Guide

Sub-Slab Depressurization: How the Standard System Works

The principle in one paragraph

A house is a weak pump. Warm air rises and leaves through the upper part of the building, and replacement air is drawn in low, including through the foundation. That pressure difference is what pulls soil gas indoors. Sub-slab depressurization does not remove radon from the soil, which would be impossible. It reverses the pressure difference so the building stops pulling.

Step by step

1. Diagnostics

Before anything is cut, a competent installer works out what is under the slab. Communication testing, drilling a small test hole and measuring pressure change at other points, shows how far suction will travel. A slab over clean gravel communicates well and may need one suction point. A slab poured directly on clay or fill may need several.

2. The suction point and pit

A hole roughly four to five inches across is cored through the slab, usually in a corner, a utility area or an unfinished part of the basement. Material is excavated beneath it to create a small pit, which gives the fan a volume to draw from rather than trying to pull through packed soil. That pit is a significant part of why one system works and another underperforms.

3. The pipe route

PVC rises from the suction point and runs either interior, up through the house and out the roof, or exterior, up an outside wall. Interior runs are quieter, less visible from the street and keep the pipe warm so condensation drains back rather than freezing. Exterior runs are less invasive to finished space. Pipe is sloped so condensate returns to the pit.

4. The fan

An inline fan goes on the pipe, and by code it must sit outside the living space: attic, garage, or outdoors on the exterior run. That is a safety requirement, not an aesthetic one. If any part of the pipe under positive pressure leaked inside the house, it would be discharging radon-laden air into the building.

5. The vent termination

The pipe terminates above the roofline, above the eave, and away from windows, doors and any other opening, so that discharged gas disperses rather than re-entering. A termination that stops short, or sits next to a bedroom window, is a failed installation regardless of what the post-test says.

6. Sealing

Accessible openings in the slab get sealed: the perimeter expansion joint, the gap around a sump pit, plumbing penetrations, visible cracks. Sump pits get a sealed lid. This is not the fix, it is efficiency. Every unsealed opening is somewhere the fan wastes suction pulling conditioned indoor air instead of soil gas.

7. The manometer and the post-test

A U-tube manometer is fitted to the pipe. When the fan is pulling, the two liquid columns sit at different heights; when it is not, they level out. That is your at-a-glance status indicator for the life of the system. A post-mitigation test then confirms the result, and that number is the actual deliverable.

What a good installation looks like when it is done

• The manometer shows a clear pressure difference and is mounted where you can actually see it
• The pipe is properly supported, sloped back toward the suction point, with no sagging horizontal runs holding water
• The fan is outside the living space and its housing is sealed at every joint
• The termination is above the roofline and clear of windows and other openings
• There is a labelled system description near the fan or the manometer, as standards require
• An electrical disconnect for the fan is accessible, so it can be isolated for service
• You have a written post-mitigation test result, on paper, with a number on it

When this is not the right system

Sub-slab depressurization assumes a slab with something drawable underneath it. A dirt-floor crawlspace has no slab, and needs sub-membrane depressurization instead. Hollow block walls, existing drain tile loops, mixed foundations and slab-on-grade all change the design, and those are covered on system types. A newer home may have a passive rough-in already in place, which is much cheaper to activate than to install from scratch: see passive vs active.

Once it is in, the ongoing story is short and is on fans and maintenance.

Common questions

Does the fan run all the time?

Yes, continuously. It is a small fan, typically drawing power comparable to a light bulb, and cycling it off lets pressure equalise and radon return. Turning it off to save electricity defeats the system.

Is it loud?

A correctly installed system is a low hum at the fan and some airflow noise in the pipe. The fan is not permitted in the living space, so it sits in an attic, a garage or outside. Noticeable noise usually means a fan that is oversized, mounted against a wall that transmits it, or failing.

Will it change my heating bill?

Slightly. The fan uses a modest amount of electricity continuously, and the system removes some conditioned air along with the soil gas. Good sealing at the suction point and around obvious floor openings keeps that loss small.