SHADES OF GREEN: Windows – a question of treatment

One of the most down-to-earth issues any householder faces in our climate are the heating bills. We would be well justified in telling any acquaintance from a tropical locale that, “yes it’s nice in Wisconsin about half of the year, in your terms, but really, our big challenge in living here is the cold.” Addressing that challenge has many aspects and one of these is the performance of doors and windows.

There is a big marketing effort for replacement windows, as we all know. The existing ones in older houses are frequently substandard. They are not tight, and they lack the technical features that most modern units now incorporate, such as low-E coatings and gas filling between the panes of sealed units.

The E in “low-E” means “emissivity.” Low emissivity means that a low emissivity surface is reluctant to radiate energy from itself, and instead reflects energy, like a mirror does. (Think of it as just being obstinate.)

It may help to visualize just the opposite thing: a highly emissive surface. Wood stoves are finished in a dull dark color because we don’t want the surface of the stove to be reflecting heat back inside the stove: instead we want the heat to radiate, (to be emitted) outward. Hence, a high-E surface.

A low-E surface, in contrast, is essentially a bright, polished metal. A mirror is usually a sheet of glass with aluminum vapor-deposited on one side.

In windows, low-E coatings are in fact mirrors, but in a clever way they reflect heat radiation without having the light-blocking property that a regular mirror would. What happens then is that in cold weather, the higher heat inside tends to be reflected back into the room. In summer it is also beneficial because the reflection works in both directions: Outdoor heat tends to be reflected back outside.

The other beneficial thing we see in many modern windows is that, instead of dry air, some other gas fills the space between the panes. It’s usually argon, a more dense gas than air, and so it acts as a better insulator than air does.

One way to compare the effectiveness of windows in keeping out the cold is to look at R-value. “R” is a measure of “Resistance” to heat transfer. One pane of ordinary glass is R-1; two panes is R-2. Three panes yield about R-2.5. With the high-efficiency windows, the total system R-values do go up, to perhaps 4.5, 5 or higher. It’s a big improvement in percentage terms when you can double the R-value of a window.

However, in relative terms, windows are still rather energy-loss centers when compared to walls, which are typically R-25 and higher, and ceilings at R-35 to R-60.

So: Replace, or not? You’ll have to decide.

There is another thing about windows, though. Windows, any windows, lose a lot of heat; but it’s not obvious because the losses are so diffuse. It’s not like somebody forgot to close a door all the way and you notice it immediately. The house is warm because the thermostat tells the furnace what to do, and nobody thinks much about it. But just the same: These losses are going on, and why let windows just sit there, especially at night, and lose all that heat? Why not defend them, help them along in some way?

There are various possibilities for this, ranging from the fantastically pricy to practically free. Insulating blinds are attractive and help significantly, especially if you can contrive to have them fit tightly in the window opening. I’m a fanatic about a snug fit; but since the blinds have to travel freely along the jambs on each side, it does take some bother to fashion some sort of chink that you can insert on each side to close the gaps after the blinds are down, and use them consistently.

Roman shades are another species of night insulation for windows. They are generally pleated cloth affairs that drop down at night and which are typically held in place at the sides with spring-loaded pieces of millwork. Upscale from that are insulated cloth coverings that run in tracks. One brand, made in Vermont, is called Window Quilt. These can even be automated and motorized, great for busy families.

Pop-in panels can work, too. Made, usually, out of pieces of rigid insulation, these can be covered or painted to blend in and are just fitted in to the window opening. Where to store them, whether hinged or freestanding, is an issue, as is the interference of any curtains present.

I sometimes work with groups of inner city teachers, and I suggest to them a solution in a situation where the energy bill for a family is a real problem and so is money for any sort of window treatment. A project could start right in the classroom, where kids could learn to glue up multiple layers of salvaged corrugated cardboard, cut to fit precisely into a window casement. They add handles, any decoration they like, maybe aluminum foil on one side, and there you are: virtually free window insulation. (If this sounds a bit off-topic in a middle class community like ours, think again: More of us than we’d like to think are already in this category economically, or could be in the future.)

So, in summary, just know the bad news, that windows are bigger energy losers than you might think, and the good news, that there are various things you can do about it.