Greenhouse floor slab insulation retrofit (C) Daniel Friedman Steven Bliss Solar Gains / Losses: Conventional Glass vs. Heat Mirror Glazing & Low-E Glass

  • GLASS vs HEAT MIRROR SOLAR GAIN/Loss- CONTENTS: Usage guide for Heat Mirror window glazing: Why we reduce the total window aperture space when using Heat Mirror glazing compared with double-glazed glass. Solar gains and losses compared between clear glass windows and Heat Mirror glazing. Solar Age Magazine Articles on Renewable Energy, Energy Savings, Construction Practices
  • POST a QUESTION or READ FAQs about solar gain and using heat mirror window glazing
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Solar Gain or Loss through different types of glass:

This article describes low-e window films that can be added to existing windows to improve window efficiency and improve solar gain in cold weather.

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Comparing Energy Efficiency of Low-E Glass vs Quad Glazing

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

Low-e Window Retrofit Film Products


I am designing a passive solar home and I am considering using Heat Mirror instead of clear double glass. Am I correct in assuming that I can increase the glazing area in relationship to the mass to compensate for Heat Mirror's lower solar transmittance? - Bob Hutchinson, Kingsville Ontario

Answer: Surprise: not according to an expert! Decrease the glazing aperture when using Heat Mirror glazing.

High passive solar gain room (C) Daniel FriedmanNot according to Timothy Johnson, passive solar specialist at MIT.

He advises just the opposite: reduce the glazing aperture.

The reasoning goes something like this: the transmittance of clear double-paned glass is around 0.89, and the transmittance of Heat Mirror 88 glazing is 0.69 - 22 percent less.

So you get less solar gain with Heat Mirror glazing than with clear double glass.

But the insulating value of Heat Mirror 88 (1 1/4-inch thickness) is R-3.84, 52 percent better than the R-2 of double-glazed glass windows.

In going from clear double-paned glass window glazing to Heat Mirror 88 window glazing, the increased insulation value outweighs the decreased solar energy transmittance.

You get less heat transmitted from the sun to indoors, but you lose still less heat from inside the building to outdoors.

To prevent overheating you will need to reduce the window aperture to about 75 percent of that you would have used for double-paned glass, suggests Johnson.

For more up to date information about the performance of films to increase solar collector efficiency, see SOLAR COLLECTOR FILMS

As we discuss in more detail at SUNGAIN, FILMS, LOW-E GLASS, here is a quick comparison of window types and their efficiencies:

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

Readers should also see SUNGAIN, FILMS, LOW-E GLASS for details about use of Sungain film on windows and window film selection and installation, and see WINDOWS & DOORS our home page for window and door information. A wide range of energy conservation and solar energy topics is found at SOLAR ENERGY SYSTEMS. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

Original article source:

Sunspace & Solar Glazing & Shading Articles


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