Prevent Black Soot on a Skylight
LESLIE: Holly in Florida, you’ve got The Money Pit. What can we do for you today?
HOLLY: Yes, I’m calling regarding a skylight that we have in our home. Several years ago we had a new roof put on and we had the skylight replaced. We have a cathedral ceiling. Since we replaced it we’ve had no leaks but on the cathedral ceiling we have something black growing. We assume it’s mold. It does not leave a stain. We can get up there on a really tall ladder and wipe it off but we get this every year …
LESLIE: It’s actually not that uncommon of a problem. What happens is because this is in your ceiling; it’s at your roof line, it tends to sort of mess with the convective loop of the air flow in your house and you’re getting a cooler spot on the roof line. So when it’s cooler, dirt and dust just sort of tends to stick because of the temperature difference …
LESLIE: … and that’s why you’re seeing the staining. It’s not a leak; it’s not mold; it’s not water. It’s just the dirt and the dust that’s in the air tends to want to magnetically stick to this area because it’s cooler.
HOLLY: Oh, OK. Alright.
TOM: Yeah, Holly, as the warm air of your house strikes that cold underside of the skylight, it basically chills and then settles; so you get this sort of – think of it as a breeze or a convective loop that’s always sort of washing the air against that surface. Depositing dirt, over time, creates that black spot.
LESLIE: Mm-hmm. That’s why it wipes off so easily.
TOM: If you could get into the attic space around that and insulate the well of the skylight, that would stop some of that …
TOM: … because it would be warm and it wouldn’t chill and sort of attach.
LESLIE: It would solve that temperature difference.
HOLLY: OK. Well, thank you so much.
TOM: Alright, mystery solved, Holly. (Leslie chuckles) Thanks so much for calling us at 888-MONEY-PIT.