Thread: Sweating ducts in crawl

The temp here was between 85-95 today. Humidity 70% + Dewpoint 70 degrees.

Crawlspace had a vapor barrier over entire area. Foundation vents open. Flexduct was not sweating, it was raining. There were puddles on the vapor barrier. There was the "M" word on some of the floor joists right over the sweating ducts.

Otherwise the crawl was dry and lovely. No water marks on foundation walls, no effervesence.

There is a raging debate about the dryness of crawlspaces. I understand the current thinking is insulate, seal and condition.

I am wondering if there is more going on here than high humidity and working A/C ducts.

I know we don't provide solutions, only problems but what are some of the potential solutions to this problem.

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Closed crawlspace design.

Air is more than likely leaking inbetween the outer cover and the insulation. The duct connections are more than likely not sealed properly. Also I did not see crawl space ventilation. Even if there is I always recommend a crawl space ventilation fan.

Also if you think about the ducts in the attic in the winter when the heat is on or any other condition. If the ducts are sealed properly at the connections you will never see them wet at the outer covering. I found this in one attic in Miami Florida and that turned out to be the case. There was also minimum attic ventilation.

That looks like a fungus infection on the 2X4 in the one photo not a mold.

That sub-structure need some ventilation.

I would get that plastic out of the sub-area. and expose the soil to a new ventilation system with lots of air.

Thats been my approch to sub-areas. I like the soil exposed to air
and vent to the exterior. this has always worked for me.
move the moisture out. the plastic traps the moisture and the soil never get any air and then you get water on top of the plastic. what a mess.

Best

Ron

Originally Posted by Bruce Ramsey

The temp here was between 85-95 today. Humidity 70% + Dewpoint 70 degrees.

Crawlspace had a vapor barrier over entire area. Foundation vents open. Flexduct was not sweating, it was raining. There were puddles on the vapor barrier. There was the "M" word on some of the floor joists right over the sweating ducts.

Otherwise the crawl was dry and lovely. No water marks on foundation walls, no effervesence.

There is a raging debate about the dryness of crawlspaces. I understand the current thinking is insulate, seal and condition.

I am wondering if there is more going on here than high humidity and working A/C ducts.

I know we don't provide solutions, only problems but what are some of the potential solutions to this problem.

I would close those vents to the crawlspace.

Open vents allow moist warm air to enter crawspace and condense on those AC supply ducts and drip/pond on the flooring vapour barrier.

Just my thoughts from a different angle/region.

Bruce,
Had a similar situation here in Charlotte area (Waxhaw, NC) just today.
Part of my situation was caused by the footprint design (two rectangular areas with one common corner) and the inability for there to be good "cross" ventilation.

I had "dead zones" that simply had little-to-no air movement, some areas of the stemwall foundation that had no vents (along rear wall where deck was attached), and some areas that were so low to the ground that the clearance of the drop girders and ductwork prohibited good movement of air.

Solution IMO (but deferred to specialist):
#1 Create a sealed crawl space. The irregular layout of this specific CS and the low/blocked areas make it hard to improve.

#2 If costs are too prohibitive (not my call), then first try installation of 1 - 3 industrial dehumidifiers set on humidistats (not talking foundation vent fans), have complete overlapping ground vapor retardant, except maybe under supply plenum or trunks (prevents ponding of any dripping that does occur), insulate condensate line (at least up close to evaporator coil), and re-check after several weeks.

Dave

IMHO I believe the closing of the crawl vents and completion of the moisture barrier will reduce the condensation seen in these pictures. The moisture barrier should be extended all the way to and even up the foundation wall a few inches to provide a complete barrier. The foundation vents are probably the ones that open when they get hot. Moisture in the air is moving to the cooler space and these vents are operating opposite of this fact. The only other thing I would check is for leakage along the plenum. I find leaks at the plenum connections will provide conditioned air and add to the condensation levels. Sealing these connections will improve efficiency and reduce moisture levels.
I realize that different regions may require different tactics, but still believe opening a crawl and ventilating is a mistake. I have seen the results of foundation vents allowing the moisture in, combined with poor control of the ground water vapor.

The biggest problem I see with sweating ducts is a HVAC system that is not performing properly and is over-cooling the ductwork. I would suggest that the unit be serviced to make sure the temperature of the air leaving the A/C coil to not too low and to make sure the air flow is not too low.

James, I married an Irish woman, everything is over cooled! I live in a meat locker.

Closing the foundation vents during the humid summer months usually solves it. Tried and trued by me. Crawl stays at a constant temp and moist air is not introduced. And no the air doesn't become stagnant unless you have moisture/water intrusion through foundation.

Different areas call for different methods. Here in the amazon of NC we need closed vents.

Another thing is the R value of the duct work. The lower the R value the more it is going to sweat. The worst duct in my area is the old gray vinyl which I believe is has a R value of 1.5. It splits under the heat in the attic and sweats and molds in the crawl.

Lose the vapor barrier or install power vents. Water will condense on a surface that is colder than its dewpoint, and many metal ducts have an exterior temperature below the dewpoint in a humid climate like yours. Suggest insulating the crawlspace walls so outside temperatures don't bleed through brick if the owner wants the barrier to remain but the crawlspace air should be moving around, especially with a high dewpoint.

Originally Posted by Robert Olson

Lose the vapor barrier or install power vents. Water will condense on a surface that is colder than its dewpoint, and many metal ducts have an exterior temperature below the dewpoint in a humid climate like yours. Suggest insulating the crawlspace walls so outside temperatures don't bleed through brick if the owner wants the barrier to remain but the crawlspace air should be moving around, especially with a high dewpoint.

I'm sure he has already given a recommendation, this thread is almost a year old.

A crawlspace needs a vapor barrier and you do not want to put fans sucking humid air into a crawlspace. Doing either one or both will only compound the problems if you do so.

This website has a tremendious amount of information on crawlspaces in the South & Southeast
Advanced Energy

From experience we used to install the foundation fans with a humidistat control and install a vapor barrier. After a year we would do our annual inspection and the problem seamed worse.
We now install a 100% vapor barrier and install a dehumidifier (vents are to stay closed). Now on our annual inspection the moisture levels are in the normal range of 12-15%.
The thinking of bringing hot air on cool ducts will not fix the problem only make it worse.
Yes, we had to eat a little crow and remove all the vent fan crap and do it the right way. We now are known in our area for our moisture control program and we give a written warranty. No call backs yet and we have been doing it this way for the last 4 years.