A: The mildew is growing because the porch ceiling gets wet, most likely from condensation that forms there because of the way a porch straddles the divide between indoors and outdoors. Air under a porch roof is warmer than outdoor air, yet the porch ceiling is cold. As the warm air rises and hits the cold surface, condensation can occur, and that liquid moisture is what allows mildew or mold to grow. (The terms mildew and mold are often used interchangeably, but mildew technically refers to types of mold that generally grow flat, while mold is a broader group of fungi.)
The type of paint could be part of the problem, because oil-based paint contains more ingredients that mildew can feed on. “It’s a buffet for mold,” said Frank Glowacki, who has studied how coatings and cleaners affect mildew for more than 20 years and is now director of brand marketing for Rust-Oleum. But even water-based paints can provide food for mildew, he said.
Scrubbing the ceiling with bleach each year or two makes the ceiling look better, but it can’t solve the underlying issue. Bleach kills mildew spores, but there are so many floating in the air that mildew will regrow if the conditions are right. Using water or water with a bit of soap would also remove the mildew, assuming the ceiling paint is still in good enough condition to be wiped clean.
When mildew forms indoors because of condensation, adding insulation or increasing ventilation often solves the problem. It’s hard to see how those strategies would work on a porch ceiling. But repainting with a finish that’s formulated to resist mildew probably would help.
For more than 20 years, through its Zinsser brand, Rust-Oleum has sold Perma-White Mold & Mildew-Proof interior paint ($13.98 a quart at Lowe’s). This paint, promoted especially for use in bathrooms, was a game-changer when it went on sale, because it carries a five-year warranty against mildew growth on the paint. Glowacki said the key feature of this paint is the way the mildew-fighting ingredients, which are proprietary, are built into the finish, so they work gradually over years, as they’re needed. “Moisture pulls the antimicrobials to the surface,” Glowacki said, adding that although the warranty is for five years, the protection may last much longer.
Perma-White is labeled as an interior paint. Could it be used on a porch ceiling, which never gets rained on or hit directly by ultraviolet light from the sun? It could, Glowacki said, but an exterior paint formulated to resist mildew might work even better. Interior paints, Perma-White included, stand up to repeated washing. The trade-off is that they are more brittle, which makes them more prone to developing cracks when temperatures rise and fall. Temperatures fluctuate more dramatically outdoors than indoors, so manufacturers formulate exterior paints to put a premium on flexibility and resistance to fading from ultraviolet light; however, they don’t stand up to as much scrubbing.
Because doing a lot of scrubbing is the very thing you and your wife want to avoid, Glowacki recommends selecting an exterior paint that’s mildew-resistant. Rust-Oleum doesn’t make standard exterior paints, but many quality exterior paints are suitable, including finishes from Behr (sold at Home Depot) and Valspar (at Lowe’s).
If you repaint, it’s essential to deal with whatever mildew is on the surface first. Glowacki said there are two options. Wash first with an Environmental Protection Agency-registered mildewcide, a category that includes chlorine bleach, which, although it can’t keep mildew from regrowing, is a good, inexpensive way to kill surface growth right before you repaint. Or, Glowacki said, you can prime the surface with a primer such as Zinsser Mold Killing Interior/Exterior Primer ($13.99 a quart at Ace Hardware). The primer delivers a quick kill; it doesn’t have the long-term protection against mildew growth needed in a topcoat, Glowacki said.
If you don’t want to repaint, Glowacki suggests spraying the ceiling with an outdoor cleaner that works without scrubbing and doesn’t need to be rinsed, such as Jomax House Cleaner and Mildew Cleaner ($18.67 a gallon at Lowe’s). “Redo it every year,” he said.