If you've seen the term "active sub-slab depressurization" (sometimes called active sub-slab suction) on a radon inspection report or a contractor bid, it can sound more complicated than it is. It's the most common radon mitigation method used in Colorado basements and slab-on-grade homes, and once you see the four or five parts that make it up, the whole system is easy to understand.
What does "active sub-slab depressurization" actually mean?
Active sub-slab depressurization (ASD) is a radon reduction method that uses an electric fan to continuously pull soil gas from beneath a home's foundation slab and vent it outdoors above the roofline, before the gas can seep up through cracks and openings into living space. The word "active" distinguishes it from passive systems, which rely on natural air pressure differences alone and don't include a fan.
The U.S. Environmental Protection Agency identifies sub-slab depressurization as the most widely used and generally most reliable radon mitigation technique for homes with basements or slab foundations, according to the EPA's Consumer's Guide to Radon Reduction. It works because most soil gas doesn't come through solid concrete -- it travels through the gravel and soil layer under the slab, then finds its way in through sump pits, floor cracks, and utility penetrations.
How is active soil depressurization different from a passive system?
The difference is the fan. An active system has an inline radon fan mounted in the attic, garage, or exterior wall that runs continuously to create negative pressure under the slab; a passive system uses only pipe routing and the stack effect of warm air rising, with no fan. Active systems typically achieve lower radon levels because the fan produces a much stronger and more consistent suction than passive air movement alone.
Some new Colorado construction includes a passive radon-resistant rough-in (a vertical pipe stub from the sub-slab gravel to the attic) that can later be "activated" simply by adding a fan and a discharge run to the roofline, which is usually a smaller job than installing a full system from scratch.
What is active sub-slab suction and how does the fan actually work?
Active sub-slab suction is the physical process of the radon fan drawing air from a pipe network embedded in or below the slab, creating a zone of lower air pressure under the house than inside it, so soil gas flows out to the vent pipe instead of up into the living space. One or more suction points are cored through the slab into the gravel or soil below, connected by PVC pipe that runs up through the house and out above the roof.
A properly installed system runs 24 hours a day, every day, because radon entry is driven by pressure differences that exist around the clock, not just during certain hours or seasons. The licensed operators in our network in Colorado typically install the discharge pipe so it vents above the roofline and away from windows, doors, and other openings, consistent with ANSI/AARST radon mitigation system standards, so exhausted gas doesn't re-enter the home or drift toward a neighbor's window.
What is a manometer and why does it matter?
Every active system should include a small U-tube manometer (or an electronic equivalent) mounted on the visible pipe run, usually in the basement or garage. It's a simple pressure gauge that shows whether the fan is actually creating suction. If the liquid column sits level or the reading matches what's noted on the install tag, the fan has likely stopped working and the system needs a service call -- this is the one part of the system a homeowner can check visually in about five seconds without any tools.
How much does an active soil depressurization system cost in Colorado?
Cost varies by home size, foundation type, number of suction points needed, and whether the fan is exposed to Colorado's freeze-thaw cycles outdoors versus routed through a conditioned attic. Rather than quote a number here that won't hold up across Jefferson County, Denver, Boulder, or El Paso County homes differently, see our dedicated cost breakdown for what Arvada and Jefferson County homeowners typically pay for a full ASD installation.
How do you know if an active soil depressurization system is working correctly?
You know an ASD system is working when the manometer reads within the range noted on the system's install label, the fan produces a faint but steady hum, and a follow-up radon test taken after installation shows levels below the EPA's action level of 4 pCi/L. A mitigation contractor should provide (or arrange) that post-installation test, since installing the hardware alone doesn't confirm the result -- only a retest does.
Colorado does not license radon mitigation contractors at the state level, but the Colorado Department of Public Health & Environment (CDPHE) radon program directs residents toward professionals certified through the National Radon Proficiency Program (NRPP) or the National Radon Safety Board (NRSB). It's worth asking any installer for their current certification number before work begins, and confirming with CDPHE's radon page whether that credential is still active.
For background on what radon levels actually look like across the Front Range before you decide whether mitigation is even necessary, see our guide to average radon levels in Arvada and Boulder. And if you want the deeper technical walkthrough of a full ASD install from suction point to rooftop vent, we cover that step by step in our piece on active soil depressurization systems in Arvada.
This article is for general homeowner education and is not a substitute for a site-specific inspection, a current radon test, or advice from a certified radon professional. Requirements, permitted methods, and typical costs vary by municipality and by home -- always confirm details with a licensed, certified operator before starting work.