That photo above is from my basement. In case you missed my last article, 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. Ultimately, I’d like to take it all the way to net-zero energy. 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 source of moisture 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 finding an opening at this location?
Actually, it was none of those. Let’s peel back the layers and see what we can find. 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 paneling in that corner where the moisture damage is. Above the paneling, you can see spray foam. But what else is going 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 surfaces 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 conditions of 75° F and 50% relative humidity, the dew point 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 significantly higher than 55° F.
I can guarantee you that duct, a good 20 ft. from the air conditioner, wasn’t much colder than 55° F. So that leaves the humidity 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-in. pipe on the left is the culprit responsible for the slow hot water delivery I mentioned in my last article.) Notice that the two pipes go through a gap that’s 1-1/2 in. high. Here’s another view of the gap.
Through that gap and on the other side of that wall is a crawlspace. The outside of that part of the house has foundation vents, indicating that the house was built with a vented crawlspace. And anyone who knows a little building science can tell you that vented crawlspaces and the psychrometric chart are not friends. Translation: The vents in crawlspaces 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 office) encapsulated the crawlspace. I don’t know the whole history of the work done on the house, but there’s also a small dehumidifier in 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 first rule for preventing humidity damage:
Keep humid air away from cool surfaces.
Had they followed that rule, the ceiling tile would still be fine. The wood paneling above it wouldn’t be falling apart. And the duct above wouldn’t have become an accidental dehumidifier.
Allison Bailes of Decatur, Georgia, is a speaker, writer, building science consultant, and the author of the Energy Vanguard Blog. You can follow him on Twitter at @EnergyVanguard. All photos courtesy of Energy Vanguard.