@article {12005, title = {Key Elements of and Materials Performance Targets for Highly Insulating Window Frames}, journal = {Energy and Buildings}, volume = {43}, number = {10}, year = {2011}, month = {10/2011}, pages = {2583-2594}, chapter = {2583}, abstract = {

The thermal performance of windows is important for energy efficient buildings. Windows typically account for about 30{\textendash}50 percent of the transmission losses though the building envelope, even if their area fraction of the envelope is far less. The reason for this can be found by comparing the thermal transmittance (U-factor) of windows to the U-factor of their opaque counterparts (wall, roof and floor constructions). In well insulated buildings the U-factor of walls, roofs and floors can be between 0.1 and 0.2 W/(m2 K). The best windows have U-factors of about 0.7{\textendash}1.0. It is therefore obvious that the U-factor of windows needs to be reduced, even though looking at the whole energy balance for windows (i.e., solar gains minus transmission losses) makes the picture more complex.

In high performance windows the frame design and material use are of utmost importance, as the frame performance is usually the limiting factor for reducing the total window U-factor further. This paper describes simulation studies analyzing the effects on frame and edge-of-glass U-factors of different surface emissivities as well as frame material and spacer conductivities. The goal of this work is to define material research targets for window frame components that will result in better frame thermal performance than is exhibited by the best products available on the market today.

}, keywords = {Fenestration, heat transfer modeling, thermal performance, thermal transmittance, u-factor, window frames}, doi = {10.1016/j.enbuild.2011.05.010}, author = {Arlid Gustavsen and Steinar Grynning and Dariush K. Arasteh and Bj{\o}rn Petter Jelle and Howdy Goudey} } @conference {1408, title = {Experimental and Numerical Examination of the Thermal Transmittance of High Performance Window Frames}, booktitle = {Thermal Performance of the Exterior Envelopes of Whole Buildings XI International Conference, December 5-9, 2010}, year = {2010}, month = {09/2010}, address = {Clearwater Beach, FL}, abstract = {

While window frames typically represent 20-30\% of the overall window area, their impact on the total window heat transfer rates may be much larger. This effect is even greater in low-conductance (highly insulating) windows which incorporate very low conductance glazings. Developing low-conductance window frames requires accurate simulation tools for product research and development.

The Passivhaus Institute in Germany states that windows (glazing and frames, combined) should have U-values not exceeding 0.80 W/(m2 K). This has created a niche market for highly insulating frames, with frame U-values typically around 0.7-1.0 W/(m2 K). The U-values reported are often based on numerical simulations according to international simulation standards. It is prudent to check the accuracy of these calculation standards, especially for high performance products before more manufacturers begin to use them to improve other product offerings.

In this paper the thermal transmittance of five highly insulating window frames (three wooden frames, one aluminum frame and one PVC frame), found from numerical simulations and experiments, are compared. Hot box calorimeter results are compared with numerical simulations according to ISO 10077-2 and ISO 15099. In addition CFD simulations have been carried out, in order to use the most accurate tool available to investigate the convection and radiation effects inside the frame cavities.

Our results show that available tools commonly used to evaluate window performance, based on ISO standards, give good overall agreement, but specific areas need improvement.

}, keywords = {experimental, Fenestration, frame cavity, heat transfer modeling, hot box, international standards, thermal transmittance, U-value, window frames}, author = {Arlid Gustavsen and Goce Talev and Dariush K. Arasteh and Howdy Goudey and Christian Kohler and Sivert Uvsl{\o}kk and Bj{\o}rn Petter Jelle} }