Some of y’all might remember this picture…
http://techno-fandom.org/~hobbit/hse/myduct.jpg
and the discussion that went around it, concerning careful backoff
[from the wall] and sealing-up of the outer duct sheath to make sure
no water infiltrated into the fiberglass between. This was for the
intake duct on my HRV setup, the coldest part, and after retrofitting
same on my HRV setup accordingly it was fine all this winter.

So I was downstairs putting the snow shovels away [wishful thinking?]
and happened to bump the end of the *exhaust* side duct, which
instead of feeling like its usually crinkly-outer-cover self, gave
sort of a soggy “thud” right where the end gets tied closed. And it
had a bit of a heavy belly hanging down. I realized the thing had quite
a lot of water sitting in it — not dripping, the cover’s intact and
very well attached to the pipe itself, but clearly some of the water
from the interior air had not fallen out at the HRV core itself and
landed here when it met the cold at the interface. Possibly during
the between-times when the HRV isn’t actually running, but that’s
a guess. The outgoing duct has a fabric backdraft damper right at
the wall, but that doesn’t say a bit of cold can’t penetrate back
inward.

Fortunately the duct comes *downward* to where the wall pipe is,
similar to the picture, so all the water pooled within the last
6 inches of run or so and was easy enough to empty out and squeeze
most of the absorbed accumulation out of the ‘glass. Now it’s got
to dry out, I suppose, after which I can put it back together and go
on with life. But it’s a bit of a mystery how *circulating* interior
air is getting in between the inner duct and the outer sheath. It’s
quite tightly put together. Of course it’s had all winter to build this
up, so even a tiny gap in, say, the inner duct might be significant.
The humidity in the place has been down around 30% all winter so there’s
not a ton of water in the outgoing exhaust air in the first place.

Poking a drain hole is not in the cards; that would defeat the whole
point of sealing the outer cover.

So if you’ve got an HRV check out its plumbing, you might uncover some
interesting long-term effects from the wikkid-cold.

_H*