Proper venting is more than blowing off steam

Q: My sewage vent on my roof gets clogged with ice when it gets really cold outside. This creates a sewage smell in my home. A cap of snow often forms on top of the vent.

I wonder if this snow contributes to the icing, either directly or by slowing down the exit of warm air. What can I do?

Sewer vents can ice up when condensation and warm air coming up through the vent meet the cold air outside. Finding a solution to this problem can be cumbersome because of its reoccurring nature, plus the fact that you have to climb on your roof in wintertime.

Simply applying an open-sided cap runs the risk that condensation might form on the underside of the cap and worsen the problem as air is now forced to take a less direct path to the atmosphere.

Another tactic is to hang a copper tube on the edge of the sewer vent.

Soldering a copper tee across the top of copper pipe will ensure that it will rest across the top of the vent and not fall in.

The theory is if the copper should extend far enough down the vent and get into the heated space, then it is conductive enough to stay warm and keep the vent thawed out all the way to the top.

However, this tactic has mixed results. This tip also requires that the vent pipe go straight down to the heated space, which is not always the case.

If you have an accessible attic, the first, and usually easiest, fix for a freezing vent would be to wrap a thick layer of fiberglass insulation around the pipe all the way to the roof deck.

This approach solves the problem most of the time.

It will keep the escaping air warmer and cut down on crystal build up inside, and possibly melt any snow that collects on top, too.

If the portion of the pipe on the cold side of the roof is really tall, you may try cutting it down so it is closer to the roof flashing, to limit the amount exposed to the exterior.

Note that the city of Fairbanks building code requires a that the top of the pipe be at least 10 inches above the penetration in the roof.

It is possible ice crystals forming inside the pipe are also serving to support the snowfall. If the vent has a two inch diameter, consider upsizing it. Current code requires a three-inch minimum stack vent.

On some occasions it may even be necessary to go to four inches.

Another method is to purchase a non-frosting vent cap.

These are well-insulated caps that use a heating element, similar to heat tape, to help warm the top of the vent and keep it free of ice.

These caps can be purchased online.

Alaska HomeWise articles promote awareness of homerelated issues. If you have a question, e-mail the Cold Climate Housing Research Center at can also call the CCHRC at (907) 457-3454.