Windows

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

Certifying window thermal performance

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

• Depends on – Type of glass– Number of panes– Tinting– Reflective coating– Shading by frame

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

SHGC

• In the cold– Higher SHGC to enable passive solar heating

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

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)