Hi foqus,

If the windows are open during the day, it could be pollen. If you close up the entire room, and this still happens, then I really can't say. Maybe someone else has an idea.

My best to ya and hope this helps.

Jay J -Moderator

I would venture to guess that when the cedar was installed to the exterior walls & ceiling that there wasn't an air tight barrier put up first. It's common that mant will insulate such a situation with kraft faced insulation. Well the kraft batting is an adequate insulation & vapor barrier, it is not an air barrier (Poly accomplishes this service). The result is that air moves through the cracks in the paneling. Over time, black, fine, sooty dust accumilates. That dust falls back through the cracks & is carried by tha air currents back inside (If air can move our through a crack, when the wind changes, it can move back in).
Another variable is if blown or poured in insulation was used (Again without poly or another air barrier), some of the insulation could be falling through.
Unfortunately, short of tearing down the cedar & doing it right, it's a tough thing to solve. You could caulk the cracks, but that's a lot of caulk & as the wood expands & contracts, the seal is lost.