Home>Discussions>INTERIORS>Molding & Carpentry>what is this dark patch on a plaster wall?
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stonemaker
what is this dark patch on a plaster wall?

I have an inside, interior wall that is plaster. There is a long rectangular dark "shadow" about 18" wide going from the ceiling down about 4'. No roof leaks that we are aware of. It doesn't feel damp. Looks like a shadow but is there all the time. The house is gas heated, built in 1984, located in Ontario. There is unheated attic space behind the wall, no access. Does anyone know what this can be? I can only think it has to be something bad.

havanagranite
Re: what is this dark patch on a plaster wall?

my first guess is that it would be an old patch, stud spacing about 16 inches by a 4ft piece of wall board

bp21901
Re: what is this dark patch on a plaster wall?

Another guess is that the area in the unheated attic space is not insulated as well on the dark patch section as the rest of the wall space. This rectangle is a cold spot would be somewhat like the dark lines from thermal transfer along studs on outside walls in some homes.

Too bad there is not access to the attic space to check.

bsum1
Re: what is this dark patch on a plaster wall?

I'm wondering what inside wall has an unheated attic on the other side? Is it an upstairs room?
It could have been an access to the attic space for something like repairs or adding insulation and then sealed up like was mentioned earlier.

stonemaker
Re: what is this dark patch on a plaster wall?
bsum1 wrote:

I'm wondering what inside wall has an unheated attic on the other side? Is it an upstairs room?
It could have been an access to the attic space for something like repairs or adding insulation and then sealed up like was mentioned earlier.

thanks for the question. the room that has the dark patch visible is a 16' high ceiling, great room. The kitchen, next to the great room, has an 8' ceiling. that leaves 8' of wall, above the kitchen that has no access. why they made the kitchen ceiling lower when the house was built, we will never know. so, if i stand in the great room 9 16' high) and look into the kitchen, the ceiling in there is 8'. It is not a patch or previous opening. this shadow has only shown up in the last year. not visible at all in the last 14 years we have lived here

goldhiller
Re: what is this dark patch on a plaster wall?

Could it be that there's insulation on the other side of this wall (FG batts maybe?), but the piece in this area has fallen down?

Any signs of mildew on the surface......or is it just "dirty"?

Does the area feel colder than the rest of the wall in the winter or warmer in summer?

Or....it *might* be that there was once an interior brick "stub chimney" in this location with a wood stove below with metal flue into the stub chimney. These sorts of locations frequently harbor soot in the plaster, which can be held back to varying degrees of success with the right primers and paint. Maybe the existing paint just can't suppress any longer.

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