I hope someone here might have some insight into what is going on. My house is a hybrid of outbuildings from an 1880 farm and additions over the years. The living room, one of the original outbuildings, is above the old basement, the dirt floor, fieldstone wall kind. The rest is on crawlspace which extends from the basement.

Last spring, I had central AC put in. The furnace, etc., is all in the basement below the living room. Prior to the installation, the crawl space was lined with plastic as a vapor barrier. The ductwork was insulated. A couple of years ago, the basement ceiling (under the living room floor) was insulated with rodent repelling insulation, the basalt kind of insulation (Roxol).

I have a dehumidifier in the basement which runs 24/7 in the warm months and keeps the basement very dry. In addition, we've had a very dry summer - no rain and low humidity, so it's been downright arid down there.

Ever since the AC and new furnace and insulation (new, not the Roxol) were installed, an odor has been wafting up into the house. It's actually worse when the basement is dry. It is not dead animals. It's not a horrible smell, but it's a strong one. This did NOT happen before all this work was done. It is strongest in the area of the old basement, but how it's getting through the Roxol, I don't know. And again, there was no odor before the AC, duct insulation, and vapor barrier were put in.

(In addition, the new AC is pulling this smell in and it's coming into the house through the AC vents. Someone will be coming this week to inspect the return duct, as we believe it is leaking somewhere, hence the AC pulling air from the basement).

Does anyone have any idea what might be causing this odor? I suspect the best way to deal with this is going to be getting rid of the odor, or at least confining it to the basement.

Thank you!