What is SHGC and Why is it Important?

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.