If you don’t get all the edges taped down tightly what you’ll be creating is like a diaper worn with impermeable foil outer pants.
The moist air will easily get in through the gaps in the tape job. Then water condense on the extra cold metal. The water will then run off and permeate the batting. But the liquid will be held in by the foil. So now you’ve got a section of permanently soaked batting. Which becomes a ginormous mold colony pretty quickly.
You don’t want to build a condensate trap. Instead you want to ensure no humid air can get in contact with the cold metal. That’s a tall order for a retrofit job on a small section of duct working inside a cramped hole in the wall.
As an HVAC tech I would have zipped up the ductwork for you that day and left you happy. Did this “specialist” check the float switch on your drain pan and the drain line itself by any chance? Who Goes on a call only to tell a customer to go to Home Depot on a wing and a prayer?
This makes sense, but I’m not quite giving up yet. Each time I tape up the seams or add another layer, I think I’m sealing up some of these gaps and there’s less condensation when I leave it for a time. I know it’s likely that there will always be a few gaps, but isn’t it possible that if there’s just a tiny bit of bare metal that can be reached, the condensation will be low enough that I’ll never have to worry about it? Hopefully, anyway.
Note that your dew points have crashed today … set the thermostat to 58ºF to continue your experiment … tomorrow the humidity will be back up but the forecast says dew points go back down in the low to mid 60’s the rest of the week …
Perhaps you could lower the thermostat in two degree steps, I’m curious as to what point your wife puts her hands on her hips, taps her foot and gives you that “other” look … all in the name of science mind you …
Yeah, I’m going to try and keep the experiment going at least until another high-humidity day, if my wife can put up with the hole in the ceiling for that long!
Condensation at the seams makes me wonder if the duct is leaking cold air. Is this problem at a joint in the ductwork? Can you feel cold air blowing in the space?
What kind of tape are you using?
Ironically, duct tape is pretty useless in this situation. I recommend aluminum tape – which is a bitch to work with, but once attached, is going to last for decades.
Latest update: there’s still dripping. Not a lot - maybe a tablespoon per day, but that’s probably too much.
I don’t think I can get all the gaps and seams with the tape, so I’m considering tearing down the stuff I added and using the spray insulation which arrives in the mail tomorrow.
Probably not 1% of houses insulate the duct in the house. (not that they’re a bad idea)
But I would want to know why. Towards that end I would want to know:
The size of the duct work
The Model number on the A/C unit outside.
The humidity level in the basement.
The return air temperature.
The supply air temperature.
Insulting may address the symptom and not the cause.
This morning I tore down all the soggy foam insulation wrapping and used the spray insulation (billed as “Anti-Sweat Cold Pipe Insulation Spray”), spraying several coats over all the exposed duct I could reach. There’s one area that I couldn’t reach with the spray can – parallel to the wall and against the back wall, with only a half-inch or so between the duct and the wall. Probably about 8 inches by 4 inches or so of bare metal.
After blowing a fan to dry the spray insulation coats, I turned off the fan and left it for the rest of the day. Luckily, today was pretty humid with a high dew point, so it’s a pretty good example day, I think – tomorrow will be even more so, I believe.
Results: the spray insulation actually WORKS GREAT! Thanks to whomever suggested it. There wasn’t a single drop of condensation on any parts of the duct that I sprayed. The only condensation was on that single bare patch that I couldn’t reach with the spray can. It looks like it’s a grand total of about a half-tablespoon of water for the whole day from condensation on that patch, based on what was caught in the bucket underneath (not even enough water to pool at the bottom – just a few splatters).
I’m not sure what to do about it. My fingers can sort of reach that part, and I could get some more adhesive foam insulation like the stuff I used before, but I’m not sure if that would improve things or just mean more soggy foam insulation. I’m very, very pleased with the spray insulation – it worked as billed, and there’s been no sweating at all over those areas sprayed, including parts of the duct that had heavy condensation earlier. I’m wondering if I’ve done enough and the little condensation left isn’t enough to actually cause damage to the drywall (once the drywall is replaced – still open for now).
It’s a 1st floor condo, so no basement – the unit itself is up on the roof of the building (5 stories). As for the other stuff, I might be able to answer some of it with a bit of effort – I’ll get back to you. Thanks!
The insulation spray is actually white and as it is now the duct looks like it’s been frosted by ice – kind of like things left in the freezer too long.
In other, other words anything that would inhibit air flow will create high temperature difference; a difference grater than 22 degrees. (and temperatures colder than 55 degrees)
That could be
An overcharged unit
A dirty filter
A dirty evaporator coil
Restricted return air registers or too many closed supply registers
A poor envelope: walls,ceilings, windows allowing too much outside ambient humidity
Fan speed set to low
An oversized unit (least likely but I see it)
If you knew how many times I make that mistake myself you would laugh.
Sounds like the spray on insulation is a closed cell insulation. Closed cell insulation is pretty good at preventing moisture migration, but I’ve only really have seen it used for pipes.
