Coefficient of thermal insulation of windows

A window is a structure where two basic parts can be distinguished that differ by thermal insulation features: a non-transparent one, i.e. a window frame (casement) and a transparent one, i.e. glazing panels. Based on that subdivision the U-factor (coefficient of heat transmission) for windows or balcony doors can be calculated by the following formula. The formula is used for technical approvals granted by the Building Research Institute (ITB) for systems of windows and balcony doors:

where:
Ug – coefficient of heat transmission through the central part of window glazing, [W/(m2·K)],
Ag – glazed area, [m2],
Uf – coefficient of heat transmission through structural profiles or joints of frame profiles [W/(m2·K)],
Af – parea of the above components, [m2],
Yg – linear coefficient of heat transmission for a thermal bridge that is formed at the contact border between glazing and frame profiles, [W/(m·K)],
L – length of a linear thermal bridge, equal to the length of contact border between glazing and frame, [m].

Rules for determining of individual areas are specified in the standard PN-EN ISO 10077-1:2002 “Thermal performance of windows, doors and shutters – Calculation of thermal transmittance – Part 1: Simplified method” [8]. According to these rules the visible area of vinyl glazing beads is included into the glazed area.

Values of Uf and Ug coefficients are calculated in accordance with the rules specified in the standard PN-EN ISO 10077-2:2005 by means of the numerical method for Y-simulation of heat penetration in a 2D area with use of purposefully developed computer programmes.

It should be noted that the formula (2) allows calculation of U-factors for whichever windows, i.e. a windows with any design, size and split.