Shutters act as windows, see-thru walls, really in-game.
No heat loss.
I had suggestion some time ago to make it possible to close and open them, similar to doors (and gates)
Given 3.63, most current release, introduced fire, and smoking changes, it might be a while before Sami goes back into that (smoke and heat) section of code the next time. But then, he’s been pretty darn productive lately.