A permanent label attached to the edge of the door slab lists the certified U-factor. A temporary label also appears in a more convenient place on the face of the door. The certification procedure is the same for doors as for windows, although the standard sizes differ. Manufacturers may choose to test products either under the window or door procedures. NFRC allowed this flexibility for manufacturers whose product lines may cross over the definitions of windows and doors.
Higher numbers mean more heat loss.
The values presented include the door slab and frame. Some door manufacturers sell only door slabs, while others sell only complete, prehung door systems. Some do a little of both. A manufacturer who sells a particular door only as a prepackaged system may test and certify it as one unit. Doors sold as slabs that are prehung by another party are tested in one of two standardized frame systems.