Seal penetrations from top floor to space above. Add insulation.

Do you have an attic or access space above highest ceiling?

Insulation and air leak sealing. Where is the building envelope, the ceiling or the roof, or are both the same thing?

Angela, I just found two excellent articles on this topic since I discovered our builder had not put the noncombustible firestop nor fire-rated sealant around our masonry heater chimney where it passes thru the attic.

The first article is "Sealing an air leak next to a chimney (by Charles Miller)." The next article covers all about air leaks in the entire home: Home Remedies for Energy Nosebleeds (by Bruce Harley)." Both are from the Fine Home Building mag. Google.

Am now planning to construct a simple 'catwalk' around the chimney, so I can manage the repair. Not an easy task, as there are two layers of 6" fiberglass batts in the attic.

Hope your attic is accessible.