SHGC, Visible Light Transmission and U-Value
The key factors to understand when judging the expected performance of a window or door are:
- Solar Heat Gain Coefficient (SHGC)
- Visible Light Transmission (Vt)
- U-Value (U-Value)
These are the standards by which most test protocols evaluate glazing from.
Solar Heat Gain Coefficient (SHGC) – is the measure of relative heat gain (compared to a piece of 1/8″ thick clear glass) that a particular piece of glass will absorb under exposure to the sun. Reducing the Solar Heat Gain Coefficient (SHGC) is a function of either tinting the glass and making it darker or coating the glass with low-emissivity coatings to reflect the amount of infrared spectrum most responsible for heating that the glass absorbs.
Visible Light Transmission (Vt) – is a measure of the amount of total visible light that will pass through the glass, again relative to a piece of 1/8″ thick clear glass. Adding low-emissivity coatings coatings to glass will affect the Visible Light Transmission (Vt) factor pretty dramatically depending on the specific glass manufacturer’s method of coating.
U-Value (U-Value) – is a calculated measure designed to represent the ability of the assembly (both pieces of glass, the air space, the frame components, etc.) to resist the transmission of cold and heat through them. This is a composite value of all the components and is not a relative measure. The lower the U-Value, the better able the assembly is capable to resist the passage of heat and cold. U-Value is the inverse of R-Value in measuring the same physical characteristics and thus the higher the R-Value, the better able to resist the passage of heat and cold.
Traps With Solar Heat Gain Coefficient – (SHGC) and Visible Light Transmission (Vt)
Looking at the Energy Star website will tell you the recommended SHGC and U-Value you should have for your specific Climate Zone. But these recommendations do not take into account daylighting and the effect that Visible Light Transmission (Vt) can have on the performance of the overall structure during the heat/cool/light cycles. If you lower Visible Light Transmission too much, the daylighting in the interior will be reduced to a level that may require supplemental electrical lighting for some functions, or to make the environment enjoyable to the occupants. In evaluating the overall structure from a green building perspective, you must look at all the factors here.
Adding electrically generated light, uses energy for the lighting, but it also generates additional cooling loads on the HVAC system adding to the overall electrical usage. There have been studies done particularly on skylights, daylighting and electrical usage in commercial buildings which attribute a very significant increase in electrical usage to this phenomenon. So make sure you do not overshoot in reducing the daylighting in the home in pursuit of the gremlin of the lowest Solar Heat Gain Coefficient (SHGC).
U-Value is the relative constant in all this. Lowering Solar Heat Gain Coefficient (SHGC) will not in and of itself have a large impact on U-Value. Nor will lowering the Visible Light Transmission (Vt). The U-Value is a function of the physical properties of the glass, wood and/or metal that compose the composite of the unit and these are not affected by changes in the other values.
U-Value and Solar Heat Gain Coefficient in Climate Zone 4
If you live in Climate Zone 4, then U-Value becomes a more important consideration in judging the window performance overall. But in this zone, higher Solar Heat Gain can add incrementally to the heat inside the home and reduce the heating demand placed on the heating system. Thus, a SHGC of .65 will be of more benefit to you . You will also finsh that the High Solar Heat Gain low-e coatings will typically have higher Visible Light transmission in the .7 range.
So, you must understand the impact of all these factors as it relates to your specific location. When you are through evaluating on this basis, you will need to look at the air infiltration ratings. I will discuss this in another article.
This added 7/7/2008
Light to Solar Gain Ratio (LSG)
I ran across a new measure that helps you guage the relationship between SHGC and Visible Light Transmission. The new ratio is known as the Light to Solar Gain (LSG) and is a ratio of the Vt to SHGC. For instance if the Vt is .66 and the SHGC is .40 then the ration of .66/.40 or 1.65. The recommended ratio is 1.25 or higher. The higher the ratio, the higher the daylighting benefit. It also means that the glass transmits more light than heat to the interior, again a positive in the Southern climate zones 1,2&3.
Solar Heat Gain in Energy Star Zone 4
If you live in Zone 4, then it is desirable to have a higher Solar Heat Gain ratio in your windows. Otherwise your heating system will work harder in the winter and diminish your energy efficiency. The standard, 1st generation pyroltyic Low-e coatings (also known as High Solar Heat Gain Low-e, is the easiest method of obtaining a higher SHGC and still retain the reflectivity of the low-e as it pertains to winter infa-red heat retention in the interior of the home.
You may run into a problem in this regard as some window companies have settled on the more efficient (for zones 1,2,3) sputter coat low-e‘s as their “standard” offerings. They prefer to narrow the choices because it is less costly to offer fewer choices. Do not be disuaded. Stick to your guns and insist on the specs that meet the Energy Star qualifications for your home. Any window manufacturer can get the pyrolytic coatings. If they tell you they cannot, move on to a manufacturer that will give you what you need. Buying windows is like anything else, you will either get what you “buy” or you will get what you were “sold”. In the end, you must be responsible for the choice you make.