It’s best to vent bathroom fans downward

Q. I am a regular reader of your column, and I have paid close attention to your recommendations for venting bathroom exhaust fans. I don’t, however, believe that I have read your advice for my situation.

I have a 40-year-old brick ranch home with a hip roof (thus no gables to vent through) in the cold Chicago suburbs. The roof soffits have a continuous 3-inch vent strip running around the entire house. All of the walls are plaster on rock lath, so they aren’t easy to cut into and patch. I have bathrooms at each end of the house, and they are vented only into the attic, which is insulated with about 18 inches of fiberglass insulation.

To date, I have not had any visible problem with moisture in the attic, but I am planning to replace both bathroom vent fans soon and the roof in the next year or two. I would like to vent the new exhaust fans properly.

You have stated you don’t like these fans vented through the roof or through a ventilated soffit. What should I do?

A. Fans vented through a roof in all but hot climates create condensation, which runs back down the vent, wets insulation and the ceiling, and can cause the fan to rust. When vented through a soffit vent, the moisture is sucked right back into the attic since soffit vents are intakes.

Hip roofs are a challenge for a number of reasons; venting bathrooms, kitchens and clothes dryers is one.

The most efficient way to vent bathrooms is to do so downward through a basement or crawl space and outside through the rim joists. This respects the laws of physics, which allows us to save energy. When a fan is vented upward through an attic to the outside, the stack effect encourages warm, moist air to continually exhaust because the flap of the outside jack is not airtight and is constantly pushed open by the exhausting air. When a fan is vented through the rim, or band, joists, the normal stack effect in the house seals the flap, thus preventing the loss of energy suffered by upward venting.

Since you have a ranch house, you may want to ask a licensed electrician how feasible this will be in your case. The duct should be run through an inside wall so as not to disturb any exterior wall insulation, and, if it is difficult to do so because of the plaster on lath, it may be run in a corner of each bathroom and boxed.

If that is not feasible, and since you have not experienced any problems, you may just decide to live with the status quo and keep an eye on it, although I agree with you that it is best to make sure not to cause problems in the future.

So here is a possible alternative: Run the ducts to the soffit vents and have a metal piece about 2 feet wide and as deep as the overhang made to cover the vented soffit. Cut holes in the middle for the jacks. Both ends should be bent down at a 90-degree angle and drop about 3 inches. The far end at the roofline should flare out at a 45-degree angle and extend out about 3 inches to deflect the exhausting air away from the roof fascia. If your soffits are white, use the white side of the metal, but if dark, use the other side. A contractor with a metal brake can easily fashion this.

If the existing ducts are not insulated, consider snugging R-15 batts on each side and on top of them to reduce condensation. If you plan on replacing the ducts, use metal or Schedule 20 bell-end PVC pipes. The bell ends should face the fan, and it is best to slant the pipes slightly toward the outside by placing small blocks of diminishing size under the ducts. Any condensation will run to the outside. Insulate them as described above.

Q. I have a split-entry house built in the 1970s. We have forced-air heat, and my wife likes to burn a kerosene heater in the living room when it gets cold. I have told her this is bad, as there is no vent for the heater and you have an open flame putting out carbon monoxide, which will make a person sleepy.

We get black dots on our ceiling and walls in the living room right where the nailheads for the drywall are, plus black streaks where the joists are. I say this is coming from the smoke from the kerosene heater since it has nowhere else to go. She believes it is from the whole-house fan in the hallway. I have explained that we do not run the whole-house fan in the winter, which is when we get the black marks where the joists and nailheads are. Would you kindly explain which one of us is right.

A. Non-vented heaters are unhealthy for the reason you mention: carbon monoxide. The only safe way to use them is if a window nearby is open to provide ventilation.

Kerosene heaters also emit large amounts of moisture, which is responsible for the black dots and the “ghost” joists you have noticed. The moisture condenses on the less insulated elements of the construction of your house: drywall nails or screws because they conduct heat through the framing members, and the ceiling joists because they do not have as high an R-factor as the insulation between them.

The heater generates combustion particulates, which, combined with dust in the environment, are captured by the light condensation on the nails and joists.

It would be best to abandon the use of the kerosene heater and, if your wife needs the added comfort of a room heater, instead purchase one of the infrared heaters available in hardware stores and home centers.

Q. I have heard it is not good to cover an exterior air conditioning unit, but I never have heard why. Can you shed some light on that for me?

Also, I am enclosing a picture of the dryer vent and the bathroom fan vent of our house. I think the previous owner took the easy way out by running these through the overhang instead of having the vent go through the exterior siding. I do not see it really affecting anything other than with the bath vent: The cover will not close, and I have had to shoo a few wasps out of there in the summer. Also, with this being open, there is a direct flow of air back to the inside. Do you know of a spring-loaded vent that would close or one that is meant for this position/angle?

I do have access to the vent, as the ceiling in that bathroom is a suspended ceiling, although I have filled the beam pockets with Dow’s froth pack liquid foam, which I think is great!

A. If you are referring to the condenser of a central air conditioning unit, it is best to cover its top and halfway down its sides to keep rain, snow, leaves and tree debris from getting into the unit over the winter. Leave the bottom half open to allow air circulation to prevent condensation.

If you are referring to window units, it is best to cover them to keep out the weather and to provide some insulation to a window that is not closed as tightly as it otherwise would be.

Both exterior jacks to your bathroom fan and dryer are inappropriate for soffit installation; they are meant for wall installation. There is no way their flaps can close without defying gravity.

You can buy soffit vent jacks in an electrical supply house. Since they are installed in the overhang of the second story and not in a vented attic soffit, there is no concern about recirculating moist air into an attic.

Q. What company did you previously mention that sells zinc (I think) or some metal to keep moss from growing on roofs?

A. Several companies sell zinc strips to control the growth of algae on roofs; the strips are not as effective on moss and lichen. These strips are installed just below the top shingle cap and should be installed by an experienced worker to make sure the roof is not compromised.

Here are a few names: Shingle Shield comes in 3-foot sections, (800) 942-3004. Fifty-foot rolls can be purchased at Z-Stop, (800) 845-5863; Home Depot; and ZincShield, (800) 440-3010.

Keep in mind that zinc strips take a long time to work on contaminated roofs. Faster results can be obtained by spraying the roof with cleaning solutions. Shingle Shield sells one, and so do others.

Ÿ Henri de Marne was a remodeling contractor in Washington, D.C., for many years, and is now a consultant. Write to him in care of the Daily Herald, P.O. Box 280, Arlington Heights, IL 60006, or via email at henridemarne@gmavt.net. His book, “About the House,” is available at www.upperaccess.com and in bookstores.

© 2012, United Feature Syndicate Inc.