Proper venting of bathroom and kitchen fans

Posted11/11/2018 6:00 AM

Dear readers: Recently I answered a very long question from a reader from the Chicago area who had sent me four photos of the most horrendous icing problem I recall ever seeing over the years.

He had remodeled two bathrooms and added fans, which were connected together and were vented through a soffit of his roof overhang. The overhang looks like it is about a foot deep and is vented by means of perforated vinyl panels.


The photos show an ice dam at the eaves, icicles coming down from the frozen gutters and massive ice buildup on the vinyl siding, including over a window in the area immediately below the bathroom vent, which indicates leakage into the house wall coming through at the window head and the vinyl siding in several areas. He reported that this is the only part of the roof where such a problem exists.

The venting of bathroom and kitchen fans is a recurring problem, which comes up every once in a while and is often misunderstood even by some builders, as I have seen so many times in my work as a consultant to homeowners and builders alike, as a former certified home inspector and as a columnist. So I think it is time to review the do's and don'ts of this important subject.

Bath and kitchen fans should never be discharging into attics. It causes excessive moisture, which can lead to mold formation and, in climates where heating is prevalent, the melting of the snowcap and the ensuing buildup of ice dams (kitchen fans also discharge grease that can create a fire risk). Nor should they be vented through high gable vents, ridge vents, vented soffits or through the roof.

Soffits are air intakes for attic ventilation. Trying to vent warm, moist air from a bathroom or kitchen out through them returns the moisture back into the attic. Venting upward to a ridge vent also has undesirable effects:

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• It can lead to frosting of the ridge vent, which can then drip back down onto the insulation and wet it.

• The condensate runs down the vent and wets the fan itself, adjacent ceiling material and insulation. Venting through the roof has the same effect.

In older houses not built to today's tight standards, it helps to leave the bathroom door wide open after showers to help dissipate the moisture into other rooms.

The best way to vent a bathroom fan in existing houses and in cold climates is through a gable wall as low as possible, and preferably those facing south, using a rigid plastic pipe having a slight slope down from the fan exhaust to the outside wall jack. Flexible, flimsy plastic venting should be avoided; condensation may pool in depressed areas and may eventually leak if the plastic envelope fails.

Instead, it is best to use one or more 10-foot sections of 4-inch schedule 20 drain pipe. Make sure the bell-ends face toward the fan and not the outside. Come up from the fan just high enough to obtain a gentle slope to the outside wall. Place small wood blocks under each section of the pipe at both ends and screw them on top of the floor joists, reducing their size as the slope progresses. Repair any disturbed insulation. Aluminum pipe may also be used.


Terminate the pipe into an aluminum or plastic wall jack as low as possible in a gable wall. Choose a hood-like wall jack; avoid a louvered plastic jack, as I have seen many with broken flaps, an invitation for mice and insects.

Once the pipe is installed, it should be insulated by placing fiberglass batts tightly on each side and on top to reduce condensation.

The most difficult venting situations are with hip roofs. Since there is no gable wall, the only choice is through a soffit in cold climates. If the soffits are vented by means of perforated vinyl panels, simply replace them with solid panels to eliminate the re-entry of the exhaust into the attic through them on the chosen side of the hip roof; you still have three sides to ventilate the attic. If the soffit venting is done differently, block it for six feet or more on each side of the exhaust pipe. You can do this with a piece of white aluminum with the outside ends turned down a couple of inches at a 45 degree angle. This will give the moist exhaust air a chance to evaporate and keep it from being sucked back up into the attic. The same principles apply to kitchen vents, but obviously the vents are of different sizes.

The above recommendations are based on the majority of situations in existing houses. But the very best way to vent bathroom and kitchen fans is downward. This is easy to do when building a new house, and sometimes possible in first-floor bathrooms, but likely to be difficult in any retrofit situation on second-story bathrooms.

In these situations, the venting of bathroom fans is done through a band joist in a basement or crawl space. A kitchen fan is also best vented downward, as specified by manufacturers for some kitchen ranges.

Venting downward respects the laws of nature. All buildings are subject to stack losses: Warm air rises and exfiltrates through any cracks and crevices it finds, creating a negative pressure in the house, which causes infiltration through similar avenues on the lower parts of the building in an attempt to equalize it.

All vents rising upward and exhausting out of the attic are perfect exfiltration paths; the stack effect causes the flap in the jack to vibrate open and let conditioned air out, exacerbating the problem. By contrast, the flaps on downward terminations are firmly sealed by the very same stack effect, which tries unsuccessfully to suck in makeup air through these openings.

Q. We live in Vermont, built our home in 2011 and had Hardie Cement Board siding installed. After the first couple of years, the boards on one side of our house started cracking and each year a couple more seem to crack (see attached picture). This particular side of the house has a southerly exposure and can get quite hot in the afternoon. None of the other sides of the house have any cracked boards. Have you ever heard of this happening before? Do cement boards have limitations of use due to sun and heat exposure?

A. Yes, I have seen this before. Even though the major manufacturer of cement board -- James Hardie -- and most discussions about cement siding, known in the trades as cem board, glow with praise as to its durability and freedom from cracking, chipping, paint peeling, etc., I have heard and seen a few cases where this type of siding failed.

Some of the failures can be attributed to poor installation by installers not familiar with its special installation procedures specified on Hardie's website. Some of the boards may be defective -- not uncommon in many other products. Nothing is always perfect.

You can file a claim with James Hardie; there is a way to do so at the end of their website,, under the title About Us -- Submit a Claim.

• Henri de Marne, a former remodeling contractor turned columnist and consultant, is the author of "About the House with Henri de Marne" (Upper Access Publishing). He continues to take questions from readers for this column and his website, Email questions to

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