This is my second winter in a 2-story, 1891 victorian townhome
in Philly. While I spent a great deal of time last year sealing and caulking and had a pro blow some modern cellulose insulation into the roof (no attic to speak of), I'm still loosing heat as my 2nd floor is roughly 3-4 degrees cooler than downstairs!

Among other things, I'm concerned I'm loosing heat through the plumbing stack. The stack (a modern 4" PVC pipe) is drawn from my basement up through an obsolete brick chimney that I assume once served a kitchen oven (the furnace/fireplace chimney is in the front of the home). This exposed chimney was simply plastered over some years back and is frigid to the touch on the first and second floors.

While I was recently up on the roof and it appears to be properly sealed with a cast-iron cap, this chimney is open in my basement and also has a 4" x 6" opening on the first floor (at a laundry sink).

What is the proper way to seal such an opening to reduce heat loss? And finally, am I losing heat through the brick chimney? If so, do I have any options beyond reframing it with several more layers?