1. Windows

2. 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

10. 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

11. 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

12. SHGC Depends on – Type of glass – Number of panes – Tinting – Reflective coating – Shading by frame

14. 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

15. SHGC In the cold – Higher SHGC to enable passive solar heating

16. 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%

17. 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

18. 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

19. 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

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

21. 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

23. 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

24. 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

25. 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)