Healthy Indoors | 27
the basement so the humidity down there is much better.
The dew point in that part of the basement now is running
at about 50° F, well below the design dew point.
In short, the problem in this basement was that someone
back in the history of the house did not understand the fi rst
rule for preventing humidity damage:
Keep humid air away from cool surfaces.
Had they followed that rule, the ceiling tile would still be
fi ne. The wood paneling above it wouldn't be falling apart.
And the duct above wouldn't have become an accidental
dehumidifi er.
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• The Lesson of the Raining, Dripping, Crying Duct Boots
Dr. Bailes has a PhD in physics from the University of Florida.
After starting off on the traditional academic path of teaching,
he found a way to use his interest in energy and the environ-
ment, as well as his love for buildings, especially homes. Af-
ter fi nishing the construction of his own green home in 2003,
he became a home energy rater, and later founded Energy
Vanguard in 2008. Energy Vanguard is more about teaching
and less about fi xing, although they do offer HVAC design
services. It's focused more on the big picture—creating mar-
kets, spreading the word (such as for Energy Effi cient Mort-
gages), expanding networks—and less on individual com-
ponents. The company is on the forefront of change—the
vanguard—in the way we see and use energy in our homes.
For more information, visit https://energyvanguard.com
is 55° F. (That's a handy fact for building science types
to know, so you might want to remember it.) Also, the
temperature of the air coming out of an air conditioner
is usually between 55° F and 58° F.
So, cold air in the duct at close to or slightly above the
design dew point should not present a huge condensation
problem. That is, this duct shouldn't have gotten enough
condensation to cause the damage you see in the above
photos unless either the duct was much colder than 55° F or
the dew point in the room was signifi cantly higher than 55° F.
I can guarantee you that duct, a good twenty feet from
the air conditioner, wasn't much colder than 55° F. No
way. So that leaves the humidity being much higher than it
should have been. But where was the water vapor coming
from? Here's another look above the dropped ceiling.
The moisture-damaged area is in the left center part of
the photo above. Those two copper water pipes go to the
kitchen. (That 3/4" pipe on the left is the culprit in the slow
hot water delivery I mentioned in my last blog.) Notice that
the two pipes go through a gap that's an inch and a half
high. Here's another view of the gap.
Through that gap and on the other side of that wall
is a crawl space. The outside of that part of the house
has foundation vents, indicating that the house was built
with a vented crawl space. And anyone who knows a lit-
tle building science can tell you that vented crawl spac-
es and the psychrometric chart are not friends. Trans-
lation: The vents in crawl spaces don't solve moisture
problems; they create them.
That was the source of the moisture. I say "was" because
about four years ago, PV Heating & Air (the same company
that put the new HVAC system in our offi ce) encapsulated
the crawl space. I don't know the whole history of the work
done on the house, but there's also a small dehumidifi er in
i
