sealed to the perimeter foundation walls and around any columns, pipes, or other penetrations. The seal quality at the edges is what separates an effective system from one that leaks soil gas around the barrier.
Step 3: suction point and fan
A suction point is created beneath the liner. A PVC pipe connects that suction point to an inline radon fan mounted outside the conditioned space, usually on an exterior wall or in the crawl space itself. The fan runs continuously on low amperage, drawing air from beneath the liner and pushing it up and out through the exhaust pipe, which terminates above the roofline or away from windows per ANSI/AARST standards.
Step 4: system diagnostics
A manometer or U-tube pressure gauge is installed on the pipe to give you a visible indicator that the system is maintaining negative pressure. If the gauge reading changes significantly over time, it signals that something in the system needs attention.
What materials go into a proper SMD installation?
| Component |
Specification |
| Vapor barrier liner |
Reinforced polyethylene, minimum 6-mil thickness |
| Seam sealing |
Compatible waterproof tape sealed at all overlaps |
| Suction pipe |
Schedule 40 PVC, sized to fan requirements |
| Radon fan |
Inline fan rated for continuous operation, sized to crawl space volume |
| Pressure gauge |
Manometer installed on vent pipe for ongoing monitoring |
| Exhaust termination |
Above roofline or per ANSI/AARST placement requirements |
Fan selection matters. An undersized fan won’t generate enough suction across a large crawl space. An oversized fan pulling too hard can create pressure imbalances in the home. Sizing is based on the square footage of the liner area and the soil permeability beneath it.
How long does the installation take, and what does it disrupt?
Most single-zone SMD installations in standard crawl spaces are finished in one day. Larger crawl spaces or those requiring multiple suction points may take a full day and a half. You do not need to leave your home during the work. The crew works beneath the house.
After installation, the fan runs continuously. It draws a small amount of electricity, roughly comparable to a low-wattage light bulb, and operates quietly. Most homeowners do not notice it is running.
Post-installation testing: how you confirm it worked
Installing the system is not the finish line. A post-mitigation verification test must be done, typically within 30 days of installation, to confirm radon levels have dropped below the action level. This test uses either a charcoal canister or an alpha track detector placed under closed-house conditions.
Certified testing matters here. Results from a properly conducted
certified radon test in Atlanta carry weight for real estate transactions and provide a defensible baseline if questions arise later.
If the post-mitigation test still shows elevated levels, the system needs to be evaluated. Common causes include incomplete liner sealing, an undersized fan, or additional entry points that weren’t addressed. A retest drives what happens next, not guesswork.
For homeowners who want to understand the full scope of environmental health services available, the
environmental health service services page covers the broader range of what is offered in the Atlanta area.
FAQs about sub-membrane depressurization in Atlanta
Does my crawl space need to be clean and dry before SMD installation?
Minor moisture is normal in Georgia crawl spaces and does not prevent installation. Significant standing water or active flooding needs to be addressed first. The technician inspecting the space will tell you directly if there is a condition that must be corrected before the liner goes in.
Can I use my crawl space for storage after SMD is installed?
Yes, with care. The liner needs to stay intact. Heavy or sharp objects dragged across it can puncture the barrier and compromise the seal. Light storage on top of the liner is generally fine.
Will the radon fan run all the time?
Yes. The fan operates continuously. That is by design. The system only maintains negative pressure as long as the fan is running. If the fan stops due to a power outage or mechanical failure, radon can begin accumulating again. The pressure gauge on the pipe lets you see at a glance whether the fan is working.
How do I know if the liner seal has failed over time?
The manometer reading will drop or fluctuate if suction is lost. Periodic visual inspection of the liner and seams is also useful, particularly after any work is done in the crawl space by plumbers or HVAC contractors who may disturb the barrier.
Does SMD work differently depending on soil type?
Soil permeability affects fan sizing and suction point placement. Sandy or gravelly soils allow air to move freely beneath the liner, which makes it easier to achieve even negative pressure across the space. Dense clay, common in parts of metro Atlanta, may require additional suction points to get full coverage.
Is NRPP or NRSB certification required for radon work in Georgia?
Georgia does not currently mandate state licensing for radon contractors, but working with a professional certified through NRPP or NRSB means the work follows ANSI/AARST standards. Those standards define installation requirements, testing protocols, and post-mitigation verification procedures. Certification is how you verify that the person doing the work actually knows the trade.
