WOOD by itself is a surprisingly good thermal insulator.
The average (normal) thermal conductivity of wood is
λ = 0,12 W/(m ⋅ K)
The heat transfer coefficient (U-value) required of external walls (in Finland) is
U = 0,17 W/m2
(They do make an exception for log houses but I think that’s cheating)
Now it would be cool to do the math myself, but I’m lazy so I will use an applet I found on the web. It turns out the required thickness of solid wood would be about 700 mm.
700 mm thick walls of solid wood! I want that! Would be one hell of a carbon sink too.
(The picture is of Avanto’s “Kyly” sauna, which of course is just a luxury folly. But I think it might be worth trying to scale this concept up into structures of socially and economically significant scale.)