Locating the Source of an Odor in an Old House?

I purchased a home built in the 1950s. After buying, I had the insulation updated and a new roof installed. Roughly corresponding to that time, I have noticed an odor. It is present during the summer, worse upstairs (3 level home - smell not in basement, just in one area of main floor, present throughout on top level). How can I identify the source of the odor and eliminate? I don't think it's mold/mildew; it has a chemical smell.