What’s the one home upgrade you regret not doing sooner?

I swear, I spent years putting off adding proper insulation to the attic because I thought it wasn’t a “priority” compared to other stuff. Big mistake. Every winter we froze, every summer it felt like a sauna upstairs. I finally bit the bullet last year and can’t believe how much more comfortable the house is — plus, the energy bills dropped more than I expected. Anyone else have that one upgrade you wish you'd done way earlier?