Building Fundamentals: Energy Efficiency Geekout – Anatomy of Windows and Doors Part 2

This article by Clarke Snell was first published in the New Life Journal.

Last month I ragged on windows and doors, pointing out that they are generally a very weak spot  in the performance of a modern, environmentally conscious building. To summarize: they don’t insulate very well, are a source of air leakage, can cause perceived discomfort, and can either let in too much solar heat when it’s not wanted or block too much solar heat when it is wanted. The obvious question is, “What can we do about it?”

Luckily, a lot of really smart people have been working on window technology in recent years and they are making big strides. If you are looking to build a new house, there are good choices to be made to improve the energy efficiency of your doors and windows. Similarly, if you want to increase the performance of your existing house, replacing windows and doors is a good place to start.

Sidebar: R-value vs. U-value

Resistance to heat flow in building materials is usually quantified as R-value. The higher the R, the better the insulation. Just to confuse us, the insulation value of windows is expressed as U-value which is the inverse of R. To find out the R-value of a window, divide 1 by its U-value. For example, U= .4; 1÷.4 = R-2.5

If you want to understand the mechanics, there’s a lot to learn. For example, most windows have two glass panes separated by a space filled with air or another gas, but triple pane windows with much lower U-values (see sidebar) are becoming more common. Then there’s the issue of low-e coatings, basically coatings that increase efficiency by reflecting heat energy. Windows can have different numbers and types of coatings configured to reflect heat in or out. Frame type is also important with choices ranging from metal to vinyl to wood to fiberglass. Glazing spacers, thermally broken frames, gas fills, closure mechanisms…the list goes on.

You really don’t need to worry about most of that stuff because all of this technology is synopsized in three quantifiable performance characteristics: U-value, solar heat gain coefficient, and air leakage rate. The National Fenestration Rating Council has created a standardized rating system that requires computer modeling and lab testing for verification of these variables. The results of these tests are prominently displayed on a label you’ll find on any new window or door. If it’s not labeled, don’t buy it. If you are talking with any professional, be sure to reference these numbers and make clear that you want values for the whole window or door unit, not just the glass. Armed with this basic knowledge, I can now offer you some simple rules of thumb summarized in the following chart:

Wind or Door Facing

U-value (BTU/hr-sf-F)


Air Leakage (CFM/sf)







East, West, North1














* My advice is for you not to go below these performance ratings

1East, west, and north facing openings. In terms of winter solar heat gain, these windows will be a net loss. No matter how much sunlight you can let in, the energy gained won’t be enough to offset the energy lost when the sun isn’t shining through the glass. Therefore, choose windows and doors with the lowest SHGC, U-value, and air leakage rates that you can afford.

2South facing openings. For glass that faces south AND gets full sun at least between 10:00am and 2:00pm all winter, choose windows and doors with the lowest U-value and air leakage but highest SHGC. NOTE: There is a new building code in effect setting a maximum SHGC for windows which is well below the desirable SHGC for south-facing windows that get full winter sun. There are ways around this glitch that are too involved to describe in this column. Just be sure to get this worked out with your builder and code officials before ordering windows.

As you start to shop for windows and doors, you may think that some of my chart numbers are wrong. According to NFRC specs, they aren’t. Right now, there are a huge range of performance levels and corresponding prices for windows and doors. Windows made in Europe, such as by the German manufacturer Optiwin, are the best, but they can cost more than $100/square foot. (Compare this to perhaps $15-20/sf for a decent off the shelf window in the US.) Canadian and US manufacturers are catching up in the performance category, so you just have to look around.

On the other hand, if you can’t afford the premium windows, there are other low-budget strategies. Covering glass openings with thick curtains anytime you aren’t in a room will increase window efficiency. If you live in an old drafty house, you can buy shrink wrap plastic to cover your single pane windows in the winter probably for less than $20. You’ll most likely immediately experience an increase in comfort due to higher radiant surface temperature (see last month’s column) and reduced air infiltration.

Regardless of the specifics of your situation, my point is simple. If you want to reduce your heating and cooling bills, improve interior comfort, and reach carbon reduction nirvana, don’t neglect your doors and windows.

A sample NFRC window label

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