If you've seen "active sub-slab depressurization" or "active soil depressurization" on a radon inspection report and wondered what's actually happening under your slab, you're not alone — homeowners from Urbandale to Ankeny to Altoona search these exact terms every week. Here's what an ASD/SSD system is, how it's built, and what drives the price on a typical Des Moines-metro home.
What is active soil depressurization (ASD) in radon mitigation?
Active soil depressurization — also called active sub-slab depressurization, or ASD/SSD — is a radon-reduction method that uses an electric fan to continuously pull soil gas from beneath a home's foundation and vent it outdoors above the roofline before it can seep indoors. It is the primary radon-reduction technique recommended by the U.S. EPA's radon program for most existing house styles.
The system lowers air pressure in the soil beneath the slab relative to the air pressure inside the living space. That pressure difference pulls radon gas — along with other soil gases — into a network of underground piping instead of letting it migrate up through slab cracks, sump pits, or utility penetrations.
How does a sub-slab depressurization system actually work?
A licensed operator drills one or more suction points through the concrete slab, removes a small pit of gravel or soil beneath the opening, inserts PVC piping into that void, and connects the piping to an inline fan that runs continuously so the vacuum under the foundation never lets up. In split-level and multi-section homes common around Ankeny and Waukee, more than one suction point is sometimes needed for the vacuum to reach every part of the slab evenly.
The number and placement of suction points depends on what's under the slab. Homes sitting on coarse gravel usually need fewer points because air moves through the fill easily; homes on dense clay or compacted soil — common across parts of the Des Moines metro's glacial till — often need extra suction pits, or a longer diagnostic communication test before installation, to confirm the vacuum actually reaches the far corners of the foundation.
Where does the vent pipe and fan go on an Iowa home?
The fan and PVC vent pipe are installed outside the living space — typically routed through a garage, closet chase, or exterior wall — with the pipe discharging above the roof eave and away from windows and doors, similar to a plumbing vent stack, so any expelled radon disperses safely outdoors instead of re-entering the house. This routing follows ANSI/AARST soil-gas mitigation standards for existing structures.
Exterior-mounted fans are common on ranch homes without attic access, while interior or attic-mounted fans are typical on two-story homes throughout Beaverdale, Sherman Hill, and East Village, where older foundations and finished basements limit routing options. A properly installed fan draws a small but continuous amount of electricity — because it runs 24 hours a day, every day — and most homeowners describe the noise as similar to a bathroom exhaust fan running quietly in the background.
How much does an active soil depressurization system cost in Iowa?
Installed cost for an active soil depressurization system in Iowa depends mainly on foundation type, how many suction points the diagnostic test shows are needed, and how far the pipe has to travel to reach an exterior wall or roofline — homes with unfinished basements or attached garages are typically less expensive to fit than finished basements or slab-on-grade homes that need interior pipe chases built out. For a full breakdown of typical price ranges and what drives them up or down, see our radon mitigation cost guide for Iowa.
Multi-suction-point systems, sump-pit conversions, and homes with block foundation walls (which sometimes need wall-cavity depressurization in addition to sub-slab suction) generally cost more than a single-point ASD system installed on a simple ranch slab. Fans also have a service life — most need replacement roughly every 5 to 10 years — so factor that into the system's long-term cost.
How do I know if my ASD system is working correctly?
A working ASD system shows continuous negative pressure on its manometer — a small U-tube gauge mounted on the vent pipe — and should be paired with a post-mitigation radon test within 30 days of installation to confirm levels have dropped below the EPA action level of 4 pCi/L, per the EPA's Citizen's Guide to Radon. If the manometer's liquid columns sit level instead of offset, the fan may have failed or a pipe joint has likely come loose.
Homeowners should glance at the manometer periodically. Most licensed operators in our network mark a baseline reading on installation day so it's easy to spot, months or years later, when something has changed and a service call is needed.
Is a passive system enough, or does my Iowa home need an active system?
Most existing Iowa homes need an active, fan-powered system rather than a passive one, because passive stack-only setups rely on natural convection and wind effect, which rarely produce enough suction to reliably pull indoor radon below the EPA's 4 pCi/L action level in a state where indoor radon levels run well above the national average. New construction sometimes starts with passive radon-resistant features that can be activated with a fan later if a post-construction test comes back high.
Radon mitigation contractors performing this work in Iowa must be licensed under Iowa Code chapter 136B, and homeowners can ask to verify a contractor's license before signing a contract. For background on why Iowa's radon levels run so high to begin with, see our explainer on Iowa's radon geology.
This article is educational and not a substitute for a site-specific radon test or inspection by a licensed measurement or mitigation professional. Radon levels and the mitigation approach needed vary by home, foundation type, and municipality — always confirm system design and licensing requirements with a qualified local provider before installation.