I built a headboard out of door slabs due to the light weight and nice finish. I just looked up the R-value which is 2.
They are really light weight, perfectly square, pretty strong, and dirt cheap. You have to use them pretty much as-is since they are hollow and once you start cutting into them or changing the shape they lose strength.
They are all 80" long, and come in widths from 18"-36"
http://www.homedepot.com/h_d1/N-5yc1v/R-100086651/h_d2/ProductDisplay?langId=-1&storeId=10051&catalogId=10053
