Christian Kohler

High-performance commercial building façades

There is a significant and growing interest in the use of highly-glazed facades in commercial buildings. Large portions of the facade or even the entire facade are glazed with relatively high transmittance glazing systems, and typically with some form of sun control as well. With origins in Europe the trend is expanding to other regions, including the United States. A subset of these designs employ a second layer creating a double envelope system, which can then accommodate additional venting and ventilation practices. The stated rationale for use of the these design approaches varies but often includes a connection to occupant benefits as well as sustainable design associated with daylighting and energy savings. As with many architectural trends, understanding the reality of building performance in the field as compared to design intent is often difficult to ascertain. We have been particularly interested in this emerging trend because prior simulation studies have shown that it should be technically possible to produce an all-glass facade with excellent performance although it is not a simple challenge. The published solutions are varied enough and sufficiently complex that we undertook a year-long international review of advanced facades to better understand the capabilities and limitations of existing systems and the tools and processes used to create them. This is also intended to create a framework for addressing the missing tools, technologies, processes and data bases that will be needed to turn the promise of advanced facades into realities. This summary, available as a PDF file and a web site, reports those findings.

Developing Low-conductance Window Frames: Capabilities and Limitations of Current Window Heat Transfer Design Tools — State-of-the-Art Review

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 that incorporate very low-conductance glazing. Developing low-conductance window frames requires accurate simulation tools for product research and development. Based on a literature review and an evaluation of current methods of modeling heat transfer through window frames, we conclude that current procedures specified in ISO standards are not sufficiently adequate for accurately evaluating heat transfer through the low-conductance frames. We conclude that the near-term priorities for improving the modeling of heat transfer through low-conductance frames are: 1. Add 2D view-factor radiation to standard modeling and examine the current practice of averaging surface emissivity based on area weighting and the process of making an equivalent rectangular frame cavity. 2. Asses 3D radiation effects in frame cavities and develop recommendation for inclusion into the design fenestration tools. 3. Assess existing correlations for convection in vertical cavities using CFD. 4. Study 2D and 3D natural convection heat transfer in frame cavities for cavities that are proven to be deficient from item 3 above. Recommend improved correlations or full CFD modeling into ISO standards and design fenestration tools, if appropriate. 5. Study 3D hardware short-circuits and propose methods to ensure that these effects are incorporated into ratings. 6. Study the heat transfer effects of ventilated frame cavities and propose updated correlations.

State-of-the-Art Highly Insulating Window Frames - Research and Market Review

SINTEF Building and Infrastructure State-of-the-Art Highly Insulating Window Frames – Research and Market Review Arild Gustavsen, SINTEF Building and Infrastructure, Bjorn Petter Jelle, Norwegian University of Science and Technology (NTNU), Dariush Arasteh and Christian Kohler, Lawrence Berkeley National Laboratory (LBNL) Project report 6 – 2007 This work is supported by the Assistant Secretary of Energy Efficiency and Renewable Energy, Office of Building Technology, U.S. Department of Energy under Contract No. DE-AC02-05CH11231 and by the Research Council of Norway within the research project MOT – «Modern Wood Window Frames with Surface Treatment» (In Norwegian: Moderne trevindu med overflate-behandling).

Research Needs: Glass Solar Reflectance and Vinyl Siding

The subject of glass solar reflectance and its contribution to permanent vinyl siding distortion has not been extensively studied, and some phenomena are not yet well understood. This white paper presents what is known regarding the issue and identifies where more research is needed. Three primary topics are discussed: environmental factors that control the transfer of heat to and from the siding surface; vinyl siding properties that may affect heat build-up and permanent distortion; and factors that determine the properties of reflected solar radiation from glass surfaces, including insulating window glass. Further research is needed to fully characterize the conditions associated with siding distortion, the scope of the problem, physical properties of vinyl siding, insulating window glass reflection characteristics, and possible mitigation or prevention strategies.