Energy Efficiency in Windows: Solar Heat Gain and U-Factor

How Window Glass Affects Energy Performance

Glass plays a very important part in impacting the energy performance of a building; as such they form a very complex part of building design. Issues such as the location of the building both in terms of climatic location, the building orientation, the type of window and window area affect the energy performance of a building. Glass absorbs heat and redirects this heat inwards in three distinct separate ways, Conduction, Convection and Radiation. When these methods of heat transfer are applied to the performance of the window they interact in a complex manner. SHGC is a general term and standard used in the US used to describe the solar heat transmittance properties of glass. Solar Heat gain refers to the amount of heat gain in a building as a result of the glass absorbing the sun’s radiation. The amount of solar gain increases with the strength of the sunlight and the ability of the glass to transfer the radiation. As the building’s glass windows and doors absorb the sun’s radiation the building heats up, this is known as the greenhouse effect. SHGC is measured between the values of 0 and 1; the lower the value the less radiation is absorbed and radiated by the glass. Solar heat gain can be beneficial in the winter as it warms up the interior of the building but for our Florida marketplace we want to minimize solar heat gain during since the majority of our calendar year deals with very high temperatures. Solar heat gain can be an issue for us as it will cause the building interior to heat up too much.

Solar Heat Gain

Solar heat gain coefficient is influenced by the glazing type, the number of panes, and any glass coatings. The non-solar heat flow through a window is a result of the temperature difference between the indoors and outdoors. Windows lose heat to the outside during the heating season and gain heat from the outside during the cooling season, adding to the energy needs in a home. The effect of non-radiation heat gain through a window is measured by the U-Factor. U-factors allow one to compare the insulating properties of different windows. Insulating glass, also commonly referred to as glazing that includes multiple glass panes separated by spaces filled with a special gas with insulating properties and assists in reducing heat transfer between the exterior and interior of the building. Typical gases used for this purpose are Argon, krypton, sulfur hexafluoride, and carbon dioxide. Glazed windows also reduce the amount of ultra-violet radiation. Ultraviolet radiation is the main component of sunlight that can fade and damage drapes, carpets, furniture, and paintings when transmitted through windows.

Microscopic Low Emittence (LoE) Coatings

In addition to the insulated glass and gas filled glazing, microscopic low emittance (LoE) coatings also aid in the energy efficiency of windows and doors. These coatings are virtually invisible and contain metal or metallic oxide coating deposited on a glazing surface. The coating limits radiative heat flow between panes by reflecting heat back into the home during cold weather and back to the outdoors during warm weather. This effect increases the insulating value of the window or door.

U Factor

Typically, solar gain through convection and conduction play’s a bigger part in the energy performance of building during winter periods, when the temperature difference between the outside and inside is much larger. So how does the impact of heat gain impact the energy performance of a building? Let’s take an example of what effects a home in Florida will feel if inadequately designed for SHGC. A building with no consideration to optimizing the SHGC will end up with a building that will overheat in the summer causing the building’s cooling system to work harder and inefficiently to counter the building’s heat gain. By using an appropriate SHGC for the glass relative to the location and orientation of the building the building energy performance can be improved by reducing the thermal load on the building during the summer. What this means for our Florida and Export customers that live is hot climates is that we would always advise to select glass with low solar heat gain coefficients without loss of light. This will ensure that a minimal amount of heat is radiated into the building while also lowering the building’s cooling requirements.

Impact Resistant Glass and Energy Efficiency

As we all know, we have extreme weather patterns with regards for hurricanes here in Florida. So how do hurricane impact windows and doors affect the energy efficiency of your new home? When selecting impact resistant glass doors or windows, it’s recommended to look for energy performance features such as glass with a low-E coating and heat-reducing glass tints. This will ensure an energy efficient building and protection against high winds during hurricanes. When selecting glass windows or doors it’s recommended to ensure they have an Energy Star Rating. ENERGY STAR labelled windows and doors meet a stringent energy efficiency specification set by the DoE and have been tested and certified by the National Fenestration Rating Council (NFRC). NFRC is an independent, third-party certification agency that assigns specific energy efficiency measures such as U-factor and Solar Heat Gain Coefficient to the complete window system, not simply the glass.