I hear you, it's a single two-story house built in 2000, not small but smaller than typical developers build these days here. I added 18 inches of insulation to the attic, not much I can do in the walls. Double-pane glass. It's not drafty but seems poorly insulated as a whole despite being relatively modern and better-than-average quality. Houses in cold climates seem much tighter and better insulated.

For ATX, this capacity is entirely about the summer. Any winter performance is an afterthought.