Sub-Slab Depressurization Explained

Sub-slab depressurization, often shortened to SSD, is the workhorse of Colorado radon mitigation. If you have a basement or a slab-on-grade home, SSD is almost certainly the method a quality contractor will propose. This page walks through how it works, the design decisions that affect whether it works well, and what to look for in a written quote.

The contractor drills a hole through the basement slab, typically 4 inches in diameter, and excavates a small pit beneath. The pit gives the system a place to draw soil gas from. A sealed PVC pipe is connected to the pit and routed up through the home (or up an exterior wall) to a fan, and from the fan to an exhaust point above the roofline.

The fan creates suction at the pit. Once it's running, the pressure beneath the slab drops below the pressure inside the home. Soil gas — and the radon dissolved in it — now flows into the pit and out through the system rather than into your basement.

The mechanics are simple. The decisions that determine whether it works well are not.

Before installing SSD on anything other than a simple single-zone basement, a quality contractor will run a pressure field extension (PFE) test. They drill a small test hole, apply suction with a portable vacuum, and measure how far the negative pressure spreads through the sub-slab gravel or soil.

If the field extends well — typically 20+ feet from the test point — a single suction point can pull the entire foundation footprint down. If the field is short, the soil under one part of the slab won't communicate with the soil under another, and the system needs two or more suction points to cover the full footprint.

Contractors who skip PFE on a tight-soil or multi-zone home and propose a single suction point are the contractors most likely to install a system that doesn't bring radon below the action level. Ask whether a PFE will be run before the install starts.

One suction point handles most Colorado basements with porous gravel under the slab. You need more than one when:

• The PFE test shows poor sub-slab communication (tight clay, dense soil).
• The home has multiple foundation zones — a tri-level, a split-level, an addition with its own slab.
• The slab is unusually large (typically 2,500+ sq ft).
• There's a structural footing or interior bearing wall that the soil gas can't flow under or around.

Each additional suction point adds $300–$700 in materials and labor and sometimes a second fan. That's not padding — it's the work being done correctly. More on quote variation →

The fan goes in unconditioned space — typically the attic or an enclosure on an exterior wall. The reason: the pipe between the fan and the exhaust is under positive pressure (radon-laden air being pushed out). If that pipe leaks in conditioned space, it would release radon back into the home. Putting the fan in unconditioned space means any leak in the positive-pressure pipe vents to the outdoors.^[1]

The exhaust point has specific AARST and EPA requirements:^[2]

• At least 10 feet above grade.
• At least 12 inches above the roof edge.
• At least 2 feet above or 10 feet horizontally from any window, door, chimney top, or adjacent building.

These distances ensure that exhausted radon disperses safely instead of recirculating back into the home or a neighbor's home.

SSD works because of the pressure difference. Sealing reduces leakage paths so the fan can maintain that pressure difference efficiently. A correct SSD install includes sealing of:

• Visible slab cracks, especially around the perimeter and around the suction pit.
• The floor-wall joint (the gap between the slab and the foundation wall).
• Sump pit covers — a tight, gasketed seal with a transparent inspection port.
• Plumbing penetrations (drain pipes, water lines, gas lines).
• Any expansion joints in the slab.

Sealing without depressurization is a myth — see why sealing alone isn't enough. Sealing as part of an SSD system is essential.

Radon fans are rated for sea-level performance. At Colorado Springs altitude (about 6,000 feet), a typical 4-inch inline fan loses roughly 24% of its airflow capacity.^[3] That has two implications for SSD design:

1. The contractor may need to specify a larger fan than national catalog specs suggest. For tight soils or multi-point systems, this can mean upgrading from an RP145 to a GP500 or HS-series.
2. Where a sea-level home would use one suction point, a Colorado home with the same soil type may need two — the fan can't pull a wider pressure field at altitude.

Contractors who don't adjust for altitude can quote less but install a system that doesn't reduce radon below 4.0 pCi/L. Full altitude correction details →

• Specific number and location of suction points
• PFE diagnostic test included (for multi-zone or large homes)
• Specific fan model — RP145, GP500, HS-series, Fantech, or equivalent — selected for Colorado altitude
• Pipe size and routing path described
• Sealing scope itemized (slab cracks, floor-wall joint, sump, penetrations)
• Exhaust point that meets AARST/EPA distance requirements
• Manometer install at the suction point, accessible and visible
• Post-mitigation test within 30 days, 2–7 day duration, closed-house conditions^[1]
• Workmanship warranty (1–2 years labor minimum)
• Fan warranty (5 years on a name-brand fan)