ThermalBuck is a window buck made from expanded polystyrene insulation wrapped in a polyurethane shell. Meant for installing windows in walls with rigid exterior foam, it comes in 8-foot lengths that are cut on site to fit, and held in place with adhesive and screws. The manufacturer, Brinc Building Products, says that unlike wood, this material won’t warp, shrink, or expand. And because it protrudes out from the framing to support the weight of the window, it will transfer wind loads from the window to the structure. It comes in four thicknesses, from 1 to 2.5 inches, with prices from $2.50 to $3.25 per foot.