Around here, southern WI, I'm seeing many instances of green mold/algae or whatever it is appearing on the outside of vinyl siding. discoloring the siding. Most of the time it is more or less localized around a specific area on the siding. Maybe it is opposite a bathroom or a high moisture area. I'm sure this is happening in many other geographical areas of the country. What causes this and what is the fix? Is it related to my previous post about exterior wall construction? Many of the homes I see this on are relatively new or newer homes ( +/- 10 years). I havn't been called on to repair any of these yet, but I anticipate it in the future. Any input would be appreciated.
It's algae if its green but you'll get mildew as well. We see lots of it here on Puget Sound because of our damp weather. It's normally found on the north side of homes here and to a lesser extent on the east side, because these are the sides that get the least sunlight.(UV is deadly to fungal organisms.) Sometimes we'll even get it on the south or west sides, when these areas are heavily shaded. It'll grow wherever moisture is able to collect and remain.
You can kill it with some house wash or some diluted deck cleaner. Vinyl is very conducive to algae growth for some reason.
It normally doesn't spread to the interior of walls, unless there are water leaks into the wall for it to follow. It's a homeowner maintenance thing. Clean the siding like you would a car and you don't have to worry about it.
The material will grow best where the conditions are ideal for what makes it grow best. I may be that the areas are where insulation is thin or missing. I've seen it more pronounced over studs where you actual see vertical lines on the walls. It often appears heavier under windows because I the additional moisture. Warmth makes it grow but sunlight and dry surfaces will kill it like Mike said.