If you don't have attic insulation, then you definitely have a major heat-sink there. When I insulated our attic, it made an ENORMOUS difference. We have a 14' attic peak and I finished the attic space into a "bonus room" area, so it's all in the roof, not the floor. When we had a plaster ceiling collapse below it 2 years ago and I re-rocked our bedroom, I stuffed a foot of fiberglass into every box sill around the ceiling. But the floors are still uninsulated except for any remnant cellulose that drifted in from the blow-in years ago. I'm contemplating a lot of drilling and Great-Stuff'ing this summer to shore up the floor box sills. I might borrow a borescope from work to double-check my work
One note about using Great Stuff (expanding polyurethane foam) - do it in warm weather. If you try to squirt it into a cold box, you'll actually see it shrink instead of expand. It's useless to apply in the cold.
Edit: added photos of the progression thru the attic from as-rec'd, to insulated, framed, rocked, and then finished. Fine, I like to show off.
Sue me.