Moisture around AC vent

One year old house. I was in the crawlspace (no basement) and noticed the subflooring was wet around one of the air vents. The line is pretty well insulated all the way to the vent. I'm not sure why that vent in particular would be a problem, but it seems to be the only one. I'm concerned the subfloor will start to rot over time. I checked it about a month ago and it was the same. There is no moisture I can detect from inside the house on the floor (pine flooring.) Any ideas why this is happening or what to do? I will call the builder, but it's a couple months beyond warranty and he might not do anything. Need to be armed with information....
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Condensation.
Just as when you sit a glass of ice water on the table for a while, moisture from the air will condense on the coat surface. It must be insulated and sealed better. Hard to say what will work best because I can't see what you have, but a wrap or fiberglass, or a spray of insulating foam will probably fix the problems. Yes, over years it can be a serious mold or rot problem.
You may also want to look at what you can do to reduce the moisture in the crawlspace. Plastic over the dirt, ventilation, etc.
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jeffc wrote:

Somebody else got the one-word answer to what it is and basics of why...
If the subflooring is what is wet, then the moisture is either condensing above it and soaking through or the vent/ducting is running in contact w/ it or there is a collection point at which the moisture is dripping onto the subflooring.
I'm guessing the problem is that somewhere they missed an area that isn't insulated and that's where the condensation is forming. As the above suggests, that point _could_ be a considerable distance from where you're noticing the symptoms so don't confine your search only in the immediate neighborhood but think of how water could have gotten there from elsewhere considering the ducting arrangement, gravity, support straps as conduits, etc., etc., ...
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jeffc wrote: ,,,

One more thought...
This kind of a problem isn't one that happened overnight--if he gives you too much grief over it on that basis I'd raise at least a moderate fuss...if he's a reputable builder one would hope there would be at least a modicum of self-respect and pride there. That, of course, assumes you've been a reasonable person to deal with up to know, of course, not a real pita! :)
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