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Building Science

Sleuthing a Basement Moisture Problem

What caused that nasty looking spot on the ceiling tiles?

This nasty-looking spot on the dropped ceiling in the basement of the house I just bought indicates a moisture problem. Was it from a plumbing leak? Capillary action in the foundation walls? Bad flashing? Nope. It was something else.

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.

Water damage on the original ceiling in our basement  [Photo credit: Energy Vanguard]
Water damage on the original ceiling in our basement.
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.

A supply duct is in the space right above the water damage   [Photo credit: Energy Vanguard]
A supply duct is in the space right above the water damage.
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.

What is the source of the excess humidity that caused the water damage?  [Photo credit: Energy Vanguard]
What is the source of the excess humidity that caused the water damage?
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.

The source of the excess humidity was the crawl space  [Photo credit: Energy Vanguard]
The source of the excess humidity was the crawlspace
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.


  1. tommay | | #1

    Looks like there is a small bend in the pipe so condensation is probably forming on the pipe and settling down to and dripping from that portion of pipe.

    1. GBA Editor
      Allison A. Bailes III, PhD | | #3

      Tom, if you're talking about one of the two copper water pipes, that wasn't the problem. The duct is much colder and right above the damaged area.

  2. DougFromMaine | | #2

    The power of convection and condensation is pretty amazing. We built a 2-story addition about 10 years ago. During construction, I put in flexible 3/4" low voltage conduit runs from the basement to a junction box or two located in each room, and also to the attic, to account for whatever changes in technology might happen in the future. (It turns out the big change was that we don't hardly need cables at all anymore, but that's another story.) About five years ago I decided to use the basement-to-attic conduit run for some CAT5 cable. I guess I shouldn't have been surprised that the loop of conduit I left in the attic was completely full of water, from cool/damp basement air rising through the 40' of conduit into the hot attic! I've since sealed the basement ends of all the conduit runs.

    1. GBA Editor
      Allison A. Bailes III, PhD | | #4

      Doug, maybe I don't understand the configuration, but it seems like you wouldn't get condensation in the hot attic. Humid air needs surfaces that are below the dew point for condensation to occur. Was something else going on there?

      1. DougFromMaine | | #5

        Hmm, maybe it was the reverse of my assumption - perhaps the condensation was occurring in the winter when the damp air rising from the basement was actually warm relative to the cold air in the vented attic...? If not that, I'm out of explanations. There was definitely no water intrusion. I added an image illustrating the situation.

        1. charlie_sullivan | | #6

          Nice picture, which makes sense to me (a cold-climate native).

  3. user-7259986 | | #7

    Many a times homeowner insurance covers the roof damage or wind damage coverage, cost, but only if you made an agreement with an insurance company for this. When you face roof damage you first think of how to deal with the damage caused. Click pictures of the damaged part and don't clean it unless and until the person from insurance company comes and see the damage as a proof. But if the Insurance company is denying the fact or asking for more proof which you don't have, then in such case you can hire a public adjuster like as a safety purpose. They will solve all your problems regarding getting higher coverage amount on the insurance policy.

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