"Understanding Your Home" by Building Inspector Mark Visser
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Latex vapour barrier paint
is hard to find in Canada but may be available at Glidden (Product code1060-1200) and Colour Your World stores. The paint is approved as an effective vapour barrier.

Ghost Marks on ceilings below the attic
Also called Shadowing or Pattern Soiling
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Attic insulation. Vapour barrier. Latex vapour barrier paint.

Ghost Marks. Shadowing or Pattern Soiling. These are all names for lines which can be seen on ceilings of the upper level in the house. The lines run where the ceiling joists are located. The same dark markings can also form along the ceiling edge. They are caused by uneven or missing attic insulation.
Dust particles accumulate more readily on cold surfaces than on warm surfaces, hence the ceiling, having wide differences in surface temperature, will develop noticeable dirt lines. During my home inspections
I have seen these ghost marks mainly on plaster ceilings in older homes in rooms located directly below the attic. Once spotted, I'd further investigate and would always find inadequate levels of attic insulation, as well as the absence of a vapour barrier.
To remedy the situation, the insulation R-value should be brought up to local codes. If possible, remove the existing insulation and install a vapour barrier. If this is not an option, just top up the insulation and provide a vapour barrier by painting the ceilings in rooms below the attic with a few coats of a latex vapour barrier paint which seems to be available in some areas.

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