GLAZING
· General Description
· Technical Specifications
General
Description
1. Radiation
2. Conduction
3. Convection
B. Glazing In ITLL
C. Energy Terms
1. ASHRAE U-Value
2. R-Value
3. Relative Heat Gain
4. Shading Coefficient
5. Solar Heat Gain Coefficient (SHGC)
6. Solar Reflectance
7. Solar Transmittance
8. Visible Light Transmittance
9. Visible Light Reflectance
Radiation
Radiant heat energy is transmitted through single-, double- and triple-glazing, causing heat loss in winter and heat gain in summer. Radiation occurs when heat, after being sent through space, travels to a distant object where it can be reflected, absorbed or transmitted. One way to minimize this effect is to use reflective film (i.e., heat mirror) or reflective coating (i.e., low emissivity, or Low E) on the glazing. Low E glass and invisible films, both transparently coated to maintain visibility, will reflect a large percentage of radiant heat energy that would otherwise escape. Similarly, outward radiant heat energy from the sun, which causes excessive summer heat gain, is reflected outward.
Conduction
Conduction occurs when energy passes from one object to another. Conduction is both a cause of heat loss through sealed insulating glass units and the primary cause of heat loss through window sashes and frames. The thermal insulation value (R value) of sealed insulating glass units can be improved by increasing the number of glass panes and air spaces; for example, from double to triple or by filling the units with gas.
Convection
Convection occurs from the upward movement of warm, light air currents. When gases (including air) are heated, they expand, become less dense, and rise. As the gases cool, they increase in density and fall. These convection currents carry heat energy from warmer to cooler areas (i.e., from the warm interior to the cold exterior of window glazing). Convection within sealed insulating glass units results in heat loss.
For maximum insulating value, the air space between panes of hermetically sealed insulating glass must be correct. Adding more space between panes than what is needed can actually cause convection currents and result in excessive heat loss. Sealed glazing units with l/2" air spaces between panes are close to optimal thermal performance and are superior in insulating value to units with less space between panes.
Glazing
In the ITLL
There are six different types of glazing installed in the basement of the southside of the ITLL building. Sensors on the interior and exterior windows measure the inside and outside temperatures.
View Data: To view the data you must first install the Run Time Engine for Windows or Mac .
The six different types of glasses are:
The standard glazing used in most of the ITLL is VE 3-85 insulating glass.
Insulating glass greatly increases a window's thermal performance. It is constructed with two or more glass plies, separated by a desiccant-filled spacer and sealed with an organic sealant. The desiccant absorbs the insulating glass unit's internal moisture.
Condensation forms on glass when the glass temperature falls below the dewpoint of the air. To prevent condensation from forming, the glass temperature needs to be higher than the dewpoint of ambient air. Insulating glass units decrease the potential for condensation formation on roomside glass surfaces by "insulating" the inboard glass ply from conductive/convective heat loss to the outside. This "insulation," using an air space between the two glass plies, results in a more stabilized interior glass temperature. Unfortunately, insulating glass alone may not totally eliminate condensation formation in extreme climates. To lessen this risk, a Low E (low emissivity) coating can be applied to the insulating unit. Low-E coating reflects invisible long-wave infrared or heat, and it reduces heat gain or loss in the building by redirecting the heat. In addition, Low E provides greater light transmission, low reflection and reduces heat transfer.
Among the six different types of glazing used in ITLL, the heat mirror is highly reflective to the near-infrared or heat component of the solar spectrum. Heat mirrors provide a clear "visual read" through the glass while reflecting heat back to its source: to the outside in the summer and to the inside in the winter.
Energy
Terms
The specifications given below for the various types of glazing in the ITLL show the various measures of energy. Following are the explanations for the various energy terms.
ASHRAE U-Value
A measure of heat gain or heat loss through glass due to the differences between indoor and outdoor temperatures. These are center pane values based on ASHRAE standard winter nighttime and summer daytime conditions.
U-values are given in BTU/(hr*ft2*°F) for the English system. Metric U-values are given in W/(m2*°K)*.
*Note: To convert from English to metric, multiply the English U-value by 5.6783.
Winter nighttime U-values are based on an outdoor temperature of 0°F (-17.8°C), an indoor temperature of 70°F (21°C) and a 15 mph (24 km/h) outdoor air velocity.
Summer daytime U-values are based on an outdoor temperature of 89°F (32°C), an indoor temperature of 75°F (24°C), a 7.5 mph (12 km/h) outdoor air velocity and a solar intensity of 248 BTU/(hr*ft2*°F) (782 W/m2).
R-Value
Thermal resistance, expressed in ft2*hr*°F/BTU). It is the reciprocal of U-value. The higher the R-value, the less heat is transmitted through the glazing material.
Relative Heat Gain
The amount of heat gained through glass, taking into consideration U-value and shading coefficient. Using the ASHRAE standard, relative heat gain is calculated as follows:
English System: RHG = Summer U-value x 14°F + shading coefficient x 200.
Metric System: RHG = Summer U-value x 7.8°C + shading coefficient x 630.
Shading Coefficient
The ratio of solar heat gain through a specific type of glass that is relative to the solar heat gain through a 1/8"(3 mm) ply of clear glass under identical conditions. As the shading coefficient number decreases, heat gain is reduced.
Solar Heat Gain Coefficient (SHGC)
The portion of directly transmitted and absorbed solar energy that enters into the building's interior. The higher the SHGC, the higher the heat gain.
Solar Reflectance
The percentage of solar energy that is reflected from the glass surface(s).
Solar Transmittance
The percentage of ultraviolet, visible and near infrared energy (300 - 3000 nm) that is transmitted through the glass.
Visible Light Transmittance
The percentage of visible light (380 - 780 nm) that is transmitted through the glass.
Visible Light Reflectance
The percentage of light that is reflected from the glass surface(s).