Whether your company is working on plans for a new building or looking to renovate or add onto an existing property, you can find energy-efficient windows, doors and skylights to fit your needs. First, though, you have to understand a few basic principles including how these glass products gain and lose heat, energy performance ratings, and energy performance characteristics such as U-factor, solar heat gain coefficient and air leakage.
Windows, doors and skylights gain and lose heat in three ways:
(1) Direct conduction through the glass, glazing, frame and/or door.
(2) Radiation of heat, typically generated by the sun, into a house, and the loss of heat out of the house through room-temperature objects such as people, furniture and interior walls.
(3) Leaking of air through and around doors, windows and skylights.
That doesn’t mean you can’t reduce the amount of heat loss or increase the amount of heat gain. By choosing and using energy-efficient windows, doors and skylights, you can protect the environment, reduce the costs of heating and cooling your company headquarters, and enjoy a naturally lit work environment.
ENERGY PERFORMANCE RATINGS & LABELS
The energy performance ratings of windows, doors and skylights indicate the product’s potential for heat gain and heat loss. Energy performance ratings also tell you how much potential the window, door or skylight has for allowing sunlight into your home.
How do you know what a window or door’s energy performance rating is? All ENERGY STAR qualified windows, doors and skylights carry labels or stickers from the National Fenestration Rating Council (NFRC). The NFRC runs a voluntary program that tests, certifies and labels windows, doors and skylights based on their energy performance ratings. These NFRC labels enable consumers to reliably determine the product’s energy properties and compare with other windows, doors and skylights.
ENERGY STAR, a government-backed program, provides information on energy efficiency and enables businesses and individuals to protect the environment. ENERGY STAR qualifications are based solely on U-factor and solar heat gain coefficient (more on these later).
DESIGN AND USING WINDOWS EFFICIENTLY
You must choose, position and size your windows in a specific way to maximize solar heat gain. In very hot climates, you want big windows facing the south to collect solar heat in the winter, as this is when the sun hangs lowest in the sky. By summer, the sun has risen high overhead. That’s when you’ll want overhangs and other shading devices on those south-facing windows so that you can prevent excessive heat gain, thereby reducing the costs of cooling the house.
On Energy.gov, it’s recommended that you minimize the use of Windows on walls that face north, east and west while still allowing for adequate daylight. This is because it is difficult tocontrol heat and light through east- and west-facing windows when the sun is low in the sky. East- and west-facing windows, doors and skylights should be shaded and/or have a low solar heat gain coefficient. North-facing windows are used solely for light purposes as they collect little solar heat.
In addition to using shading to help control solar heat gain, you can purchase windows with low-emissivity (low-e) glazing.
HEAT LOSS AND HEAT GAIN
Using the folloing energy performance characteristics, you can measure and rate the heat loss and heat gain properties of windows, doors and skylights.
– U-factor is usually expressed in units of Btu/hr-ft2-oF. This is the rate at which a window, door, or skylight conducts non-solar heat. A U-factor may refer solely to the glass or glazing, but NFRC U-factor ratings refer to the entire window or door’s performance. This includes frame and spacer material. Windows, doors and skylights with lower U-factors are more energy-efficient.
– The fraction of solar radiation admitted through a window, door or skylight is called solar heat gain coefficient. This solar radiation is either transmitted directly and/or absorbed. It issubsequently released inside the home as heat. The amount of solar heat a window, door or skylight transmits and the greaterits shading ability, the lower the solar heat gain coefficient will be. “A product with a high SHGC rating is more effective at collecting solar heat during the winter,” according to energy.gov.The lower the product’s solar heat gain coefficiency rating, the better it will be at reducing the amount of cooling needed in the summer. Your building’s climate, orientation and external shading determine the optimal solar heat gain coefficient for a particular window, door or skylight.
– Air leakage, expressed in units of cubic feet per minute per square foot of frame area (cfm/ft2), is the rate of air movement around a window, door or skylight in the presence of a specific pressure difference. The tighter the product frame, the lower the air leakage will be.
There are two energy performance characteristics that can be used to measure and rate a window, door or skylight’s ability to allow sunlight into a building: visible transmittance and light-to-solar gain.
– Visible transmittance: A fraction of sunlight’s visible spectrum (380 to 720 nanometers) that enters a building through the glazing of a window, door or skylight. The higher the visible transmittance, expressed as a number between 0 and 1, the more visible light flows through a window, door or skylight. The visible transmittance you need for a window, door or skylight is determined by your building’s daylighting requirements. Another consideration is how much reduction of interior glare the space needs.
– Light-to-solar gain is the ratio between the solar heat gain coefficient and visible transmittance, which is a gauge of how efficient different glass or glazing types are at allowing daylight in while blocking heat gains. The higher the light-to-solar gain rating, the more light is allowed in while avoiding excessive amounts of heat. Light-to-solar gain ratings aren’t always provided on products.