Current practice is to declare in CE marking / DoP the Uw for a representative window of a window type (Table F.1 of EN 14351-1:2006+A2:2016) in size 1230 x 1480 mm according to Table E.1 No. 4.12 of EN 14351-1:2006+A2:2016. The value is determined by a Notified Body via either measurement in hotbox or more often via calculation according to EN ISO 10077-1 and -2. Some manufacturers (primarily SME’s) are also using the option of tabulated values according to EN ISO 10077-1, Table H.1 or H.3 which are also based on reference size 1230 x 1480 mm.
EuroWindoor input for declaration of Uw values of windows based on reference size (December 2023)