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Why are stud lines and nail heads showing up on the wall?
10 March 2007


Q. We have a 30 year old split foyer house. Eight years ago we had new roof, siding, gutters, soffitt, vinyl windows and all new doors installed. Just in the last three to four weeks we have noticed that on three walls upstairs that you can see the nail marks on the drywall and darker lines where the studs run up and down. The upstairs is open with a living room, dining room and a kitchen.

The front of the house faces east and is brick. The north and west sides are vinyl siding. The problem lies only with these three walls. I have checked the gutters and flashing.They are clean and good. I have checked the attic and itís dry and clear.

We washed all three walls today and it helped only a very little. We mixed a solution of bleach and simple green to wash and rinse the walls. Weíre going to paint soon. Last paint job was 2003. Do you think we should use an oil based sealer first like a Kilzģ? Should we run a fan constantly? We keep the house cool, 68ļF most of the time. Any advice or thoughts?

A. Yes, youíll need to seal the walls with a primer prior to top coating. Before you go to the trouble and expense of painting these walls you need to know whatís causing this and what to do about it or youíll be doing it again and again. When you told me which direction the walls are facing that suggested to me you are on the right track to figuring it out.

I've seen "shadow lines" of roof rafters, wall stud nails etc many times. The phenomenon has a name and it's called ghosting. It's primarily due to temperature differentials on the surfaces of the building materials and tiny bits of airborne stuff collect and it not only looks like soot, it usually is to a great degree.

What happens is that at times a thin layer of moisture will form on the surface of a ceiling or wall just over the framing and it picks up fine bits of dust or soot that float around. Over time it builds up and drywall nail heads and framing lumber lines start to appear. This only occurs on outside walls or ceilings under unheated attics.

The phenomenon appears sooner in houses where the occupants smoke or the house furnace, water heater and cooking are gas or oil. Toss in a smoky fireplace or an unvented gas log and that will speed things up in the ghosting department.

Similar perplexing problems have shown up in recently constructed homes heated and cooled with heat pumps. There appears this mysterious "soot" that attaches itself to walls, carpet edges, on HVAC grilles, and other surfaces such as appliance surfaces and plastic surfaces. It drove both homeowners and building professionals nuts tracking down the source both for the purposes of identification but also for prevention.

When I first saw this sooting problem in newer homes I naturally went to the heat pump and its distribution system. But heat pumps don't use any sort of fuel that would generate soot. We speculated that the cause might have been an oil leak that vapori zed on the electric resistance coils but we could never isolate or prove this as the cause. We eventually concluded the problem wasn't the air system. It's candles, unvented gas logs, open flame cooking, burning incense and tobacco use. Itís just spread by the air system.

I was recently called to a house with this problem and was completely flummoxed as to the cause because the owners had a heat pump, no gas, didnít burn the fireplace, cooked with electricity, didnít smoke or burn candles -- none of the usual suspects for a case like this. Then I spotted one of those plug-in air freshener devices-- the type that electrically heats up an aromatic oil to dispense the desired odor. When I finished taking inventory, the number of those things in this small condo unit came up to over twenty. I told the owners to get rid of them and then paint the walls. That worked.

My recommendation to anyone who has encountered this problem in a home, regardless of its age, is to not use anything with a flame inside the home and be careful of any device that puts anything into the air. If you do have a fossil fuel heating system have the heat exchanger checked. If you've got the problem then all you can do is prime, paint and take future precautions. Youíve already washed the walls so you are on your way.

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