Actually, people have started using fungus as a building and insulation material. Folks get bags of substrate (things like sawdust, etc.) and let the 'root structure' of the fungus permeate it entirely. The resulting brick is very strong, can be grown into any shape, is very light, and excellent at insulating. I imagine you need to keep it quite dry, however, so it might require sealing with dried clay, or if you've got a lot of bugs, then shellac would likely work really well!

http://inhabitat.com/phillip-ross-mo...than-concrete/

Also, I say 'root structure' but the fiberous growth is actually the real fungus itself - the bits we see on the surface that look like traditional mushrooms are actually the fruiting bodies. Fungus can be a huge organism - your fungus biome could be one single organism that covers a whole forest-sized area of land if you wanted! Only the