Q: We own a 12-unit apartment building in San Francisco near the Golden Gate Bridge. One of the units has a mildew problem. This apartment is on the top floor and has a north/northwest exposure.

There has been a mildew problem ever since the current tenant moved in five years ago.

The unit is about 750 square feet and is filled with enough furniture to fill a house. There are easily 75 pictures and wall hangings. The windows are never opened and the heater is on constantly.

When in this apartment I have noticed severe condensation on the windows with moisture running down the glass. The blinds are never opened, so no sunlight or air is ever let in. The question: Could this situation be causing the mildew?

A: Yes. Mold and mildew flourish in warm, dark and damp climates.

A cluttered top-floor apartment, facing north/northwest, near the bay, kept at a constant warm temperature without sufficient air circulation and light, produces a microclimate ideal for growing mold and mildew.

So, what to do? Of course you could change tenants. But that is problematic and it would not solve the underlying causes of the problem – poor ventilation and condensation.

To address these causes, you and your tenant must take steps to improve the airflow in the unit.

You don’t say, but we suspect the apartment is not heated with a central forced-air heater. Central heating systems are equipped with a fan that constantly circulates the air in the area being heated when the heater is in operation. Space heaters, such as floor and wall heaters, do not do this. Installing a central heating system to address these issues is an expensive option, and it would only reduce the problem.

There are a few simple things your tenant can do to alleviate the problem. First, on nice days open a couple of windows and allow fresh air to circulate. On the top floor, security shouldn’t be an issue. You may lose a little heat, but in our opinion the trade-off is well worth the extra expense.

Second, open the blinds and let the light shine in. This will circulate air between the windows and the blinds as well as provide some radiant heat from the sun. This should limit condensation on the windows.

Finally, try to decrease the use of the heater. We suspect that your building has single-pane windows, and the difference between the outside temperature and the temperature in the apartment is enough to cause significant condensation on the interior of the window panes.

Something else you should consider is installing an exhaust fan to move some of the stale air in the apartment to the outside. Today, building codes require methods to ensure adequate ventilation in interior spaces in addition to the forced-air heating systems common in most homes. This includes either a window, an exhaust fan or both in bathrooms and laundry rooms.

Bill and Kevin Burnett will attempt to answer your questions, although the volume of e-mail sometimes makes this impossible. Contact them at sweat-equity@comcast.net.

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