Radon Mitigation Design for Multifamily Buildings

A radon mitigation design for a multifamily building is a plan-specific engineering package that shows how soil gas is collected beneath the slab and vented above the roof, specified to ANSI/AARST CC-1000-2018 so your plan reviewer and your lender can approve it. It is a design deliverable for your permit set, not an installation contract. This page covers what that package contains, the standard it answers to, the two system types you choose between, and where HUD financing changes the requirement.

Multifamily projects sit outside the prescriptive residential codes that cover single-family homes. CC-1000-2018 applies to new construction of any building intended for human occupancy other than one- and two-family dwellings, which puts apartments, condominiums, congregate housing, schools, and commercial occupancies squarely in its scope. The design has to be drawn for that standard from the start, because a reviewer checks the package against it line by line.

The package is the document set a contractor builds from and a plans examiner stamps. For a multifamily building it covers the full footprint, not a single foundation. A complete package includes:

• A sub-slab collection layout: the gas-permeable layer, suction point locations, and the spacing that ties them together under each slab-on-grade and below-grade area.
• A continuous soil gas barrier specification: the membrane, the laps, and how it is sealed at every penetration, footing, and construction joint.
• Vent stack routing and sizing from each suction point through conditioned space to a discharge above the roofline, clear of windows and air intakes.
• Pressure-field and fan provisions: where the design is passive, where it is built to convert to active, and the electrical rough-in for any fan.
• Code references and the certified radon professional's review, so the credential the compliance chain expects is built into the deliverable.

The point of the package is that it travels with your permit set as a standalone document. Your contractor installs from it and your examiner checks it without you having to bundle design into a full installation bid.

CC-1000-2018 is the consensus standard for soil gas control systems in new construction of large buildings, and the EPA recognizes the ANSI/AARST consensus standards as the radon standards of practice. The standard is prescriptive: it sets the minimum requirements for the soil gas barrier, the gas-permeable collection layer, suction point design, vent stack routing, and the inspections that confirm the system was built as drawn.

Two requirements drive most of the design work. First, the barrier has to be continuous, forming an unbroken layer between the soil and the occupied airspaces above. Second, the collection and vent system has to be able to depressurize the soil under that barrier, either passively or with a fan. The 2023 revision tightened the construction-stage inspections from optional to required, which means the package now has to anticipate what an inspector verifies before, during, and after the pour.

Every CC-1000 design starts passive and is engineered so it can go active without rework. The two modes differ in how the soil is depressurized:

• A passive system relies on the stack effect. Warm air rising in the vent stacks creates enough draw to pull soil gas up and out without a fan. It runs silently and uses no power, and on many sites it holds indoor radon below the action level on its own.
• An active system adds an inline fan to each stack. It is specified when the building is large, the soil is tight, or post-construction testing shows the passive draw is not enough. Because the design already locates the fan position, the wiring, and the labeling, conversion is a field change rather than a redesign.

Designing for both from the outset is the difference between a one-line field upgrade and tearing back into a finished slab. The package shows the passive layout and the active provisions on the same sheets.

If the project is HUD-financed, radon control stops being optional. HUD recognizes ANSI/AARST CC-1000-2018, “Soil Gas Control Systems in New Construction of Buildings,” as the applicable standard for new-construction radon control, and HUD Multifamily requires radon to be addressed in the environmental review. In practice that means the lender and the plan reviewer both expect a design keyed to CC-1000-2018, signed by a certified radon professional, in the package they approve. A design that only gestures at radon, or one bundled into an installer's scope without its own stamped sheets, slows the financing down.

The same logic applies to schools and commercial occupancies inside the standard's scope. The cleanest path is a design built to the governing standard and carrying the credential the reviewer is looking for, so the radon question is answered before it is asked.

The design is drawn from your building, so it starts with your plans. Send a vector PDF architectural set exported from CAD or Revit. The workflow reads the foundation geometry, lays out the collection and vent system across the footprint, specifies the barrier and sealing details, routes the stacks, and references the governing standard. A certified radon professional reviews and signs the result. You receive a code-referenced package ready to drop into your permit set, quoted up front with no retainer and no bundled installation contract.

If you are scoping a new apartment, condominium, or mixed-use building, start with the multifamily building-type page for what the package covers, then request a quote with your plans and we return a fixed price before any work begins.