SGM-MFLB-2023 Radon Mitigation in Existing Large Buildings

If you own or manage an existing large building that has tested high for radon, the standard that governs the fix is ANSI/AARST SGM-MFLB-2023. It sets the minimum requirements for mitigating radon and other soil gas in existing multifamily, school, commercial, and mixed-use buildings, the properties too large to fall under the single-family home standard. It is the mitigation counterpart to CC-1000-2018, which governs radon control in new construction. This page explains what SGM-MFLB-2023 requires, how mitigating an occupied building differs from designing a new one, and what a compliant mitigation design package for an existing large building has to contain.

SGM-MFLB stands for Soil Gas Mitigation Standards for existing Multifamily, School, Commercial, and Mixed-Use Buildings. It is one of the 2023 ANSI/AARST releases, and it consolidated two older documents into a single standard: the large-building mitigation standards previously published as RMS-MF and RMS-LB were combined into SGM-MFLB-2023. The compliance date is December 1, 2023.

The retitling from “Radon Mitigation” to “Soil Gas Mitigation” is not cosmetic. The same sub-slab systems that pull radon out of the ground also control the intrusion of chemical vapor contaminants, so the standard now speaks to soil gas broadly rather than radon alone. That matters for commercial and mixed-use sites where a vapor concern often sits alongside the radon reading.

SGM-MFLB-2023 carries weight because it is a recognized standard of practice, not a vendor preference. The EPA treats the ANSI/AARST consensus documents as the radon standards of practice, which is why lenders, HUD reviewers, and school districts point to them when they ask for a design.

The cleanest way to place SGM-MFLB-2023 is against its new-construction sibling. CC-1000-2018 governs soil gas control designed into a building before the slab is poured. SGM-MFLB-2023 governs the retrofit, the system you install in a building that already exists and is already occupied.

That difference drives every design decision:

• CC-1000-2018 works with an open excavation. A gas-permeable layer and suction points go in under the slab as it is built, so the collection network is effectively free.
• SGM-MFLB-2023 works over a finished, occupied floor. The design has to reach the sub-slab space through cores drilled in existing slabs, route piping through tenant and common areas, and hold the building in operation while the work happens.

Because you cannot rebuild the sub-slab, the existing-building design leans harder on diagnostics: measuring how far a single suction point can depressurize the soil before committing to a fan and a pipe layout. Get that wrong in new construction and you add a suction point on the drawings. Get it wrong in an existing building and you are coring another hole through an occupied lobby.

A design that meets the standard is not a fan on a wall. It is an engineered sub-slab depressurization system sized to the building, and it specifies:

1. Diagnostics first. A sub-slab communication (pressure field extension) test establishes how many suction points the building needs and where they go, before any fan is selected.
2. Suction points and fan sizing matched to the measured sub-slab conditions, not a rule of thumb, so the system actually holds a negative pressure field under the whole footprint.
3. Vent routing that carries soil gas from the suction points to a discharge above the roofline, clear of operable windows, doors, and fresh-air intakes.
4. Sealing of major slab openings, sumps, and penetrations so the fan pulls on the soil rather than short-circuiting to indoor air.
5. System monitoring and labeling so building staff can confirm the fan is running, plus a post-mitigation testing plan to prove every affected area now reads below the EPA 4.0 pCi/L action level.

Our multifamily radon design page walks through how this comes together for an apartment or condominium building.

The trigger is almost always a measurement. Common paths to a mitigation design include:

• An existing multifamily, school, or commercial building tested above 4.0 pCi/L, whether through a routine testing program or a specific complaint.
• A HUD refinance or acquisition of existing multifamily housing, where a failing test moves the building into required mitigation.
• A property transaction where a lender or buyer's due diligence turns up an elevated radon result.
• A school district running a district-wide testing program that flags one or more buildings.
• A vapor intrusion concern on a commercial or mixed-use site, where the soil gas system does double duty.

In each case the deliverable is the same: an engineered mitigation design keyed to SGM-MFLB-2023, not a handyman quote to “add a radon fan.”

An existing-building project does not clear a lender, a school board, or a HUD environmental review on a design that only gestures at radon. The package a reviewer expects:

• Is keyed explicitly to ANSI/AARST SGM-MFLB-2023 and references the standard where it applies.
• Is reviewed and signed by a certified radon professional, the credential the compliance chain looks for.
• Travels as standalone sheets in the construction documents, so the installing contractor builds from it and the reviewer checks it directly.
• Includes a confirmation testing protocol that names the action level and requires re-testing after the system is running.

A package that meets those tests answers the radon question before the reviewer has to ask it, which is what keeps a refinance or an occupancy decision on schedule.

If you are dealing with an elevated radon result in an existing apartment, school, or commercial building, send your building information and we return a fixed price for a stamped SGM-MFLB-2023 mitigation design. You can request a quote with your floor plans and building details, and we scope the diagnostics and the system before any work begins.