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  1. Windows

  2. Windows • Huge variety of available building components and several important roles • Thermally most important they admit solar radiation • Advantageous in the winter but disadvantageous in the summer • Despite improvements glazing still has the lowest R value (highest U factor) of all components of an envelope • Also provide daylight and ventilation

  3. Glazing • Glass part of the window • Control • the amount of daylight • Quality of light • Amount of solar heat gain • Determine the thermal and visual comfort of the space

  4. Certifying window thermal performance

  5. Windows • Fenestration – any opening in the building envelope • What that is covered with a translucent or transparent surface it’s called glazing • 3 of the most important properties of the materials, coatings and constructions • Thermal conductance (U-value) • Solar Heat Gain Coefficient (SHGC) • Visible Light Transmittance (T vis)

  6. Glazing • Appropriate values for glazing vary by climate, size, and placement of the aperture • Not unusual for a building to have 3, 4, or 5 different kinds of glazing for apertures at different sides/heights on a building

  7. Thermal Conductance (U-factor) • Sensible heat flow due to temp diff through windows and skylights is a function of the U-factor • Measures how well/poor glazing insulates • Measures the rate of heat transfer per unit area, per temp difference from the hotter side to the colder side • R-values are 1/U-factor

  8. Thermal Conductance • Measured for the glazing only or entire assembly (most often referenced) • http://www.nfrc.org/ • Size of air gap, coatings on the glazings and gas fill between them as well as the frame construction influence the U-factor

  9. Thermal Conductance • In cold climates – low U-value (.35 or less) • Warmer climates – Solar Heat Gain Coefficient is more important • Due to gains from direct solar radiation being more important that conduction through the window

  10. Solar Heat Gain Coefficient (SHGC) • Measures how much of the incoming heat from sunlight gets transmitted into the building vs. how much is reflected away • Heat from the sun is long-wave radiation (infrared and other non-visible light) • Also typically based on the entire unit not just the glass

  11. SHGC • Depends on • Type of glass • Number of panes • Tinting • Reflective coating • Shading by frame

  12. SHGC • A dimensionless number between 0 and 1 • 1 represents no resistance (all heat from the sunlight comes through) • 0 representing total resistance • Typically range from .9 to .2 • Especially important in hot sunny climates • Cooling is the dominant thermal issue • Generally use glazing with SHGC lower than .4

  13. SHGC • In the cold • Higher SHGC to enable passive solar heating

  14. Visible Light Transmittance • The % of visible light that passes through a window or other glazing • Opaque wall would have a Tvis of 0% • Empty opening would have 100% • Untinted is around 90% • More is often not better • Causes glare and overheating • Common values are often 30-80%

  15. Adaptive Properties • Some advanced glazing systems can change their visible light transmittance, solar heat gain and other properties • Liquid crystal windows – change from clear to frosted or dark when a voltage is applied by a control system • Improves privacy by not solar heat gain

  16. Adaptive Properties • Thermochroic coatings – turn from clear to dark at high temps • Reduces Tvis and SHGC • Photochomic coatings – turn from clear to dark when struck by light (sunglasses/glasses) • Electrochromic coatings – clear to dark when a voltage is applied by a control system, also reduces Tvis and SHGC

  17. Other Considerations • Infiltration – air leakage. Standard rates are .3cfm/ft2, .02 to .01 is better • Light distribution angles – direction light is transmitted into the building. Important for skylights • Condensation – occurs in units with large temp diff from inside to out • Acoustic damping – Multi pane with diff thicknesses and layers of diff material in frame

  18. High Performance Windows • Lead to higher energy performance, reduce the U-factor • Inert gas fills • Low-e coatings • Selective transmission films • Thermal breaks

  19. Air Gaps/Inert Gases • Simplest way to reduce U-value • Reduces convection within glazing unit • Denser gases like argon and krypton can be used • Reduce conduction and convection • Less conductive gases greatly reduce heat transfer by convective currents

  20. Low-Emittance (low-e) Coatings • Invisible thin layers of metal or metallic oxide particles • Hard coat- durable, less expensive by not as thermally effective • Soft coat- better performance but more expensive and subject to degradation • Typically applied to one glass surface facing into the air gap

  21. Low E coatings • Blocks a great deal of radiant transfer between panes • Reduces the flow of heat through the window • Almost as effective as adding another layer of glazing • Reduction of UV transmission – reduces fading of objects and surface finishes • Works like a thermal mirror • 3 types

  22. Selective Transmission Films • Controls the wavelengths of light • Allow visible light in while block most other wavelengths • Block short wavelengths and reflect long wave radiation into the room (from warm objects in a room)