How to calculate solar gain for a building:

This article discusses the method for calculating solar heat gain through windows.

We include definitions of solar input, shading coefficient, solar heat gain faactor, solar heat gain coefficient, and solar transmittance.

Accompanying text is reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

Steven Bliss served as editorial director and co-publisher of The Journal of Light Construction for 16 years and previously as building technology editor for Progressive Builder and Solar Age magazines. He worked in the building trades as a carpenter and design/build contractor for more than ten years and holds a masters degree from the Harvard Graduate School of Education.

Solar Gain Calculation: Heat Gain Through Windows

The question-and-answer article below paraphrases, quotes-from, updates, and comments an original article from Solar Age Magazine and written by Steven Bliss.

Figuring Solar Heat Gain: Calculation Procedure, Definition of Shading Coefficients, Solar Heat Gain Factor, Solar Heat Coefficient, & Solar Transmittance

Question: What is Solar Heat Gain and How is it Calculated?

I am having a hard time determining which specification to use when calculating solar heat gain through windows.

Does one use visible light transmission or some other figure? 3M's Sungain Film brochure mentions "Solar Input," which represents the shading coefficient times 0.87.

What exactly does that give you? How do visible light transmittance, and shading coefficient relate to one another? - Jeff Pendl, Enervision, Cincinnati OH

Answer: the basics of calculating solar heat gain.

Solar heat gain is calculated with the solar shading coefficient and the solar heat gain factor (not to be confused with the solar heat gain coefficient - see below).

Definition & Uses of Solar Transmittance

The solar transmittance of a window glazing or window is the percentage of visible and near-infrared light that passes directly through a window. Solar transmittance is used to compare different types of glass. But solar transmittance does not include the solar heat that the glass absorbs and re-radiates into the building, so alone it is not accurate enough for calculating total solar gains through windows.

Definition of Solar Input - Solar Heat Gain Coefficient & Calculating the Shading Coefficient

The "solar input" in the 3M Sungain film brochure, also known as the solar heat gain coefficient, is the solar transmittance modified to account for re-radiated heat.

If you divide by 0.87 (the coefficient for 1/8-inch clear glass), you get the shading coefficient you need to calculate solar heat gain.

The NFRC (National Fenestration Council) describes Solar Heat Gain Coefficient (SHGC) as:

Definition of Solar Heat Gain Coefficient (SHGC)

measures how well a product blocks heat from the sun. SHGC is expressed as a number between 0 and 1. The lower the SHGC, the better a product is at blocking unwanted heat gain. Blocking solar heat gain is particularly important during the summer cooling season.

This is a different figure from visible transmittance (VT) of light, also defined by the NFRC as follows:

Definition of Visible Transmittance (VT)

measures how much light comes through a product. VT is expressed as a number between 0 and 1. The higher the VT, the higher the potential for daylighting.

Watch out: other very important factors need to be considered in arriving at an accurate picture of solar heat gain through windows. Here are added factors defined by the NFRC as of 2005:

Definition of Air Leakage (AL)

measures how much outside air comes into a home or building through a product. AL rates typically fall in a range between 0.1 and 0.3. The lower the AL, the better a product is at keeping air out.

AL is an optional rating, and manufacturers can choose not to include it on their labels. This label displays AL in U.S. units. Labels on products sold in markets outside the United States may display AL in metric units.

See ASHRAE Fundamentals, Chapter 27,for lists of solar heat gain factors. Calculating solar gains by hand is a laborious, imperfect process. Many computer programs make the job easier by calculating solar heat gain for you.
(2005 ASHRAE Handbook : Fundamentals at Amazon)

The higher its solar transmittance or solar heat gain coefficient, the better a glazing is for solar heating use.

The lower those numbers are, the more suitable a glazing is for cooling.

Solar-Heat-Gain Coefficient Rating for Windows: SHGC

As we discuss in detail at CHOOSING WINDOW GLAZING Based on Climate, (a chapter in Best Practices Guide for Windows & Doors):

A window with an SHGC of .70 captures about 70% of the available solar energy falling on the window. Clear double glazing has an SHGC of about .75 versus .60 to .70 for standard low-E and about .40 for spectrally selective low- E. W

hich type of glazing is optimal for a given project depends on the climate, summer and winter fuels costs, and how glass is used in the house design.

• Low SHGC.
  
  Low-solar-gain glass blocks unwanted solar gain and provides significant savings in both peak and annual cooling loads in hot climates.
  
  For example, switching from clear double glass to low- SHGC glazing can reduce air-conditioning bills by 15 to 20% in a typical home in Phoenix or Miami (See Figure 3-10 above). Spectrally selective glass , introduced in the 1990s, combines very low solar gains with high visible light transmittance and high R-values (up to R-4 with gas fill).
  
  While this yields the greatest savings in hot climates, it is also a good choice in any climate with significant cooling loads or large amounts of un shaded west-facing glass.
  
  See Spectrally Selective Window Glazing.
• Moderate SHGC.
  
  In northern cities like Boston or Chicago or mixed climates with more-or-less equal heating and cooling costs, moderate-gain glass is a reasonable choice, balancing moderate solar gains in winter with moderate blocking of solar gains in summer.
• High SHGC.
  
  High solar-heat-gain glass is a good choice in cold climate homes with enough south glass to take advantage of passive solar gain, called “sun tempering.”
  
  Savings on winter heating bills will be partially offset by the increased cooling load in summer, however. To avoid overheating, south-facing glass should range from 4 to 7% of the total floor area (avoid sloped glass, which tends to overheat in summer and fall).
  
  With south glass in excess of 7% of floor area, thermal mass may be needed to prevent daytime overheating and to store heat for nighttime use. A designer with expertise in passive solar can help determine the right amounts of glass and thermal mass.

The question-and-answer article above, quotes-from, updates, and comments an original article from Solar Age Magazine and written by Steven Bliss.

The link to the original Q&A article in PDF form immediately below is preceded by an expanded/updated online version of this article.

• Q&A on Figuring Solar Heat Gain- PDF version, use your browser's back button to return to this page. Original article, Solar Age Magazine, November, 1985, adapted and updated for InspectAPedia.com December 2010.

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Continue reading at SOLAR GAIN TYPES, DEFINITIONS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

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