What Sub-Slab Depressurization Is
Sub-slab depressurization is the method used to mitigate radon in the large majority of Bismarck-area homes with a basement or slab-on-grade foundation. One or more suction pits are drilled through the concrete floor into the gravel or soil layer beneath it. A pipe connected to that pit runs to a continuously operating fan, which pulls soil gas from beneath the slab and vents it above the roofline before it has a chance to seep upward into the living space.
Why It Works For Basement And Slab Homes
Most homes in and around Bismarck have either a poured concrete basement floor or a slab-on-grade foundation, both of which provide one continuous surface that can typically be depressurized from a single strategic suction point, or a small number of them. That is different from a crawl space, where exposed soil across a much larger and less uniform surface calls for a membrane-based approach instead.
How Many Suction Points A Home Needs
The number of suction points a home needs depends largely on how permeable the material beneath the slab is. Homes with coarse gravel fill under the slab often need fewer suction points, since air moves relatively freely through that layer. Homes built on the clay-rich glacial till common across Burleigh and Morton counties can need an additional suction point, or a stronger fan, because clay is less permeable and restricts how far a vacuum extends from a single point. This is one of the more important local variables in system design, and it's why a diagnostic step matters more here than it might in a region with uniformly sandy soil.
Diagnostic Steps Before Installation
Before finalizing suction point locations, installers typically run a diagnostic check — using a method like a smoke pencil or a temporary vacuum test at a test hole — to see how far measurable vacuum extends across the slab from that point. This step also accounts for any sump pit, since sump pits are frequently tied into the mitigation system with a sealed lid rather than left as a separate opening that could undercut the system's suction.
Sealing Sump Pits And Floor Cracks
Sealing is a companion step to suction, not a substitute for it. A sump pit is commonly fitted with a sealed lid connected into the depressurization system, and visible cracks or gaps at the floor-wall joint are sealed to help the system hold consistent negative pressure across the whole foundation rather than losing suction to the easiest nearby opening.
When A Home Needs More Than One Zone
Additions, multi-level foundations, and homes with a block foundation wall sometimes need more than a single sub-slab suction point. A block wall's hollow cores can carry soil gas differently than solid poured concrete, which occasionally calls for a block-wall suction approach in addition to standard sub-slab suction. Homes that combine a basement with a later addition built on a different foundation type may also need a second zone, similar to the mixed-foundation situations covered on the crawl space mitigation page.
Fan Sizing And Selection
Fan selection depends on how much vacuum needs to be maintained across the sub-slab area and how much resistance the system has to work against, which ties directly back to soil permeability. A home on more permeable, gravel-heavy fill beneath the slab may only need a lower-capacity fan, while a home on denser clay till may call for a higher-capacity fan to pull enough vacuum across the same footprint. Oversizing a fan doesn't necessarily produce a better result and can add unnecessary noise or energy use, so sizing is generally matched to the diagnostic readings taken at the suction point rather than defaulting to the largest available fan.
Signs An Existing System May Need Attention
Homes that already have a sub-slab system installed, whether by a previous owner or from new construction, can still develop issues over time. A manometer reading that has flattened out or dropped to zero usually means the fan has stopped running. Unusual noise, visible cracking or gaps that have reopened around a sealed sump lid, or a retest that comes back higher than expected are all reasons to have an existing system inspected rather than assumed to still be working correctly. The system installation page covers what a properly functioning system looks like day to day.
Sub-Slab Depressurization And Sump Pits
A sump pit is one of the more common wrinkles in a Bismarck-area sub-slab project, since many area basements include one for groundwater management. Rather than treating the sump as a separate issue, most installers tie it directly into the mitigation system: the pit is fitted with an airtight lid with a pass-through for the sump pump's discharge line and float switch wiring, and the pit itself often becomes the suction point, since it already provides an opening down to the sub-slab material. This approach avoids drilling an additional hole through the slab when a usable one already exists.
How Sub-Slab Suction Interacts With Radiant Heat And In-Floor Systems
Some newer or renovated Bismarck-area homes include in-floor radiant heat, which adds another layer of planning to suction point placement, since installers need to avoid drilling into or near embedded tubing. A diagnostic review of available plans, or a careful physical check before drilling, helps identify safe suction point locations in homes with radiant floors, and may shift where a suction pit ends up relative to a home without that system.
Older Foundations And Block Wall Construction
Some older homes in established Bismarck neighborhoods have concrete block foundation walls rather than poured concrete, and block wall construction has hollow cores that can carry soil gas separately from the slab itself. In these homes, sub-slab suction alone sometimes isn't enough, and a supplemental suction point drawing from the block wall cavity is added to address that separate pathway. Identifying whether a foundation is poured or block construction is one of the first things worth noting when describing an older home in a request.
Retesting After Installation
As with any mitigation approach, a post-installation retest confirms whether the result has dropped below the EPA's 4 pCi/L action level. See the system installation page for the full installation process and the cost guide for what affects pricing based on suction-point count and other factors.