I highly suspect that the substantial humidity created with your unvented gas fireplace log may be contributing to the problem. Your marks on the walls & ceiling are partially caused by moisture condensation at these slightly less insulated points in your building envelope. Then the dust sticks to the moisture at these points over time and shows as a stain. Prolonged moisture on these areas also promotes mildew growth on these long-lasting moist spots just like the darkness that grows in a bathing shower when it remains wet.
Keep the relative humidity in your house below 55% and I think you will substantially decrease this problem. By super-hydrating the room with your unvented log set use you raise the dew point temperature at which water vapor condenses into liquid to a warmer temperature. This causes far more condensation and then the problems begin to compound quickly.
Insulation or lack of insulation may also be a contributing factor to these black streaks. Insulating the top of exterior walls is difficult in traditionally built houses because of the extra wood plates on exterior load bearing walls. There is a double top 2×4 wood plate on these exterior walls which transfers cold through the wall more than the insulation.
High performance rigid board insulation like EPS expanded polystyrene such as that made by DOW and Owens Corning, or spray foam insulation must me used in these compact locations. I suggest raking back the loose attic insulation and placing rigid board insulation pushed down to the ceiling material from the top to increase the coverage over the wood framing members with as much thickness as the ceiling framing allows. The same would be done from the exterior on the side exterior walls to cover the top of the walls up inside a soffit overhang if you have one. Access to these work areas is difficult on both the top attic space and the exterior wall space. Both locations involve dismantling the roof sheathing from the top of the roof or from the top of the exterior wall to remove the exterior cladding (siding or brick) from the exterior. These improvements are typically done when the house is built or during later remodeling.
Most all black streaking shows itself where the insulation is lacking and condensation therefore occurs at the drywall board joints near the exterior walls. Other contributing factors for sources of the black soot that often sticks to these wet condensed places are recorded from our consulting visits show the leading causes of black wall and ceiling streaking as follows:
• Unusually high humidity from unvented gas appliances like gas logs and stoves, sometimes from excessive operation of humidifiers in the cold months.
• Inadequate insulation in exterior walls and ceilings at top wall-plate locations that cause condensation.
• Regular and persistent burning of incense or candles inside the house.
• Very dirty furnace filters or furnace filters not being regularly changed.
• Excessive air infiltration through wall cavities of both exterior and interior walls that carry substantial dust which causes ghosting or black marks on interior walls.
• Houses lacking regular cleaning of carpets and floors contributing to high dust circulation.
• Gas burning furnaces that are not properly adjusted or may have cracked heat exchangers which is a carbon monoxide health threat that can cause death of house occupants.
These are some of the possibilities of causes for black marks on the walls and ceilings. I hope this helps you address the black marks.
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