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Building Science

An Interesting Moisture Problem in a Trendy Restaurant

Observations by astute diners give clues as to the source

These air conditioning diffusers near the windows in a trendy Atlanta restaurant show signs of a significant moisture problem. But what's the cause?
Image Credit: All photos: Energy Vanguard

I’ve got the curse, you know. I can’t walk into a building and not check out what’s going on with ductwork, windows, and anything else that lets me apply what I know about building science.

Recently, I went to lunch at a trendy restaurant near Emory University and of course looked up at the ceiling. You can see what caught my attention in photo at right. The restaurant is only three or four years old, so I’ve been watching this problem get worse for a while now.

I have a few ideas about what’s happening here. Do you?

The clues

Before we get into the speculation part, though, let’s lay out the facts of the case. First, those are supply diffusers for the air conditioning system. They’re in a bulkhead over the windows on an exterior wall. The dark areas around the diffusers have been growing over the past few years. It’s hard to tell what those dark areas are, but it’s probably either dirt or some kind of microbial infestation (e.g., mold).

It looked like there were a few drops of condensation on the diffuser closest to our table, which I’ve shown in the closeup photo (see Image #2, below). I really didn’t see much on the others in that row of diffusers by the windows. The outdoor weather that day was fairly dry, with a dew point of about 61°F, so maybe there’s more condensation when the dew point is higher.

Another set of diffusers near the kitchen, however, seemed to be covered with condensation. Unfortunately, I didn’t take a photo of those diffusers, but there wasn’t as much discoloration on the drywall surrounding the diffusers. More drops of water on the metal; less…

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  1. Jon R | | #1

    IMO, when the AC is running,
    IMO, when the AC is running, the building should be positively pressurized. This provides complete control over what happens to moisture from outside.

  2. user-6805179 | | #2

    I believe that the main issue
    I believe that the main issue is that the makeup air fan is not working on the kitchen hood. It would appear that all the make up air is coming through the front door along with the moisture from outside.
    You should also check the discharge temp. on the supply ductwork, if there are loose belts or clogged filters you will get a low discharge temp. at the grilles and they will sweat.
    Quick check is get a temp. reading on the return duct and a temp. on the supply and they should be roughly 15-18 degrees different

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