garage duct sweating, causing mold

[V0X]

i purchased a 50's era small cape cod home last year. The garage has a section of duct work that is about 6 feet in length that is flush with the ceiling and flush with the interior block wall.

options for insulating this to stop it sweating in hot humid days? there are only two surface areas of this duct, the bottom (when looking up) and the side. The other side is directly (no gap) attached to the block and the top is directly attached (also no gap) to the ceiling. I was thinking framing a simple 2x4 triangular enclosure with cellulose or pink insulation, but really don't want to have wet insulation if i do it wrong.

[Southern Mech]

get a dehumidifier for the garage.

[V0X]

so dont insulate the duct and just dehumidify during the summer?

[Southern Mech]

that's what I reccomend, humidity is your problem. insulation will not stop infiltraion of moisture from humidity. I would highly reccomend a service tech to make sure all is running ok first.

[V0X]

well, the infinity system is just over 1 year (furnace, coil and condenser all new) so i think im safe there. Is there any specifics I should consider regarding a dehumidifier? this integral garage barely fits my subaru forester about a foot in front and back when pulled in... also, do i run this year round, say, when Im using the furnace?

[tedkidd]

Hit the duct with 2 part foam. Do your rim joist while you are at it.

[beenthere]

Foam as Ted said, or insulated chase. Hard to dehumidify a garage.

[genduct]

that the 2 sides that are flush, will still sweat? I really don't know but sounds like the short piece of garage duct needs to be removed and replaced with ductboard so problem really solved

[teddy bear]

A dehumidified garage would be nice, but I would suggest a 1/2 inch of rubetex with adhesive be added to any of the sweating surface as the simplest solution. You need an insulation with a good vapor barrier on the the warm side attached to the any sweating surface. If this was your basement or in your home, the dehumidifier would be the best solution.
Regards TB

[geosdomain]

Ducts in garage - bad! I hope you don't really park the car in the garage.

I'll wager that even if you use the best material on the two surfaces that you can see - there will still be condensation collecting on the surfaces you can't see (or access). Though reduced, the problem remains that warm humid outdoor air will still find the duct and it will be coming through infiltration at other points like the foundation rim, joist ends, etc. At least the mold will be out of site (not good).
The duct work ought to come out, air-seal the foundation rim and other points of infiltration, and re-install duct work and then using a vapor-impermeable coating (such as 2-part foam) on the duct to complete the seal. Since the duct is coming out it would be good to replace it with insulated duct board which has both insulation and vapor barrier. Also duct board is generally easier to seal from air leakage too. If duct board used no additional coating needed. In all cases, meticulous air sealing is needed or you will end up with condensation where you can't see it and you run a good chance of noxious air from the garage mixing getting into the home.

Better yet would be to create a very strong air (and a decent vapor) barrier to separate the garage from the duct work and the living quarters. That's another thread maybe but it still would mean that you have to get the duct down in order to seal behind.

Best solution is to get the duct out of the garage. (then air-seal the conditioned space enclosure to separate the living spaces from the garage).

[motoguy128]

If you encapsulated it with 2" of closed cell spray foam up to the ceiling, it will be sealed to the ceiling, so those exposed surfaces won't get moisture or warm air reaching them. The spray foam will also air seal any joints in hte duct.

I agree that while your at it, have them spray any rim joists that are exposed in the basement.

[energy_rater_La]

interior block wall= uninsulated wall that wicks moisture.

what climate..florida..up north..??
where is the mold? exterior of duct..wall or inside duct?

I wouldn't foam ductwork. foam is NOT approved for sealing ducts.
its an easy fix all, but insulation for ductwork is fiberglass or ductboard which
is also fiberglass. then there is a reason for that. doesn't offgas or burn
and is an approved product.


If you do choose foam..do other work while crew is on job.
2 hours to set up fo foam such a small area...can't imagine
you'd get a decent price, or that any company would take so small
of a job.

mastic sealing any leaks in the duct,
caulking around duct through wall and insulating with
R-8 ductwrap. this would be my choice.

of course first you clean the mold, and make sure you get it all
before you encapsulate it to the duct/wall.

best of luck.