Indoor Systems...

When we install a radon-mitigation fan and most of its exhaust system piping inside a home's attic or attached garage, it's called as you might expect, an indoor mitigation system.

The primary benefit of an indoor system is cosmetic. The only thing that can be seen from the outdoors is an exhaust pipe protruding from the roof (12"+ by code), nothing more obtrusive than a plumbing stand pipe. But this pipe can be an issue.  If anyone other than the original roofing contractor puts in the hole through the roof for this pipe, the roofing labor warranty may be voided.  If the roof is newer, it's always best to get the roofer's approval in writing before moving forward, or even have him do the roofing work involved so the warrantee is not effected. 

Other disadvantages of indoor mitigations are related primarily to noise and condensation. As the fan is ordinarily sited in a garage or house attic, there's a good chance of hearing the system running... especially at night.  There's no way to predict how the noise will carry through the home's wood joists, roofing or drywall.

Condensation can also be a problem. The system's exhaust pipe carries the ground gasses out at an ambient temperature of 50 degrees. That can mean sweating within the pipe in the winter which rolls back through the system, and externally on the pipe's surface in the summer when heat and humidity rise. This can cause dripping from the pipe which can be a nuisance, especially in a garage.  Run above a ceiling, it can damage drywall or plaster.

What's more, indoor systems can be more expensive because they typically require more materials and workmanship. If fan servicing is required, someone will have to be home to allow entrance to the house and fan access. And because they typically involve more piping than outdoor systems, there's more resistance to the fan.  Theoretically, the greater the resistance, the lower the performance of a system.

It's because of these drawbacks that only about 10% of our installations are indoor systems.

Like any radon-mitigation system, an indoor installation must comply with plenty of protocols and codes. For instance, EPA/ASTM protocol demands that the fan be in a non-livable space (typically the garage or house attic; the basement is off limits). The exhaust discharge through the roof or siding must end at least 10' off the ground and at least 2' above any window.

If there are second-story windows, the exhaust must be at least 10' away from any window that can be opened. Electrical codes also mandate that an electrical shut-off be provided within 6', or arm's length of the fan. Building codes further require a fire-collar on the piping when it runs from the house to the garage, or if there is a garage ceiling, where the pipe goes through the ceiling to the attic.  This is to prevent fire from traveling up or down the PVC pipe from one area of the house to the next should a fire break out.