Why We Don't Use Rain Caps - American Radon Mitigation

Why We Don’t Use Rain Caps

People often ask why we don’t use rain caps on radon mitigation systems. Leaving a pipe open to precipitation may seem counterintuitive; however, radon mitigation systems are designed to handle Minnesota weather. The system not only pulls radon from beneath the home but also draws out moisture, so the small amount of water that falls down the pipe is vented back out.

Leaving the vent pipe open is preferred. Let’s look at a few different exhaust caps and see how they affect exhaust trajectory and airflow. Please note that this article was written in 2022. For the most up-to-date information, please check the current ANSI/AARST standards at https://standards.aarst.org/

Testing Vent Pipe Options

We’ll use a mock-up radon system with a RadonAway RP145 and a 3-inch pipe for this demonstration. In addition, we will use a smoke puffer to help illustrate the exhaust trajectory.

Radon System Mockup

No Cap

First, let’s take a look at what happens without any cap. You can see that the air vents straight up. Without a cap, the system moves 83 CFM.

Without a rain cap

B Vent

Next is a B Vent that we took off a mitigation system. Here, you can see that it blows the smoke down and to the side. This discharge can blow that radon-laden air onto a patio or through a window and back into the house. The system moves slightly less air at 82.2 CFM with the B vent. This cap is not permitted by code.

With B Vent

Different Exhaust Angles

A 45 is allowed. The air exhausts upward at a 45-degree angle. The system moves 81.5 CFM.

With a 45


A Tee also moves 81.5 CFM and vents the air directly to the side. The Tee is only allowed in certain circumstances.

Exhaust With a Tee

Like the Tee, a 90 also blows air directly to the side, but the system only moves 80 CFM. But, again, this is only allowed in certain instances.

Exhaust with a 90

There are also U-turns, which are not allowed as it blows the air straight down to the roof.

Exhaust with U-turn

Critter Guards

Another option is a critter guard cap with a half-inch screen. This is permitted by code, but it increases ice accumulation in the winter, so we only use it if there’s a likelihood that debris will fall down the pipe.

Critter Guard Cap

The air moves straight up with the half-inch screen, and the system moves 79 CFM.

Exhaust with .5 inch screen

Quarter-inch screens are not permitted. The air exhausts in a similar trajectory, but the system only moves 77.5 CFM.

Exhaust with .25 inch screen


As you can see, leaving the pipe open allows for the most airflow and safest exhaust trajectory, making it the ideal option in most circumstances.

If you’d like to reduce your radon levels as much as possible, contact us for a free estimate.