While obviously unsightly, the presence of mildew on interior or exterior surfaces is more than just a cosmetic problem. It’s almost always an indicator of a deeper and potentially more destructive moisture condition.
Painting over it won’t cure the condition, as the mildew will return sooner or later, and in the meantime the hidden moisture is working away at insulation, wood framing and other parts of your house.
Mildew, a form of mold, is most often seen as a black or sometimes white or greenish growth on siding, drywall, roofing and in other areas. In order to grow, mildew first of all needs a food source.
Because mildew likes organic materials, the typical home offers lots of choices, including drywall, wood, paper, wallpaper paste, cotton, linen, leather, wool and many other materials and surfaces.
In addition to the food, mildew will grow best in areas where it’s moist and warm, and where there’s a general lack of sunlight and air circulation.
Interior
Inside your house, one of the most common areas where mildew growth is seen is in the bathroom, where warm, moist air is at its most concentrated. Typically, the only thing needed to combat moisture here is the installation of a ventilation fan.
Make sure that it’s ducted all the way to the outside of the house, not just into the attic! And, of course, the fan needs to get used, both during and immediately after using the bath or shower.
If you’re having trouble getting people to follow this rule, then you can ensure that the fan gets used by having it wired to the bathroom’s overhead light so the two come on together, or to a timer control. If the fan is used regularly, you’ll remove the moisture and circulate the air, and you shouldn’t have any trouble containing the moisture before mildew can start.
If a ventilation fan isn’t enough, then you have more moisture being generated in the bathroom than just that from the shower.
Common hidden moisture sources include a leak in the tub or shower valves or supply pipes; loose and leaking drain lines; a bad wax ring seal below the toilet that’s allowing seepage; or moisture buildup on the floor around the tub or shower from bad caulk joints or excessive splashing from tub users.
Detecting this moisture can be difficult, as the source is usually concealed and you typically won’t even know you have a problem until it becomes bad enough to show visible signs — a buckled floor, crumbly drywall, etc. Here’s one place where the presence of mildew is a blessing in disguise, because it tells you there’s moisture present before it causes real damage.
Once you begin to see the mildew, your best bet is to contact a contractor who specializes in water damage restoration; most have sophisticated moisture meters that can help you track down the problem.
Other interior mildew problem areas can arise in closets, and behind beds and other furniture — especially those placed on exterior walls. This is typically the result of poor air circulation, combined with high humidity.
In the closet, try removing some of the clothes so that they’re not as densely packed. Leave the door open or replace the door with one that’s louvered to allow air to circulate. There are also moisture-absorbing chemicals such as silica gel that can help you get rid of excess moisture in specific trouble areas.
In bedrooms and other areas, move furniture away from walls so air can circulate. Keep clothes and other items from accumulating on the floor and in piles on furniture, and keep things as clean as possible to keep food sources and accumulated moisture to a minimum.
Exterior
On the outside of your house, the two areas you’re most likely to see mildew is on the siding and on the roof shingles, and here again it’s the early warning sign of a moisture problem.
If the problem is localized to one or two patches of siding or roofing, then the moisture source is typically easy to track down. Some possibilities for localized mildew on siding include sprinkler heads that are misaligned, leaking or improperly adjusted; an underground leak in a water or sewer line; moisture accumulation from a dryer vent or exhaust fan vent; trees, shrubbery or other landscaping that’s overgrown; and other similar "spot" sources.
Do a little detective work in the areas of the mildew, and the problem is often easy to find and correct.
On the roof, mildew often forms in areas where trees overhang the roof, or where leaves or pine needles accumulate. Clearing debris off the roof and trimming overhanging branches will often be enough to solve the problem.
You may also need to open up the areas around your house by removing or trimming closely packed trees which will allow sunlight and prevailing winds to reach mildewed areas and dry them out.
Widespread areas of mildew on walls and roof indicate a larger moisture problem, one that can usually be traced back to a lack of ventilation.
If you have a high level of humidity inside the house that is not being adequately dealt with, normal pressure and convection will move the moisture into wall cavities and attic spaces. Once it’s there, a lack of ventilation will contribute to the moisture, forming mildew.
To combat this, first you need to deal with moisture inside the house through the use of ventilation fans in the kitchen, bathroom and laundry; mitigation of high-moisture sources such as indoor spas and hot tubs; reducing the number of house plants; or perhaps installing a dehumidifier.
In the attic, be sure you have an adequate number of roof vents to allow moisture to dissipate naturally to the outside.
