Low-e glass, first introduced in 1979, has transformed window energy performance. Low-e glass is manufactured with a microscopically thin and transparent layer of metal or metal oxide that reflects infrared “heat” energy back into the home, greatly enhancing the thermal performance of the window.
In the simplest of terms, there are basically two kinds of Low-e glass:
- Low-e glass with a low Solar Heat Gain Coefficient (SHGC) that also reflects and keeps much of the Sun’s heat energy out of the home. This is the best choice in climates dominated by cooling.
- Low-e glass with a high Solar Heat Gain Coefficient (SHGC) that allows the Sun’s heat energy into the home. This is the best choice in climates dominated by heating or for south facing windows in climates with a mix of cooling and heating requirements.
All this seems pretty straight forward until you actually attempt to purchase a window with Low-e glass. But before I get into that let’s take a look at the climate zones in the U.S. used by Energy Star to determine the qualification criteria for windows and skylights in an Energy Star rated home.
The Northern zone is the perfect candidate for a Low-e glass with a high SHGC and the North/Central and even the South/Central can greatly benefit from high SHGC Low-e glass on southern exposures. Unfortunately, even though the average American family spends far more on heating than air conditioning, both Energy Star, LEED, and the International Energy Conservation Code (IECC) seem to be color blind when it comes to space heating and the benefits of Low-e glass with a high SHGC.
Energy Star requirements for SHGC seem to be all about cooling as the following requirements for SHGC for each climate zone indicates. The IECC is no better, only requiring a SHGC of ≤ 0.40 for any residence with less than 3,500 Heating Degree Days (HDD). LEED only takes the cooling bias further by requiring even lower SHGC’s in the South/Central and Southern climate zones.
As an unintended consequence of the regulatory bias in favor of cooling, window manufacturers have all but abandoned Low-e glass with high SHGC’s. The result is a nation divided, with more than half of the country left out in the cold without ready access to high performance windows that take advantage of the free solar energy that strikes our windows everyday. With very few exceptions you just cannot find a window manufacturer willing to give you the glass options we need and require in heating dominated climates.
So what’s the consequence of the current regulatory bias? First of all it makes no sense, according to the U.S. Energy Information Administration’s (EIA) 2001 Residential Energy Consumption Survey, Americans consume more than 7 times more energy for space heating than for air conditioning. To get an estimate of what’s on the table for high SHGC, Low-e glass in terms of the potential energy savings I assumed a 10% improvement in heating costs for homes in the U.S. with more than 4,000 Heating Degrees Days. Based on the same 2001 EIA Survey, that would amount to an annual savings of 0.362 quadrillion(1 followed by 15 zeros) Btu’s per year. In monetary terms, that’s about $475 billion dollars worth of natural gas!
When the government gets serious about Global Warming maybe they’ll fix the cooling bias in the regulations, but until then here’s where to go to get a Low-e windows with a high SHGC:
- Accurate Dorwin, Manitoba, Canada
- Alpen Windows, Boulder, CO
- Fibertec Windows, Ontario, Canada
- Thermotech Windows, Ontario, Canada
- INLINE Fiberglass, Ltd., Ontario, Canada
- Vinyl Window Designs, Ontario, Canada
- Marvin Windows & Doors, Warrroad, MN (you will need to call their technical support for architects and design professionals @ 1-800-346-3363 and ask to specify your own glass)
Just want a source for the glass? Try Pilkington North America.