Mildew on Wallpaper

House Doctor | Gary Dymski

April 13, 2000

Q. We have a problem with an interior wall at a summer home in Mattituck. We've painted and wallpapered, but we keep getting these mildew spots on the wall and the paper. Can you help?

-Sandy Spar, Smithtown

A. We spoke over the telephone, so I learned a little more about your problem. You're getting these mildew spots on the wall in a family room/den area. On the other side of this wall is a bathroom with a tub and shower. On the other side of the family room is an open kitchen area.

I'd say your problem is a lack of ventilation, especially in the bathroom where you're not sure if you even have a bathroom fan. These ventilation fans are designed to remove the moisture created from running hot water for showers and baths. The fans usually are fitted into the ceiling and vented through the attic to the roof (or another exterior wall) where the moisture is released into the outside air.

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If you have such a fan, my guess is that there is no ductwork that allows moisture to be released to the exterior. The moisture is simply being diverted into the attic above the bathroom ceiling. If you have ductwork, then perhaps it is clogged or has been disconnected.

The first suggestion is to have a bathroom fan properly installed. Since you are planning some remodeling soon, be sure to have the bathroom fan properly vented. If you do a lot of cooking, you might want to have a ventilation fan installed on an exterior wall in the kitchen, too.

Another bit of advice is to have the right size fan. This is rather easy to calculate. Measure your room to get the square feet. A 10-feet-by-10-feet room is 100 square feet, for example. Then multiply that by the height of the ceiling (usually 8 feet).

In this case, you have a room with 800 cubic feet of air. Since air in a bathroom should be changed eight times every hour, you need a fan that will move 6,400 cubic feet of air (800 cubic feet by 8). Since that air must be moved every minute, divide the 6,400 cubic feet by 60. The result is 106.6, so you need a fan that has a CFM (cubic feet per minute) rating of more than 106. (A quicker way to roughly calculate this fan rating for a room is to take the square feet and multiply by 1.1.)

Do the same for your kitchen, only remember that kitchen air should be replaced 15 times an hour.

I didn't make up these numbers. The Home Ventilating Institute, a division of the Air Movement and Control Association, provides such information. Most manufacturers of ventilation fans use guidelines provided by these groups to design their fans.

Although it mostly serves the needs of contractors, builders and designers, the Home Ventilating Institute does assist interested consumers. Write the Home Ventilating Institute, c/o The AMCA, 30 West University Drive, Arlington Heights, Ill., 60004 (847-394-0150; www.amca.org).

Until your remodeling begins, you can circulate air in rooms with box fans and oscillating fans. To remove the mildew on the painted walls and wallpaper, try cleaning with a mild bleach solution (one cup of household bleach to a gallon of water). Rinse clean with cool water and dry thoroughly.

If and when you do decide to paint that problem wall, you should consider using a paint designed especially for kitchens and bathrooms. These paints usually include a mildew-cide. Rick Slutzky of Gregory Paint Shoppe in Manhasset advises that mildew-fighting additives are made for paints, too. Just mix a packet into a gallon of paint. They are relatively inexpensive and help fight mold and mildew on problem walls.