Highly Insulating Windows

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Presentation by Christian Kohler, Lawrence Berkeley National Laboratory …

Presentation by Christian Kohler, Lawrence Berkeley National Laboratory

On Thursday June 11th, the Alliance to Save Energy hosted a webinar for Alliance Associates and others interested in opportunities for window energy efficiency. Moderated by the Alliance’s Vice President for Programs Jeff Harris, speakers representing research, industry and low-income weatherization highlighted options that can minimize window heat loss far beyond common practice. The focus was on high-end R-5 window technologies, but lower-cost products, such as low-E storm windows, and the specific needs of low-income weatherization programs were also discussed. The five presenters’ different perspectives converged in the message that there is a great need for more energy-efficient windows and that advanced technologies and their integration in incentive and weatherization programs can bring far greater savings within reach.

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  • 1.
    • Highly Insulating Windows
    • Christian Kohler
    • Windows and Daylighting Research Group
    • Lawrence Berkeley National Laboratory
    • June 11, 2009
  • 2. Windows and Daylighting Group
    • 10-15 researchers dedicated to windows research. Mostly DOE funded.
    • Engaged with industry since 1976
    • State-of-the-art user facilities for testing and evaluation
    • Software used by over 8,000 users worldwide
  • 3. Performance Indices
    • Key performance indices
      • U-factor
        • Thermal resistance
        • Units Btu/hr-ft2-F
        • R-factor is inverse, U=0.2, R=1/0.2 = 5 hr-ft2-F/Btu
      • SHGC
        • Solar Gains
        • Ranges from 0-1, higher means more solar gains
      • VT
        • Visible Transmittance
        • Ranges from 0-1, higher means more daylight
  • 4. Heat Transfer in Windows Conduction Radiation Conduction Convection Low-e coatings Special gas fills Multiple cavities Low conductance spacers Better frames
  • 5. Whole window metrices
    • Whole product vs center of glass
    • Window components
      • Framing (structural)
      • Glazing (vision)
    • Frame area can be 25% of total area
    • NFRC and ENERGY STAR require whole product numbers
  • 6. Highly Insulating Windows - range Whole window U-factor 0.10 0.20 0.30 0.50 0.40 No heat transfer Standard double-pane windows Typical ENERGY STAR windows Highly insulating windows 0.35 = Northern ENERGY STAR benchmark
  • 7. Performance Goals
    • Heating Climates:
    • static high solar, hi-R (U=0.1 Btu/h-ft2-F) can meet ZEH goals
  • 8. Benefits
    • Areas near windows are often places of great temperature variation and discomfort
    • Conventional practice to avoid discomfort is to provide perimeter heating near windows
    • Perimeter heat may not be necessary with highly insulating windows
    Thermograms comparing a conventional dual-pane with a highly insulating window
  • 9. LBNL / DOE Research
    • Triple glazings
          • Develop lower-cost, non-structural center layers
    • Spacer interactions
    • High Performance Frames
          • Collaboration with European researchers
          • Focus on air leakage
    2 sealed gas gaps at different temperatures and pressures with standard glass, unit is thicker and heavier low-e thin glass or plastic held by spacer spacer low-e only 2 paths for gas loss
  • 10. Highly Insulating Frames
    • Mostly driven by PassivHaus Institute in Germany
    • 5 Windows being tested and simulated in Norway and US
    • Verify performance with US rating criteria
  • 11. Low-e storm windows
    • Pyrolytic Low-e coating (hard coat)
    • Does not degrade in non-sealed cavity
    • Identical installation cost to clear storms
  • 12. Savings
    • Whole house heating energy savings over a winter season in Chicago for new storms:
      • Clear storm windows 8-18%
      • Low-e storm windows 19-27%
    • Estimated U-values:
      • Clear storm windows: 0.49 Btu/h-ft2-F
      • Low-e storm windows: 0.36 Btu/h-ft2-F
  • 13. Cost effectiveness – Low-e Storms Total Window Cost Annual Energy Savings Simple Payback (yrs) House 2- Low-E $1,738 $490 3.5 House 3- Clear $1,344 $111 12.1 House 4- Clear $2,661 $317 8.4 House 5- Low-E $1,738 $341 5.1
  • 14. Thank You Christian Kohler, CJKohler@lbl.gov Windows and Daylighting Research Group Lawrence Berkeley National Lab