July, 2007

Avoid Moisture Build-up in Homes

Good fans and careful installation will help avoid moisture problems.

By Steve Easley

A dealer in North Carolina asks, “Do you have any suggestions for helping to control moisture build-up in homes?”

Moisture inside homes comes from two primary sources—water leaks that originate outside, and moisture vapor generated by everyday activity inside the home.

The key to solving both problems is to control moisture at its source. Indoors, this is best done by installing good quality kitchen and bath fans.

While that may sound simple, builders and building designers often make mistakes when designing and installing bath fans. The bathroom is one of the largest sources of moisture in the home, and it is also one of the most ignored. I’ll focus on this room because if you can solve moisture problems here, chances are high you can solve a majority of the indoor moisture problems that affect the whole house.

To control moisture at its source in a bathroom, you need to quickly and efficiently exhaust the high levels of humidity generated in that room. That way, mirrors won’t immediately fog up, and delicate finishes, such as those protecting high-quality cabinets and faucets, won’t bubble, flake, or dull so quickly. Most importantly, you will be controlling the high relative humidity in the bathroom, which will help reduce the potential for mold on damp surfaces. In an existing home, you may not be able to change the bathroom layout, but start by placing a fan as close to the toilet and shower as possible.

Install the Right Fan

Perhaps the most common mistake in trying to mitigate moisture problems in a bathroom involves poor fan selection and sloppy installation. Typically a builder chooses the least expensive fan model, often one that doubles as a light. He then hooks the unit up to a long, tangled duct that vents into a soffit.

While good fan/light combinations are available, most inexpensive units don’t pull enough air once they are hooked up to an exhaust duct, and they are so noisy that homeowners tend to avoid using them.

Another common mistake is tying the bath fan switch to the light switch. The rationale for doing this is that the fan will always get turned on when the bathroom is occupied. But no fan will exhaust the humid air from the shower all at once. A fan should run for 20 minutes or more after the shower is turned off to clear the room, and to provide enough air flow to help dry surfaces where water vapor has condensed while the shower was running.

Air Flow

To move moisture, you need ample airflow. In small bathrooms, the Home Ventilating Institute (HVI) recommends at least one cubic ft. of air per sq. ft. (1 CFM/sq. ft.) of floor area. I usually advise builders to install a minimum 80-CFM fan in any bathroom with a shower. In large bathrooms (more than 100 sq. ft.), HVI recommends using a fan that can pull at least 50 CFM at each fixture (toilet, shower, and tub) for a total of 150 CFM. If the tub is jetted, HVI recommends at least 100 CFM at that fixture for a total of 200 CFM in the whole bathroom.

As always, it’s important to talk to your hardware store expert or your contractor to verify what you need.

Duct Installation

Even a reliable fan has limits on the amount of air it can move, so care must be taken when installing the ductwork to reduce the static pressure as much as possible. Choose the shortest possible path through the attic or floor cavity. In cold climates, the ductwork should be protected with insulation to prevent the condensation that forms inside the duct from freezing. However, keep in mind that insulated flex duct adds the most resistance to airflow, while smooth metal ductwork adds the least.

If possible, ensure that the duct ends at a gable end (best option) or through the roof, but do not exhaust it into the soffit.

Noise

The sound of rushing air can be surprisingly loud, and in a really cheap unit, this bothersome blast is often accompanied by a steady rattle of motor bearings and vibrations from the metal fan housing. Better fans are engineered to control the vortex of air to minimize sound, and of course, all the parts run quietly when the fan operates. More expensive fans also have less vibration. The motors are isolated from the housing, which creates less noise. A fan that is constantly switched off because it’s just too loud does no good.

In summary, bathroom ventilation is one important element to whole house moisture control. Providing good ventilation is not difficult: Place the fan near the moisture source, select a strong, quiet model that is Energy Star-rated, minimize the length of duct runs, and make sure the duct install is free from kinks and vented to the outdoors. To make sure the fan operates long enough, hook it up to a motion sensor and timer.

With these simple steps, indoor moisture levels can be safely and efficiently managed.