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Window Retrofit Dilemma – Low E or No Low E? Alistair Jackson, CSBA, LEED for Homes AP responds:

AlistairJackson-web Barbara Loomis, Marketing Coordinator of Wood Harbinger would like to upgrade four, 12-year-old casement windows on the upper floor of her historic home located in Western Washington. Since the windows were installed by a previous owner before the house was placed on the Historic Register they can be replaced with "like kind." Suppliers are telling her she should buy Low-E windows. She's concerned about losing the benefit of the sun's energy and light, precious commodities in the Norhwest. What are her options? Should she surrender to Low E?

IMG_0104 The windows in question include one window that faces a few degrees East of South East, two windows that face a few degrees South of South West, and a fourth window that faces North East. For this purpose we'll focus on the southeast/southwest facing windows, all of which are shaded from the summer sun through much of the day by two mature, deciduous trees.

The answer is yes, Barbara, yes to Low-E in your circumstances, but not the one the vendors are trying to sell you. In the end, that means the best, and most logical choice for your windows will cost you more than the standard Low-E package.

IMG_0368 It is a quirk of market dynamics and politics that has resulted in the common misunderstanding that a “Low-E” window keeps the house heat in and the solar heat out. Window distributors and installers prefer to offer the same window packages from the Mexican border to the 49th parallel. Since most of the construction activity in that territory in recent years has been in areas where air conditioning is common, windows distributors have tended to focus on Insulated Glazing Units (IGU)s with low U-factor and low Solar Heat Gain Coefficient (SHGC). (For those who crave the details, see "technical details," below.) Unfortunately, that's not the best combination for a heating dominated climate like ours in the Pacific Northwest.

In the wake of the recent “economic softening,” a group of politicians compounded this problem (in the language of tax credits that were part of the American Reinvestment and Recovery Act) by requiring that windows that qualified for credit must meet the 30/30 rule; that is, they must have both U-factor and SHGC values below 0.30. The result has been that window marketers have even less incentive to mass-market windows with a SHGC above 0.30. A smart business decision, perhaps, but not good for energy efficiency.

But high SHGC windows can be found! Major glass suppliers, including Pilkington, Cardinal, PPG,  and Guardian, all offer Low-E glass that has a high SHGC. Pilkington for example, claim that double glazed windows using their Energy Advantage  Low-E glass can provide whole window u-factor below 0.30 and whole window SHGC of 0.50 or above.

These – U-0.30, SHGC-0.50 – are good numbers to use as a performance specification for this window replacement.

(Be sure to check that the U-factor and SHGC are listed as “whole window,’ not “center of glass.” The former is the type of data that will be listed on the NFRC label (National Fenestration Rating Council – NFRC) on the windows. The latter only addresses the glass performance, not the frame, and will tend to give lower u-factors and higher SHGCs, so can be deceptive.)

Unfortunately, from most window suppliers, this glazing specification will be a custom order – meaning more time and cost.  At risk of being unpatriotic, it might be worth looking to Canada for an off-the-shelf Low-E/High SHGC window. Our neighbors to the North seem to have cold climate glazing performance figured out.

If a custom window is the answer, at least it will also allow Loomis to select a window design that is more consistent with the style of her home. Good luck!


Lowe_highSHGC Lowe_lowSHGC Technical details for those who want them: Heat energy passes through windows in two primary ways.

  • Thermal conduction: Glass is a good conductor of heat so if air one side of a pane of glass is warm and the other side is cold, heat will flow by conduction from warm to cold. In energy efficient windows, this characteristic of glass is generally addressed by using two or three layers of glass, separated by a gas- and moisture-tight spacer at the edge and a void filled with inert gas that is a poor thermal conductor. This is what we call double- or triple-glazing but is more technically known as an Insulated Glazing Unit or IGU. Thermal conduction is expressed as a U-factor between 1 and 0 – a low U-value means less thermal conduction. U-value is the inverse of the more common R-value (resistance to thermal transfer) used for insulation. So if you divide 1 by the U-value of a window (or other material), you get its R-value, and vice versa.
  • Thermal Radiation: Glass also allows heat to pass through it via infrared radiation, simply illustrated by sitting in a window seat on a sunny day, feeling the warmth of the sun through the glass. We want that warmth to come in during the heating season, but perhaps not during the summer when it might overheat the house. This radiation effect also works in reverse. In the heating season (Fall, Winter, Spring) heat from the warm surfaces (including your skin) inside of your house escapes through the windows as infrared radiation to the cold surfaces outside your house. This affect is addressed by coating the glass or installing thin films between the panes of glass that are made from materials that have low emissivity – hence Low-E. These materials are designed to reduce the amount of long-wave length infrared radiation that can pass through the glass, while allowing the shorter wave-lengths of visible light to pass.  A window’s ability to allow or prevent solar radiation from passing through is expressed as a Solar Heat Gain Coefficient or SHGC between 1 and 0 – the more radiation that can pass, the higher the number.

Low-E Terminology: The fact is that Low-E coatings don’t just reflect heat in one direction. They tend to have an effect on the rate of thermal transfer in both directions, though the extent of this effect varies with the type of coating used (research Pyrolitic and Sputtered coatings for more information). It also depends which surface of the glass the Low-E coating is located on. Typically, this has been limited to the protected surfaces facing into the cavity between the panes, but some treatments are hard enough to withstand being located on the outer surfaces, creating more performance options. It is quite possible to create an IGU that reduces thermal transfer by conduction – a low U-value – but allows solar radiation to pass – a high SHGC. For those interested in even more detail, there’s a great resource at the Efficient Windows Collaborative.

Alistair Jackson, CSBA is a Principal at O’Brien & Company. He was involved in the development of the original LEED for Homes Pilot Rating System and is, among other things, the Quality Assurance Designee (QAD) for O’Brien & Company’s role as LEED for Homes Provider. He is a strong advocate for market-led innovation and market transformation towards sustainability. He is also a father of two – and impatient for change towards a better future.

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