What is Solar Heat Gain Coefficient (SHGC)?

So you're shopping for new vinyl windows, and you're trying to figure out what all the window ratings mean.  In your research, you come across something called SHGC.  Below is a guide to solar heat gain coefficient, to help you better understand this particular window rating.

So, what is the Solar Heat Gain Coefficient?

Technically speaking the solar heat gain coefficient (SHGC) is the fraction of solar radiation admitted through a window, both directly transmitted, and absorbed and subsequently released inward.  Since it is a percentage of total solar radiation, it is expressed as a number between 0 and 1.

Is the SHGC the same thing as Visual Transmittance (VT)?

Although they may seem to be measuring the same thing, they are not.  Solar radiation comes in a wide spectrum that includes ultraviolet, infrared, and visible light (among others).  These parts are defined by their particular wavelengths.

The solar heat gain coefficient measures the entire spectrum of energy, not just visible light.  Visual transmittance, on the other had, is only a measurement of the visible light that passes through a window.  And visible light only makes up a small fraction of the entire solar energy spectrum.

Does window tinting help with SHGC ratings?

Yes, but not nearly as efficiently as low-E.  Low-E is designed to let in as much visible light as possible, while attempting to reflect as much of the rest of the light spectrum as possible.

Window tinting only aims at lowering the visible light penetration.  So it lets in less visible light, but lets in much more of the solar radiation (from other wavelengths) than low-e.

So what SHGC rating should I look for in a replacement window?

The answer to this question depends on what region you live in.  There are different climate zones defined by Energy Star.  You can click here to find out which zone you live in.


Windows Plus is here to help answer any of your questions.  Visit Windows Plus of Cincinnati or Windows Plus of Dayton to get more information.