Hi All,
I have started a remodelling project on my home and have just discovered a real problem with condensation coming from the ac ductwork. The house is on a conventional foundation with approximately 24″ clearance on one side and 36″ clearance on the other. It is approximately 25 yrs old, wood frame, cedar siding. The condensation problem was discovered while repairing termite/water damage to the mud sills and rim joist. Underneath the floor looks as if it has been constantly sprayed with a water hose. Floor joist and subfloor are completely wet with water dripping off of them. AC ductwork was done about 12 yrs ago and wrapped with insulation which is now completely saturated with water and falling off. As you can imagine, my remodelling project has now turned into a major repair project as many floor joist and sections of subfloor need replaceing.
I thought that there was adequate ventilation in the crawlspace with standard screened foundation vents in the perimeter wall. I had an energy survey done at the time the hvac unit was installed and the technican advised not to install a vapor barrier on the ground under the house. I’m in rural north Mississippi – hot and humid – and a real lack of technical experts in this area. Any advise or help on how I might could solve this problem would be much appreciated.
Replies
Your "technician" was wrong. You've got moisture from the ground evaporating in the crawl space. And you've got warm, moist air coming in from ouside through the screened vents.
First, you need to reuce the moisture in the crawlspace. Put down a vapor barrier and close off the exterior vents. Then open up an AC duct or 2 into the crawl space.
That should keep your crawlspace a lot drier.
"Blessed are the meek, for they make great scapegoats."
Thanks for your reply.
I've read Joe Lstibureck's (Building Sciences Corporation) articles on sealing the crawl space and essentially making it a conditioned space in order to prevent moisture problems. Sounds like you subscribe to the same theory.
Although unconventional in this part of the country - given the common sense factors of high humidity + significant temperature differences = condensation, seems like this should work.
Is the sub-floor insulated?
Is the sub-floor just wet near the ducts?
What was the R factor of the duct insulation?
Mike
It's O.k. to think out of the box, Just don't walk off of the plank!
Edited 7/30/2002 1:38:26 PM ET by Mike S
Mike,
Thanks for your reply.
The crawl space is not insulated.
The main trunk line of ductwork runs approximately down the middle of the crawl space 48'. Branch lines come off of each side to the exterior walls in various rooms. The water is worst directly under the main trunk and branch ducts but extends 4 to 5 feet on each side, therefore if two branch ducts are within 10 feet or so, there may be 20 feet of water damage.
I'm not sure exactly what the r-factor of the insulation was. It looks like foil backed r-11 that had been wrapped tightly around the branch ducts. The main trunk line has no exterior insulation.
After reading Boss Hog's reply and BSC article its more clear how the problem evolved. Wondering though, what are the odds of sealing the ductwork adequately enough not to require sealing off the entire crawl space. I'm just old enough to think that there's supposed to be ventilation down there!
Sorry it took so long to get back to you, I've been a bit pre-occupied.
I'm assuming that the duct work is metal.
Let's see, at first I thought that because the floor was not insulated it was cooled enough from the air-conditioned interior surface of the floor to cause condensation, but since the damage appears to be primarily near ducts i'd try to seal them with a paste type sealer, if you want to try duct tape use the foil type, I'd also re-insulate the branch lines as well as the trunk line, foil backed insulation made for duct work is available, tape the joints with foil tape. My guess is that the condensation from the uninsulated trunk help to saturate the insulation on the branch lines.
One of the reasons i'd seal the ducts is to keep any mold spores from possibly being drawn into the living space.
If you don't condition the crawspace as you have considered and you have repaired the damage wood I'd be tempted to insulate the bottom of the floor to save some energy. Because of your local weather I'd check with the local building inspector if a vapor barrier should be against the living space or perhaps on the unconditioned side of the insulation.
You might want to condition the space as Ron suggested for the rest of the summer, it might help dry out the wood before you put any insulation against it.
Mike
It's O.k. to think out of the box, Just don't walk off of the plank!
Edited 8/1/2002 11:37:52 PM ET by Mike S
Edited 8/1/2002 11:39:49 PM ET by Mike S
Before sealing the crawl space from outside ventilation check on radon. If you have unacceptable levels that require mitigation you'll save yourself from doing a job twice.
Check this out: http://www.rlcengineering.com/csfallacies.htm (sorry for no active link). If you have further questions, contact Dr Dewitt.
Interesting siteMike
It's O.k. to think out of the box, Just don't walk off of the plank!
I have a 33 year old house, tract built split level. Crawl space had a thin concrete slab poured when house was built. Uninsulated HVAC trunk runs though the crawlspace for about 30'. Foundation vents have been sealed off for the life of the house. We use this area for over flow storage for everything from clothing to papers, name it. Everything is in perfect shape. Joists (SYP) are rock solid, no mold or any other problem. All we do that I haven't seen mentioned is we run a dehumidifier down there year round. It drains into a floor drain in the adjacent laundry room. At some point I might insulate the floor above it as it does get cool in winter, but it's not a pressing problem. BTW, this is near Washington, DC, where hazy, hot, & humid are the summertime rules. Winter highs, 30 +/-, lows in the teens.
Al
I live in humid, I shoulda said HUMID, NC. Got a 6' crawl (ok, walk) space with poly vapor barrier, and my my ductwork would rain when the ac was on. Mold city...
I finally went to wal-mart and bought a cheap dehumidifier, stuck it down there...no more problems. Talked to some heat 'n air guys, they suggested sealing off the vents....even better...I'm not an expert here, just someone who has to live and deal with this stuff, but unless you have some drainage problems (which I do, and this definitly helps)...I am not even really sure why the poly is necessary if all your other ducks, I meant ducts, are in the right row...
Edited 8/2/2002 7:17:57 PM ET by bucksnort billy