Avoiding mold on vents from condensation in an old house
June 4, 2015 8:36 AM   Subscribe

I have mold that grows on the vents in my ceiling. The duct cleaners came and left because my ducts have no mold.

The technician said that the mold was due to condensation that forms when the 160-degree heat in the attic meets the 75-degree air in the main house. I have a craftsman bungalow that's about 100 years old and there are spaces in the wood in the register boxes that allow the condensation to form and drip onto the vents. He did not have a good idea to prevent it, but I suspect there is a good work-around. Any ideas?
posted by *s to Home & Garden (8 answers total) 1 user marked this as a favorite
Total guess, but I wonder if you could spray the vents with that mold and mildew resistant spray they sell for you to use on camping equipment.
posted by phunniemee at 8:44 AM on June 4, 2015

Better insulation in/around the registers in the ceiling so that hot/humid air isn't seeping into the living areas and condensing.

Whether it's putting better piping and registers in, blowing more insulation into the attic, or the simpler can of expanding foam insulation around the holes in the attic floor.
posted by k5.user at 9:07 AM on June 4, 2015

You need to do something about your attic, it should not be allowed to get that hot. I do hope you're exaggerating the temperature up there. Get a solar attic fan, and put in more insulation.
posted by mareli at 9:09 AM on June 4, 2015

How long has this been happening? (i.e. at the start of this season and since you've been using the a/c) Are the vents for central air?

A lot of mold problems around the vent area indicates a problem with the a/c unit, and you may want to contact an HVAC company to do an inspection on your a/c unit. It's possible the a/c unit isn't turning on long enough to appropriately dehumidify the vents.

Gaps into the ducting and around the ducting may also, as you indicated, be a problem, as humidity is pulled into the spaces around the register.

Ducts should be well insulated, spaces like the one you described insulated and covered with mastic foil. It also sounds like your attic is running extremely hot.
posted by Karaage at 9:11 AM on June 4, 2015

Response by poster: The duct cleaning service tech estimated 160 degrees when it's 100 degrees outside. I don't think he was exaggerating, but it's not 100 every day -- probably 1-2 weeks per summer. The attic is vented but there is no fan up there. I go up there when it's 70-80 degrees out and it's not terribly hot in the attic.

The vents are for central air. The a/c was just inspected and all is up to date. The tech today didn't see any a/c issues that would readily explain it. I do know from other service providers that the a/c is "overpowered" for the house so it cools the house quickly and doesn't stay on long enough to dehumidify much. I never noticed the mold until my boyfriend pointed it out, but it's probably something that happens really slowly and has been there since last summer at least.
posted by *s at 9:20 AM on June 4, 2015

You probably have metal vent covers. If you can switch to plastic vent covers you may find that your mold problem goes away. It has to do with the greater ability of the metal to hold the cold than plastic will. The cold metal attracts condensation better.
posted by Midnight Skulker at 9:30 AM on June 4, 2015

I am having trouble picturing this condition clearly: "there are spaces in the wood in the register boxes that allow the condensation to form and drip onto the vents", but I think you should view the condensation as a symptom of a larger problem, not as the root problem to address. The root problem is that there is a break in the insulation between the conditioned space of your house and the unconditioned attic space.

It is unclear if the ductwork that runs through your attic space is insulated or not. (If not, it should be.) If it is, but the insulation on the duct just doesn't snug up tightly to the back of the register, some additional insulation (either duct wrap, or maybe yellow spray foam type?) to fill that discontinuity would be helpful. What is the insulation on the floor of the attic, between the attic and the ceiling? Likely some kind of blown-in insulation. Was that insulation pushed away from the register locations when the ductwork was installed? You may just be able to push the insulation around so that it does not "thin up" around the registers.
posted by misterbrandt at 1:17 PM on June 5, 2015

If you own the place add some more turtle vents on the roof or get a powered vent up there that turns on when the attic is above x degrees. Put some reflectix up there too. Aside from the condensation, running cool air through 160 degree pipes isn't helping your efficiency at all...

Getting a dehumidifier in the house probably wouldn't hurt either.
posted by no bueno at 6:17 AM on June 7, 2015

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