Abstract: An electrical heater for footwear which is battery-powered and insertable into the footwear. The electrical heater includes a heatable insole, an electrical conductor, and a battery pack. The heatable insole includes an upper outer fabric layer, a heater insert assembly, and a lower outer rubber layer. The heater insert assembly is very thin and flexible and includes highly heat conductive top and bottom layers with a central layer sandwiched therebetween and having a heat conduction coefficient less than the heat conduction coefficients of the top and bottom layers, therefore acting as a momentary heat dam and storage core. A heating element is coupled to the bottom layer and has a surface area considerably smaller than the surface area of the bottom layer.