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An R9 Window -- Hypothetical Design

PHIUS Senior Scientist Graham Wright undertook the following hypothetical design exercise to demonstrate how a window could be designed and constructed to meet PHIUS' performance recommendations for climate Zone 8. The goals of the exercise were to achieve the following levels of performance:

  • An R9 whole-window installed (including accounting for some installation thermal bridge coefficient)
  • Solar heat gain coefficient (SHGC) ≥ 0.5 at center of glass without the use of any cost-prohibitive technologies


Design Notes

  • The glass is set deep into the frame in order to isolate the conductive part of the assembly in the middle of the window.
  • The glass is surrounded by insulation on both sides, with more insulation used on the outboard side of the assembly.
  • The outboard edge of the frame is articulated to lengthen the heat loss pathway and provide structure.


Overall Design













Glazing Details





  • The design exercise demonstrates that achieving the desired level of performance is not overly difficult and could be accomplished with off-the-shelf materials and components.
  • While performing the design exercise it took several attempts to get the glazing right; the starting parameters used were a 4-pane Cardinal catalog unit with Argon measuring 2 inches thick overall. Although this assembly barely met the requirements of climate Zone 8, the frame was easier to nail down and we got it right on the first try.
  • Although the resulting frame is a bit thick, aesthetically it does not appear to be too oversized or impractical.
  • Achieving the same levels of performance with an operable window would be more difficult.

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