Several issues contribute to icy vents and that nasty smell


By CCHRC Staff

The “Ask a Builder” series is dedicated to answering some of the many questions Fairbanks residents have about building, energy and the many other parts of home life.

Q: My upstairs bathroom roof vent keeps plugging with ice. This causes a sewer smell in the bathroom. I have to go up on the roof and clear the ice about once a week when it gets below zero. I just put a 90-degree pipe on the roof vent but it still plugged up. Any suggestions?

This is one of those problems that plagues many people, especially as it gets colder.

Proper plumbing venting is critical to the operation of the drain system.

The vent allows pressure to equalize in the system by providing a path for air to be drawn in when wastewater is being sent down the drains.

It also allows the septic system to vent outdoors, rather than into the house — this is the problem you have, and it often becomes more pronounced in winter.

As hot air rises and escapes, or if fans are exhausting air, the replacement air has to come from somewhere. If the vent is plugged, replacement air can sometimes be drawn into the house from the septic system. Usually this happens when a trap is allowed to run dry. If you haven’t used a sink, a shower or a garage drain in a while, the water in the trap can dry out.

This will open up a path for sewer gases to enter the house. The other place it can occasionally happen is with your heat recovery ventilator.

Your HRV system drain may be tied into your plumbing system and have its own separate trap, which is an easy place to overlook, especially if it is in the crawlspace.

Often a roof vent is sized too small, especially in older homes. As a general rule, the size of the roof vent should be the size of the main sewer vent, usually 3 inches. There are many exceptions to this rule, however a 3-inch vent is going to be less likely to freeze up than a smaller vent. In some cases it may be necessary to upgrade to an even larger 4-inch vent, depending on the number of fixtures (such as toilets, sinks and so forth) that are part of the line.

How far the vent travels is something to consider. You may also want to inspect the attic to see how much insulation is around the pipes. If a lot of piping is exposed, it can contribute to freezing, as the outgoing air is going to be more likely to condense and freeze in the pipe.

So to summarize the solutions: Make sure the traps have water in them, insulate the pipes in the attic well and make sure the main vent pipe is large enough in diameter.

Q: I recently upgraded the outside venting range hood in my kitchen. I am wondering if you have any suggestions on how to minimize the amount of cold air that comes in through the vent?

You will likely need some kind of back draft damper to keep the air from being drawn in. Local sheet metal shops carry inline back draft dampers.

They are made to fit snugly inside your pipe with compressible foam around the edges. They have sensitive spring-loaded “butterfly” flaps that open under pressure when the fan turns on.

You will want to install the damper where you can get to it in the future, usually from the outside.

Be sure to put it far enough into the pipe from the outside wall that it is in heated space so that it can’t freeze when moist air hits it. Inline dampers are also good for bathroom fan vent pipes. You never want to use them for dryers though, as they can obstruct the duct, which will collect lint and cause other problems.

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