Well, I've got to revise my list of favorite building materials, if I may without penalty. Instead of corrugated iron coming third, the ordering would in truth be, after wood and clay, flax, and then corrugated iron. Flax is just so versatile. You've got the fibers for all kinds of reinforcement, from a floor scree to a filler in the plaster or mortar mix. It makes a good insulation itself or in additive form along with other cellulose material. In spun and woven form it can be spanned across a wall attached to batons as a base layer for glueing wall paper onto, and the seeds can be pressed for oil for a whole other range of applications in and related to building. Yes, flax is number three.