What is SHGC and Why is it Important?
Various standards and estimations are used to compare the insulating properties of different windows and doors. These estimations are essential for designers and building owners to make informed decisions that will directly affect their energy efficiency programs and, ultimately, their bottom line. One measure of energy transfer through a building component is significant and often drives decision-making in building design areas such as air conditioning and heating. This ultimate factor is the SHGC.
What is SHGC?
Glass absorbs and redirects heat inward in three different ways: conduction, convection, and radiation. Solar Heat Gain Coefficient (SHGC) is a standard used in the US to estimate solar radiation that passes through glass relative to the amount of solar radiation hitting the glass. Solar radiation tends to be the primary method of energy transfer through fenestration products. Factors such as reflection, absorption, and transmittance will affect the SHGC. Expressed as a number between 0 and 1 – the lower the SHGC, the less solar heat transmitted and the greater its shading ability. SHGC is tied to visual light transmission (VT) related to how well the glass allows sunlight to pass through it. Therefore the lower the VT, the lower the SHGC tends to be as well.
Why is SHGC Important?
Solar heat gain, being the primary mode of energy transfer through most fenestration products, can lead to overheating in the summer and provide free heat in the winter. It’s essential to consider climate, the sun’s direction, and typical shading conditions at your building’s location to balance and maximize the potential benefits. A building with little or no consideration to optimizing the SHGC will end up with a structure that overheats when sunlight passes through the fenestration products. This causes the building’s cooling system to work hard and inefficiently to reduce the building’s heat gain. When using the appropriate SHGC for the glass, given its location, orientation, and angle relative to the sun, the building’s energy performance can be greatly improved by reducing its thermal load during the summer and gaining heat during the cold winter months.
What is the difference between U-factor and SHGC?
U-factor is the total heat transfer number. It considers radiation, conduction, and convection and therefore encompasses all modes of heat transfer into or out of a building. U-factor, or U-value, is used similarly to the R-Value for insulation when calculating the building’s overall thermal envelope. Heat can be lost (or gained) through glass by the processes of conduction, convection, and radiation. SHGC, on the other hand, is only considering heat transfer into the building coming from solar radiation. Both U-value and SHGC follow the model of lower values being better for insulation, increasing the potential for energy savings, and decreasing the heat loss in a building.
How can the SHGC value be reduced?
There are several ways that SHGC can be reduced, including outside shading such as pergolas, Venetian blinds, trees, and bushes, etc. Other ways that SHGC can be reduced are Low-e coatings. Virtually invisible, low-emissivity (Low-e or LoE) coatings can significantly reduce the amount of solar heat that passes through the glass. Low-e films are made from a coated polymer and are applied to the inside of the window or door lite to block or slow incoming solar radiation transmission. This solution to lowering the SHGC of a fenestration product is perfect for locations where other shading modes are not applicable. Low-e films reflect between 70 and 80% of the solar heat gain in the summer while conserving over 50% of interior heat in the winter. Low solar heat gain makes for a more energy-efficient door.
How do these values determine which door to install?
In warmer climates, designers tend to choose a door with a low SHGC (.06-31) to keep their buildings cool while reducing their energy costs. Colder climates lean toward a higher SHGC (.45-.57) to benefit from the free solar heat. The numbers vary depending on temperature, shading capabilities, and orientation.
In some geographic areas where there are drastic swings in the amount of sunlight a building sees between the warm summer months and the cold winter months, designers will use a hybrid shading model. This includes applying a Low-e coating to the glass to control the amount of solar radiation that is allowed to pass through the fenestration product and installing small visors above all of the glass to control the amount of sunlight that the fenestration product is exposed to (depending on the angle of the sun at that time of year).
This building is located at the Rochester Institute of Technology. It uses this hybrid model for shading and improving the building’s efficiency by controlling the incoming solar radiation so that it does not enter during the summer when the sun is high in the sky but is allowed to join in the winter when the sun is lower in the sky.
Find out how a thermal break can play a role in the energy efficiency of your doors.