Building Science Detective Needed!

I took this photo at a recent inspection. The switch was in a bedroom, next to a bathroom door, on the second floor of a 10 year old home. Any guesses as to what it shows? The staining has the look of something being ‘sprayed’ out from the switch box. Scorching from an electrical short circuit perhaps… but it doesn’t look dark enough for that, and scorching usually travels up from a box with a problem. Scorching that has been painted over? Maybe, but the staining is not evenly shaded, so that’s unlikely. It looks a bit like a watercoloring, with the edges of the stains darker than the middle, so a water stain? Could be, but water runs downhill and unless there is water under pressure in that electrical box, that’s not what we are seeing. Best quess… and my thanks to my inspector colleague Charlie Buell in Shoreline for putting my on to what I think is the right path… is the wall ‘breathing’ through the electrical box. Whenever there is air flow over a house part, the ghosting of dust is created. If you keep your bedroom door closed and it fits rather tightly to a carpeted floor, you will eventually see a dark area on the carpet as it filters the air flowing from room to room. In this instance, it is likely that this outlet is in a wall cavity that has an air leak… maybe into the attic, and every time something occurs to create negative pressure in the room, air is drawn through the electrical box, depositing dust particles that mimic the airflow. Making this explanation even more likely is that the switch is next to the master bathroom. This means moisture which would help the dust stick, and with the bathroom fan on, negative pressure is created. Voila… evidence of an air leak and the breathing of a wall cavity. Am I certain about this answer? No. But at this point it is the most plausible answer, and without using more sophisticated tools to analyze the issue, it’s the answer I’m sticking with. Building Science is a wonderful thing!

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