Hehe, sorry for the slow response guys, things are fairly hectic :-)
To answer the question about doors and walls, walls will act to keep heat trapped inside a room, and different wall materials will keep heat trapped at different rates, but there will always be some seepage. Doors are a necessary evil, as you want peons to be able to get inside a room but all doors will let out heat at a much faster rate than walls. Also doors help complete a room - for a room to be counted as "inside" it must be sealed by voxels and have a door. Peons prefer to be inside :-) And yes the HP should be fun (to play with, not necessarily balance!) - all items will have wear and tear associated with them, and peons can get moody as well as other "things" do damage. Generally the first tier (wood items) are things you should consider as temporary as they are incredibly flimsy - but they are useful for quickly changing peon moods in bad situations.
And yup, there has been quite a lot :-) I've been focusing a lot on the UI - (which has taken a long time, as I have very little aesthetic sense!) - and been getting some awesome responses about it. I've changed the very generic and boring dark grey into a more paper based UI - think an ancient magical tome of esoteric knowledge
And just for fun, a gif showing the evolution of the UI :-)
It's all very much still a WIP, but it has drastically altered the feel of the game (for the better!) :-)