26 | August 2019
T
hat photo above is from my basement. In case
you missed my last blog, I just bought a 1961
ranch-style house in Atlanta and am embarking
on the wonderful building science adventure of
turning it into a high-performance home. Ulti-
mately, I'd like to take it all the way to net zero energy, too.
My father-in-law, who sold us the house (Thanks, Stewart!),
had some work done to improve it several years ago, so
that water spot you see above is dry now. It just looks really
nasty.
Until this week, though, I didn't know what the source
of moisture was that created the spot. Was it rainwater
from outdoors leaking in through a penetration in the band
joist? Was it a plumbing leak from the nearby kitchen? Was
it groundwater climbing up the foundation wall by capillary
action and fi nding an opening at this location?
Actually, it was none of those. Let's peel back the layers
and see what we can fi nd. Here's another photo, this time
with the ceiling tile pulled away.
Here you can see that the current dropped ceiling isn't
the original one. At some earlier time, there was wood pan-
eling in that corner where the moisture damage is. Above
the paneling, you can see spray foam. But what else is go-
ing on up there? Let's take a look.
Well, there's more foam, but if you look closely on the
left side, you can see a bit of grey metal. That's part of the
duct that goes into the wall and delivers conditioned air to
the den above.
Ah, now we're getting somewhere! In summer, that duct
gets very cold when the air conditioner runs. And cold sur-
faces can condense water vapor out of the air if the dew
point is high enough.
Now, when you run an air conditioner and are able to
achieve the ACCA recommended indoor design condi-
tions of 75° F and 50% relative humidity, the dew point
Finding the Source
of a Basement
Water Problem
