One of the key parameters determining window quality is thermal insulation parameters of its structure. Heat transfer due to temperature difference usually cause heat loss and is sought to minimize by all means possible.
Heat loss through a product is specified by the heat transfer rate U [W/(m2*K)]. Its value equals to heat flow through 1 m2 of product surface area over a set period, when temperature difference is 1 K.
U value of a product is determined by laboratory tests and is confirmed by official certificates.