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(C) J Wiley, S Bliss Solar Heat Gain Calculation

  • SOLAR GAIN CALCULATION - CONTENTS: How to calculate solar heat gain through windows. Figuring Solar Heat Gain: Calculation Procedure, Definition of Shading Coefficients, Solar Heat Gain Factor, Solar Heat Coefficient, & Solar Transmittance. Define solar input and shading coefficients in solar heating design. Definition of solar heat gain factor & solar heat gain coefficient. Definition of solar transmittance. Solar Age Magazine Articles on Renewable Energy, Energy Savings, Construction Practices
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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.



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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:

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:

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:

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. Which 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.

(C) J Wiley, S Bliss

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.

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