Insulating blankets of ionomer foam are helping to assure crack-free, evenly cured concrete in the Libby Dam, now under construction on the Kootenai River near Libby, Montana. Ionomer foam blankets were chosen for protective insulation during the summer because of the thermal conductance properties of the resin-based material. It has a "C" factor of .649, which exceeds the US Army Corps of Engineers standard by 15 percent. The blankets have a capillary value of zero. They meet standards for water vapor permeability not exceeding 2 perm-inches. They come in thicknesses between .38 and .40 inch. Installation of the blankets is comparatively simple. Strips measuring 94 and one-half inches by 100 feet long are attached with wire to the inside of the steel forms. The flexible foam is shaped around protrusions and form contours, and then cut and trimmed as required. Wooden strips are used to affix the blanket at the top of the monolithic block to be placed. After placement of the concrete, the blankets remain in place for at least 14 days, staying on after forms are removed. Removal of the blankets is equally simple. The wooden strips are removed and the wires are clipped. The blanket is then peeled from the surface.