THE WASHINGTON POST – 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 mould to grow. (The terms mildew and mold are often used interchangeably, but mildew technically refers to types of mold that generally grow flat, while mould 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 mould,” 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 re-painting with a finish that’s formulated to resist mildew probably would help.
Glowacki said an exterior paint formulated to resist mildew might work even better. Interior paints, 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 want to avoid, Glowacki recommends selecting an exterior paint that’s mildew-resistant.
If you repaint, it’s essential to deal with whatever mildew is on the surface first.
Glowacki said there are two options. Wash first 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.
“Re-do it every year,” he said.