Black stains on FG insulation

Hey all,
I keep seeing black stains on FG insulation and need to validate what I THINK I beleive it is… (sorry no photo handy)
Common scenarios are in garages or outbuildings with VB installed but no wall or ceiling finish.
I used to think this was soot that found its way through holes and overlaps in the VB and attached to the insulation now having condensation thereon, since it was fairly common in garages with wood stoves.
Often newer similar buildings that have (or never had) combustible units (woodstoves, oil/gas furnaces).
I’m stumped… help!


Without picture it is difficult to tell (sometimes with picture), but most FG insulation I find that has black stains is from moisture.

The vast majority (99% +) of stains in fiberglass is “filtered dust” from air leakage through the unsealed vapour retarder and wall/attic system. Remember fiberglass is used as a filter medium on forced air systems!!

I have many pictures of this phenomenon. I tell my customers when talking about energy conservation (airsealing) and possibly saving their roof sheathing from moisture/mould /rot “to lift up the fiberglass batts and at each dark spot/line on the lower side of the fiberglass batts will be a leak of air containing moisture and heat from below” Seal it!!!

The black stain marks are due to leaks in the ducting. For supply ducts, it’s the venturi effect which sucks outside air into the ducting. Nothing serious, you might want to make a recommendation for duct sealing to improve the overall efficency of the system.

There would be no venturi effect on round duct supply pipe it has static pressure and would be called air duct loss. On a return yes it would draw non-conditioned air into the system and yes I would consider that as serious waste of energy

You support my original theory. I will stick to that, now I know I’m not just dreamin this up! Thanks!!

Nice to know info for insulation. Thanks for sharing, I am sure it will come up and with your information this ensures proper answer for client :cool: