Typically, lofts are insulated by strips of insulating material laid between rafters. Class II insulation means that the device is "double insulated". A refrigerator consists of a heat pump and a thermally insulated compartment. All ventilation ducts are insulated and sealed against leakage. Once liquefied, it can be maintained as a liquid in pressurized and thermally insulated containers. The other walls of the box are thermally insulated, reducing convection and radiation. The bridge is typically electrically insulated from the substrate. Roller doors cannot be effectively insulated. Large amounts of heat are wasted if a boiler is not insulated. In colder climates, rammed-earth walls can be insulated with Styrofoam or a similar insert. The electrodes are electrically insulated from each other by the PEM. Capacitors made by twisting 2 pieces of insulated wire together are called gimmick capacitors. They had double walls insulated by earth packed down in between. The plug has insulated sleeves on the pins. Such boots may also be insulated for warmth.