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My attic is 530 sqft, with soffits, ridge vent and some 18"X18" window with mesh over it (I cant recall its name). The house was built in 1960 and has batt insulation on the attic floor. The batt insulation is about 6" deep. It somewhat looks like the top left of this diagram

My question is, is it ok that the top of the batt insulation is dirty since my attic has soffit vents? As I've read, dirty means there's air movement, but doesn't it make sense that there's air movement where there are soffits and ridge vent? I'm trying to figure out if I need to air seal and insulate more.

Thanks

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A layer of dust or dirt on top of your insualtion would be quite normal in a vented attic. If the dirt was inpregnated in the actual insulation, so that it was acting like a filter, then that would indicated air movement from one side of the insulation to the other. This would be seen most often around holes in the ceiling from light fixtures, plumbing/electrical or any areas open to air leakage from the interior to the attic. I think what you are seeing laying on top of your insulation is just plain old dust from outdoors entering through the venting. not a problem.

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More insulation = good. Doing away with the ventilation = bad.

As far as dirty insulation goes: dust is in the air and tends to settle, particularly near major roadways. Also if your roof has ever been replaced it's likely that roofing grime would be all over the insulation even if your roofer was fastidious. Increasing the R-value of your insulation will be good for your house and good for your pocket book over the long term, but it's worth a cost-benefit analysis to decide how much energy you would need to save at current rates to justify any expense for more insulation.

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