Category Archives: Windows

The Road to Energy Zero Homes – Understanding Your Energy Baseline

If you’re setting out to convert your home to a “zero energy” building standard, then it’s a good idea to know your baseline. In other words, where does the energy go in a typical home?

As you might expect, especially for heating cooling requirements, it depends on where you live, the age of your home, and to some extent (since the wealthy tend to consume more) how much money you make. However, one thing is clear, the bulk of the energy used in a home is for heating and cooling. According to the DOE, Energy Information Administration’s 2001 Residential Energy Consumption Survey anywhere from 38% to 70% of a homes energy consumption is spent on heating and cooling. The average values across all home sizes, home ages, and climate zones from that survey are shown in the following pie chart.


To give you and idea of how the energy consumption varies by where you live, the following charts show consumption by climate region for the United States.


The key point of all this data is that heating and cooling consumes the bulk of a home’s energy, and all of that energy is exchanged through the home’s shell or envelope. As a result, the starting point for any attempt to convert an existing home to a zero energy building standard must be an upgrade of the building envelope. In general, energy losses through the envelope are evenly divided between infiltration (air leaks), windows & doors, and conduction through the wall, ceiling, and floor. I’ll cover each of those in later posts.

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Low-e Glass … A Nation Divided

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:

  1. 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.
  2. 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.

Energy Star Climate Map

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.

Energy Star Window Criteria

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:

Just want a source for the glass? Try Pilkington North America.

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What’s the Greenest Window?

Approximately 1/3 of the heat loss or gain in a home
is through the windows.

LEED for Homes Pilot Rating System, U.S. Green Building Council

Remember when all we needed to know about windows was whether or not to choose wood or aluminum, and whether to spend the extra money for dual glazing? Well times have changed and window and patio door design is now a sophisticated science with its own set of specialized jargon. Choosing the greenest and most energy efficient window is now a much more complicated process and depends in large part on your climate, whether or not your home is going to be passively heated using the sun, and even on what direction the window is facing.

Unfortunately, in order to make an intelligent choice and compare products, you’ll need learn some of the industry jargon used to rate today’s high performance windows.

U-value – In the winter we want our windows to keep heat in the house, but in the summer we expect them to do their best to keep heat out. A window’s ability to do this via the conduction mode of heat transfer is based something called the U-value. In the U.S. the U-value is a measurement of how quickly heat conducts through the entire window assembly including the frame. The lower the U-factor, the more resistant the window is to heat transfer. For example, a single-glazed aluminum window does a great job of losing heat because it’s U-factor is pretty crummy, about 1.30. A typical, good quality, double-glazed wood window in the U.S. today does a much better job with U-values in the range of 0.30, and some of the best windows have U-values less than 0.20.

Solar Heat Gain Coefficient – The solar heat gain coefficient (SHGC) is a measure of how much solar “radiant” energy can pass from the outside through the window. If a window has a rating of 0.50, that means that the window admits 50 percent of the heat energy that strikes the window. This measurement is typically for the entire window, so the amount of solar energy or heat that gets through depends on the type of glass, low-e coatings, and the area of the frame. A typical value for a double-glazed window would be about 0.25 on the low side and over 0.50 on the high side. A 0.25 SHGC window would admit half as much solar heat energy as a 0.50 SHGC window. For passive solar designed homes, you’ll want to specify high SHGC windows on the southern exposure.

Low-e Coatings – A low-e (the “e” stands for emissivity) coating is an extremely thin, metallic layer applied by vapor deposition to the surface of the glass. The coating is thin enough to see through, and has the ability to reflect energy that would otherwise escape from the home . In a cold climates these coatings help keep the house warm by preventing the escape of infra red energy from the inside of the home. In a hot climate their role is to keep solar energy from entering the home by reflecting it back outside at the window surface. A wide variety of low-e coatings are available. Spectrally selective coatings are especially good for cooling climates where they reduce solar heat gain without blocking an excessive amount of visible light. Low-e coatings typically lower SGHC ratings, but some specialty coatings are designed for passive solar applications in colder climates and will let beneficial solar radiation in but still block or reflect longer wave infra red energy from escaping from the home.

Now that you know the basic jargon, what makes for a good high performance green window? Every window is made up of six basic elements:

  • the frame
  • glass panes (glazing)
  • the gas between the glass panes
  • the spacers that separate the glass panes
  • and seals and hardware for operable window elements

High performance window frames are typically made from either rigid extruded vinyl (PVC), aluminum clad wood, or pultruded fiberglass. Vinyl affords the lowest cost but has some environmental issues with the release of VOC’s such as phthalates, the leaching of lead and cadmium fillers, and the release of dangerous toxins such as dioxin when burned. Some would argue that the cost benefits outweigh the risks, but from a “green purist” point of view its hard to make a strong argument for vinyl.

Aluminum clad wood is relatively sustainable, is a mediocre to fair insulator and esthetically pleasing, but the most expensive of the three options. Because the wood used in quality windows must be highly stable, it tends to be vertical grain, all heart wood which adds pressure on our old growth forests. Fiberglass is priced somewhere in between wood and vinyl and according to at least one independent Canadian study done for the Waterloo Region Green Home Assessment, foam filled fiberglass windows are the greenest of the three choices.

Window Type Comparison Table

Glass or glazing is basically the same for any high performance window, but the type and quality of the low-e coating can vary with manufacturer and greatly effect both the U-value and SHGC.

The gas fill between the window panes can be air, argon, or krypton. Argon and krypton improve performance because they have lower conductivity than air. Argon is less expensive than krypton and more commonly used.

Spacers are used to separate the panes of glass and can be made from aluminum, stainless steel, or foam silicone. All spacers have a desiccant included to absorb any moisture introduced during manufacture that may condense on the inside after installation. Because metal spacers dominant the market, and metal is a great conductor, spacers are one of the weak links in a window assembly.

For operable windows like sliders, casements, and awnings, some type of rubber or other seal is used to prevent air leakage. In general, casement and awning windows perform better than sliders, because the hardware allows the window to be “cam-locked” against the seal for a more air tight fit.

So what’s best? What is the greenest window? Weighing all the factors, for my money its a foam filled fiberglass frame, dual or triple paned with one or two low-e panes, argon filled, silicone (non-metallic) spacer window in an turn & tilt, awning, or casement configuration. If you’re looking for U-values less than 0.20, the best performing windows I’ve found in North America are from a small company in Canada called ThermoTech.

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