# Radon Mitigation Pipe Size

- **URL:** https://airsenseenvironmental.com/radon-mitigation-pipe-size/
- **Published:** 2026-09-07

Skip to content Radon Mitigation Services ▾ Radon Testing Commercial Radon Mitigation Commercial Radon Testing Indoor Air Quality Crawl Space Encapsulation Service Area Resources FAQ Blog Podcast Reviews & Projects About Us Contact 314-664-9807 Free Estimate Radon Mitigation Services Radon Testing Commercial Radon Mitigation Commercial Radon Testing Indoor Air Quality Crawl Space Encapsulation Company Service Area Resources FAQ Blog Podcast Reviews & Projects About Us Contact Call 314-664-9807 St. Louis Metro · Missouri & Illinois Radon Mitigation Pipe Size Sub-Slab Depressurization Radon Mitigation NRPP-Certified Radon Pros MO & IL Radon mitigation pipe size is most commonly 3-inch or 4-inch schedule 40 PVC, and the correct choice depends on the airflow the system needs and how much resistance the soil and pipe run present, not on what fits the fan. Undersizing the radon mitigation pipe adds resistance the fan has to overcome, which costs airflow at exactly the point the system needs it. Oversizing wastes nothing but material, which is why 4-inch is the safer default on longer runs. Pipe size is one variable among several. Suction-point placement, fan sizing, slope, and sealing all interact, and the target is the same either way: bringing the home below the EPA action level of 4.0 pCi/L. Call 314-664-9807 Get a Free In-Home Estimate IEMA Radon License #RNM20232346 Free Written Estimates Post-Mitigation Radon Testing What the Pipe Serves EPA action level4.0 pCi/L Our design target< 2.0 pCi/L Systems are engineered to bring your home below the EPA action level of 4.0 pCi/L. 3 in. or 4 in. PVC The Answer What size pipe is used for radon mitigation? Radon mitigation uses 3-inch or 4-inch schedule 40 PVC in nearly all residential installations, with 4-inch being the more common choice on longer runs and higher-airflow systems. The general pattern: 3-inch PVC: adequate for short runs, smaller footprints, and systems moving modest airflow through permeable sub-slab aggregate 4-inch PVC: preferred for long vertical runs, multiple suction points tied together, larger footprints, and any system where the fan is working against real resistance 2-inch PVC: occasionally seen on very small systems or old installations, and generally too restrictive for a modern fan Schedule 40 is the standard wall thickness. It is rigid enough not to deform under continuous vacuum and it accepts standard fittings, which matters because every joint in a radon pipe has to be sealed against leakage. Why It Matters Does radon mitigation pipe size affect performance? Yes. Radon mitigation pipe size directly affects how much air the system moves, because a narrower pipe imposes more friction on the airflow and the fan has to spend part of its capacity overcoming that instead of pulling on the soil. The practical consequences of an undersized pipe: Reduced suction at the pit, which shrinks the area of slab the system actually depressurizes A fan running harder than designed, which shortens its service life More noise, since air moving fast through a narrow pipe is audible A manometer that still looks fine, because pressure in the pipe is not the same measurement as coverage under the slab That last point is the trap. An undersized system can pass a casual inspection while leaving part of the house untreated, which is why post-mitigation radon testing is the only real confirmation. Installation How should radon mitigation pipe be routed and sloped? Radon mitigation pipe should be sloped back toward the suction point so condensate drains into the soil rather than pooling in the pipe, and it should be supported so it cannot sag into a low spot over time. Slope: a continuous fall back to the suction pit, with no dips that would trap water Support: strapped to joists or the exterior wall at regular intervals to prevent sag Joints: solvent-welded and sealed, since any leak on the vacuum side pulls room air instead of soil gas Freeze risk: exterior runs in the St. Louis climate can accumulate condensate that freezes, which is one argument for interior routing or for larger diameter A pooled section of water in a radon pipe acts like a partial blockage, and it can freeze solid in winter, which is exactly when radon levels are highest. Interior vs Exterior Should radon pipe run inside or outside the house? Either routing works, and the choice is a trade-off between appearance, freeze risk, and serviceability rather than a code question. Interior runs stay warm, which reduces condensate freezing, and the pipe is hidden inside closets or utility chases. They consume interior space and are harder to service later. Exterior runs are faster to install, easy to service, and keep the fan outdoors, but the pipe is visible on the side of the house and exposed to freeze-thaw cycles. Either way the discharge clearances are the same: at least 12 inches above the roof surface, at least 10 feet from any window, door, or air intake, and at least 10 feet above ground level. Those requirements are covered in the radon mitigation system diagram. Local Conditions Radon mitigation pipe sizing in St. Louis homes Radon mitigation pipe sizing across the St. Louis metro is driven mostly by what sits under the slab. Homes built over loose gravel aggregate, common in newer St. Charles and Jefferson county construction, present less resistance and often run comfortably on 3-inch pipe from a single suction point. Older St. Louis City and County homes are a different case. Compacted clay under the slab, stone foundations, and multiple foundation sections joined by additions all raise resistance, which usually means 4-inch pipe, a stronger fan, or more than one suction point tied together. Missouri and Illinois both follow the same discharge clearances, and Illinois requires radon professionals to be licensed through IEMA. Air Sense Environmental holds license #RNM20232346 and sizes each system from a pressure-field test rather than a rule of thumb. See sub-slab depressurization for the system it serves, or radon mitigation for the full service. Common Questions Frequently Asked Questions About Radon Mitigation Pipe Size Is 3-inch pipe enough for radon mitigation? Three-inch schedule 40 PVC is enough for many residential radon mitigation systems, particularly short runs over permeable sub-slab aggregate with a single suction point. It becomes marginal on long vertical runs, larger footprints, or systems tied to multiple suction points, where 4-inch reduces resistance and lets the fan deliver more of its capacity to the soil. Can you use 4-inch pipe with a 3-inch radon fan? Yes, and it is common. Reducing from a larger pipe down to the fan's inlet is normal practice, because the larger pipe lowers friction along the run while the fan connection stays at its rated diameter. The reverse, running 3-inch pipe on a system that needs 4-inch, is what causes performance problems. What type of pipe is used for radon mitigation? Schedule 40 PVC is the standard for residential radon mitigation. It is rigid enough to hold its shape under continuous vacuum, accepts solvent-welded fittings that seal reliably, and is readily available in both 3-inch and 4-inch. Flexible duct and thin-wall pipe are not appropriate, because leaks and deformation both undermine the vacuum. Does radon pipe need to be sloped? Yes. Radon mitigation pipe should slope continuously back toward the suction point so condensate drains into the soil rather than collecting in the pipe. A low spot that holds water acts as a partial blockage and can freeze in winter, which is precisely when radon levels are at their annual peak. Can radon mitigation pipe be reduced in diameter partway up? It can be, but it usually should not. Any reduction in diameter adds resistance at that point, and the fan has to overcome it for the life of the system. Reductions are sometimes unavoidable where a pipe passes through a tight chase, in which case the fan should be sized with that restriction accounted for. References: EPA radon action-level guidance and A Citizen's Guide to Radon. Sized From a Test, Not a Rule Get the system sized properly Pipe size follows from a pressure-field test on your foundation. We measure first, then specify the pipe and the fan together. 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