Linear thermal transmittance of the assembly of the glazing and the frame in windows

The thermal transmittance or U-value of windows can be found by calculation according to the standards EN ISO 10077-1/2 (CEN,2000). The window U-value is calculated from the U-value of the glazing and the frame as well as the linear thermal transmittance of the assembly of the glazing and the frame. The U-value of the glazing and the frame can be calculated separately while the calculation of the linear thermal transmittance includes the design of the edge construction of the glazing unit but also the design of the frame and the glazing unit. The edge construction of glazing units is made up of a spacer profile and a sealing. The spacer profile is typically made of metal and acts as a significant thermal bridge. Spacer profiles with improved thermal characteristics have been developed and are being introduced as ‘warm edge’. They are typically based on thin layers of stainless steel or combinations of polymers and metallic foils. The modelling of edge constructions in detailed calculations is time consuming and may give problems with the accuracy of the finite element calculation. The paper describes a methodology to characterize the thermal performance of edge constructions in glazing units by use of an equivalent thermal conductivity of a box of a fictive material that replaces the detailed spacer profile. A number of typical spacer profile products have been used in a test of the methodology where the linear thermal transmittance of the assembly of the glazing and the frame was calculated with the detailed spacer profile and with the equivalent box. The results proved that the method is accurate and easy to use. The equivalent thermal conductivity of spacer profiles can also be used in simplified calculations of windows and in this way make it easy to document the effect of use of warm edge profiles in glazing units. The equivalent thermal conductivity of spacer profiles have also been used in a method to express the linear thermal transmittance for a specific frame in combinations with all types of glazing units and edge constructions. By use of this method window manufacturers can establish a general valid documentation of the linear thermal transmittance for their frame profiles for all glazing units and edge constructions and in this way make it easy to document the U-value of windows with all types of glazing units and edge constructions.