windows. huge variety of available building components and several important roles thermally most...
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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
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
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)
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
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
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
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
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
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
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%
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
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
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
High Performance Windows
• Lead to higher energy performance, reduce the U-factor– Inert gas fills– Low-e coatings– Selective transmission films– Thermal breaks
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
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
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