Should a radon vent pipe have a cap?

Short answer: Often no. Radon exhaust should discharge straight up and unobstructed. The national ANSI/AARST standard for existing homes treats a rain cap as something that may be warranted only where debris, torrential rain or high winds block the pipe, and prefers a 1/2-inch wire mesh screen for debris or animals. Some states, including Pennsylvania and Ohio, do not allow rain caps at all. Check your state's radon rules before adding one.

If a cap is right for your system

1. Check your state rules. Some state radon programs prohibit rain caps or set exhaust height and distance rules. Your state radon office or your installer can confirm.
2. Match pipe size and schedule. Measure the exhaust pipe: 3-inch or 4-inch, and Schedule 40 or the thinner Schedule 20 (often printed on the pipe). Vent caps are sized to the pipe.
3. Keep the discharge clear. The cap must not substantially restrict airflow or push exhaust toward windows, doors or people. Clear snow, ice and nests from the pipe end.
4. Check the manometer afterward. With the fan running, the U-tube reading should be about what it was before. A big change can mean the cap or pipe is restricted.

Why caps are discouraged

A cap that deflects air sideways or down can send radon-laden air back toward the house, and caps can ice up in cold climates. That is why standards prefer an open, upward discharge above the roof line and away from openings.

Vent caps and checks

• PVC Vent Cap — 3" Schedule 40 PVC — $38.99
• PVC Vent Cap — 4" Schedule 20 PVC — $41.14
• PVC Vent Cap — 4" Schedule 40 PVC — $44.16
• Handmade VM2 U-Tube Manometer — $21.99
• RadonAway Easy Read U-Tube Manometer - 0-6.5" Blue — $23.99

Common questions

Does rain going into a radon pipe hurt the fan?

The ANSI/AARST standard requires radon fans that let rainwater and condensation in the pipe pass through or around the fan while it runs, so some water in the pipe is expected. A screen or diverter can help where heavy rain or debris is a real problem.

What size vent cap do I need?

The same nominal size and schedule as your exhaust pipe, for example a 3-inch Schedule 40 cap for 3-inch Schedule 40 PVC.

Can I use a screen instead?

Yes. A rodent or insect screen with mesh no smaller than 1/2 inch is the option the national standard recommends for debris and animals.

Shop radon mitigation parts How to read a radon manometer

Related categories: Radon Mitigation Supplies. General information only; follow the equipment maker's instructions and local codes. Updated 2026-11-14.