--- title: Sub-Membrane Depressurization (SMD) Explained canonical: "https://radonmitigationyorkpa.net/radon-guide/sub-membrane-depressurization/" pubDate: "2026-10-01T00:00:00.000Z" updatedDate: "2026-10-03T00:18:00.000Z" author: Radon Mitigation York PA description: "How sub-membrane depressurization lowers radon in crawl spaces and dirt floors: the membrane, sealing, suction point, fan and how to tell it works." --- # Sub-Membrane Depressurization (SMD) Explained Sub-membrane depressurization (SMD) is the standard [radon fix for crawl spaces](/services/crawl-space-radon-mitigation/) and dirt floors. A heavy plastic membrane is sealed over the soil, and a suction pipe under it connects to a fan. The fan pulls radon from under the plastic and vents it above the roof. It works like a slab system, with plastic in place of concrete. ## The basic idea Radon is a radioactive soil gas. It moves up through the ground and into any space above it. In a house with a basement slab, the standard fix is sub-slab depressurization: a fan pulls air from under the concrete, creating lower pressure there than in the house, so soil gas flows into the pipe instead of the home. A crawl space usually has no slab. The floor is dirt, gravel or a thin layer of loose plastic. SMD creates a new "floor" out of a sealed membrane and applies the same suction under it. The EPA's [Consumer's Guide to Radon Reduction](https://www.epa.gov/radon/consumers-guide-radon-reduction-how-fix-your-home) describes this approach as an effective way to reduce radon in crawl space homes. ## The parts of an SMD system ### 1. The membrane The membrane is a heavy plastic sheet, usually polyethylene and often reinforced, that covers all exposed soil. Radon standards set a minimum material requirement, and many installers use heavier liners that resist tears from crawling, storage or pests. ### 2. Sealing Sealing is what turns plastic into a radon barrier. A good SMD system has: - **Overlapped and sealed seams**, using tape or sealant made for the material. - **Edges sealed to the foundation walls**, often with sealant and a fastened strip. - **Sealed piers, posts and pipes** that pass through the membrane. If the edges and seams leak, the fan pulls crawl space air instead of soil gas, and the system loses strength. ### 3. The suction point A pipe enters under the membrane and connects to the soil space. Many installers lay a short length of perforated pipe or a collection mat under the plastic so suction spreads across the area. Larger crawl spaces may get more than one suction point. ### 4. The fan The fan is mounted outside the living space, either outdoors or in an attic, so any leak in the pressurized pipe after the fan is not inside the house. It runs all the time. ### 5. The discharge The pipe vents above the roof edge, away from windows, doors and other openings, so the radon disperses into the outside air. ### 6. The gauge A manometer (U-tube gauge) on the pipe shows the suction level. If the liquid levels are uneven, the fan is pulling. ## How to tell SMD is working You can usually see it. With the fan on, the membrane gets pulled down slightly against the soil, and you may see it tighten around rocks and dips. The manometer shows steady suction. The real proof is a test. EPA guidance says to run the post-mitigation test no sooner than 24 hours after the system starts and within 30 days. After that, retest every two years or after big changes to the home, as the EPA recommends. ## SMD vs. encapsulation These two get confused often. Encapsulation is a moisture treatment: a heavy liner sealed to the walls, vents closed and often a dehumidifier. It is built to keep a crawl space dry, not to lower radon. Encapsulation alone can reduce radon a little, but gas still collects under the liner and finds the seams. SMD adds the suction that makes it a radon system. Many homeowners combine the two. Our guide on [does crawl space encapsulation reduce radon](/radon-guide/does-crawl-space-encapsulation-reduce-radon/) covers that in more detail. ## SMD vs. crawl space ventilation Some crawl spaces are treated with ventilation: opening vents or adding a fan that blows air through the space. That can dilute radon, but the effect depends on the weather, and it brings in humid summer air and cold winter air. SMD treats the source, under the plastic, and works the same year-round. ## Where SMD is used - **Full crawl spaces** under the whole house. - **Partial crawl spaces** under an addition, sunroom or porch. - **Dirt-floor basements** and old coal rooms. - **Mixed foundations**, where a crawl space sits next to a basement and both share a fan. ## Common SMD problems - **Torn membrane.** Pets, pests, stored items or service work can puncture it. Tears should be taped right away. - **Unsealed edges.** If the liner was only laid loose at the walls, the fan pulls crawl space air. - **Too few suction points.** Large or chopped-up crawl spaces may need more than one. - **Blocked collection.** Wet or very packed soil can limit airflow under the plastic. ## SMD in York homes York homes have crawl spaces under many rear additions, sunrooms and some mid-century houses, often next to a basement. York County is in EPA Radon Zone 1, so any crawl space home here should be tested. In Pennsylvania, only a PA DEP certified mitigator can install an SMD system for you. ## Key takeaways - SMD is the main radon fix for crawl spaces and dirt floors. - It uses a sealed membrane, a suction point, a fan and a roof-line discharge. - Sealing seams, edges and piers is what makes it work. - Encapsulation alone is a moisture fix. Adding suction makes it a radon fix. - A clearance test, 24 hours to 30 days after start-up, confirms the result. ## Frequently Asked Questions ### Can I walk on an SMD membrane? Yes, carefully. Crawl on knee pads and avoid dragging sharp items. Patch any tear right away. ### Does SMD make the crawl space colder? Not by itself. It pulls air from under the plastic, not from the crawl space air. Insulation and vent sealing affect temperature more. ### How long does an SMD membrane last? A heavy, well-sealed membrane that is not disturbed can last many years. Check it during routine visits to the crawl space. ## Get an SMD system installed Want to know if SMD is right for your crawl space? See our crawl space radon mitigation in York, or call [(717) 356-3035](tel:+17173563035) any time for a same-day callback and a free written quote.