The Joy of Curtains: Eco Living Articles: Eco20/20

The Joy of Curtains

Dr. Robert Buxbaum,
President, REB Research & Consulting.

In our northern climates, most people have high-tech, double-paned windows that cost a fortune but are a lot better than plain glass. Unfortunately, they are still a lot less insulating than the equivalent area of wall. It should be obvious why this is so: the typical double pane windows are only about ¾" thick, including glass panes, and the space between them filled (typically) with air. By contrast, the wall is typically 6" thick and filled with air and glass -wool insulation. Given the comparative thicknesses, we can expect the window will let through about 10 times more heat than the equivalent area of wall, and that is about exactly what we find: the "R" value of a typical outer wall is about 20 in our climates (20 °F ft²/BTU), while the R value for a typical double pane window, about 2. The difference in R value is so great that, for many outside facing rooms, far more heat is lost through the window than through the wall, though typically the window area is much less than the wall.

If your windows are only single paned, or drafty (don't seal well), the heat loss is much more than this, but even with "good", well sealed double paned windows, we often find that the easiest way to cut ones heat bill, and make a room more livable, is to fix up the windows. One easy thing we can do in this regard is to put plastic on the windows. To get an advantage from this, the plastic should not be put too close to the window, nor too far away from it. If the plastic come too close, or touches the window, the insulating effect of the plastic becomes negligible. That's because conduction heat transfer rates are inversely proportional to the distance between the window and plastic, call it ?.

Q = ?T/R = ?T kA /?

Where Q is the heat lost per unit time, ?T is the temperature difference between the window surface and the room, k is the thermal conductivity of air, about .024 BTU/ft. hr°F, A is the window area, and ? is the air gap thickness; the glass and plastic itself adds almost nothing to the heat transfer resistance. If ? becomes small, the heat loss can become very large.

Based on the equation above, we can define a value of R for any air gap of this sort:

R = ?/k = ?(feet)/.024

We can see that the R value for a 2-pane glass is going to be about 2 when the air gap of the window reaches about 9/16", or 1/20 foot. We should expect that we can add a second R value of 2 by adding a plastic layer with another 9/16" gap to the glass. These two resistances combine, in an ideal world, leading to total resistance of 4. This summation doesn't work exactly for reasons that will be obvious later, but it's close, experimentally. We might expect that we could raise the total resistance much higher than this, and really reduce the amount of heat leaving our homes, by the using a very large gap space ? between the plastic and the glass. Sorry to say, heat resistance doesn't work that way. When the gap space ? exceeds about ¾" the insulating value drops dramatically as gas motion in the gap (free convection) drives heat transfer. The ideal size varies for different climates and gas choices, but a gap between about ½" and ¾" is something of an ideal average for most window sizes and climates we are likely to find.

Now we get to curtains. Curtains are a simple, attractive way to augment the plastic, or replace it on windows where plastic looks bad. A simple cloth curtain hung between ½" and ¾" from the window will reduce heat loss nearly as well as the layer of plastic will, and it typically looks a lot better. A window shade does about the same, but looks worse (in my opinion). Either way, though, the layer of cloth interferes with the sinking free convection of cold air, the mechanism that transfers most of the cold from the window to the room. Without a curtain, the air nearest the window or plastic gets cold and sinks, convecting the cold from the window to a puddle of cold air near your feet. This heat loss mechanism is sometimes called "shedding" because the window feels like it's shedding cold. With a curtain or shade at the right distance this shedding motion is greatly reduced. Add a second curtain another ½" - ¾" from the first, and shedding heat is reduced even more.

One of my favorite types of curtains is called a "cell." These are folding curtains of two or more lengths of stiff cloth with an air gap (or two) in between. The cell curtains are typically made to be ½" to 2 ½" thick with air-gap cells in-between. This thickness of the cells is chosen to maximize the insulating power of the shade, and done right the cell curtain will add another R value of 3 or more. It should be noted, though, that most of this advantage goes away if the cell, or curtain is set at more than about 1" from the layer of glass or plastic. At a greater thickness that this, convection between the window and the shade starts to dominate the heat loss and it will be as if you didn't have a shade or curtain in the first place.

An important option to avoid with cellular curtains, I think, is one that opens from the top and bottom - a popular option these days. I have not checked, but it seems to me that they will have very minimal insulating power when both top and bottom are open. To have both open is really inviting large shedding flows. I prefer to have one that opens from the bottom only, and is translucent. This provides privacy and extra insulation to the extent that it is closed. Behind this, I like to have a double-pane window, backed in some cases by a layer of plastic or a insulation, or a thin, see- through shade near the window. The plastic or see-through shade allows one to see out the window (or let light in) without ruining the thermal insulation too badly. After all, being able to see out, or having natural light in was the reason you had a window in the first place. Other options for privacy include blinds and regular, non-see through curtains. Thick, insulating curtains seem like a waste to me - they are not thick enough to add any significant R-value, and if they end up far from the window, the shedding heat loss will more than offset any small advantage from the thick cloth.

At this point, one's window insulation R value is up to 5 or 6, and it may be time to stop adding. One last possibility that's worth mentioning though is a reflective coating. A surprising amount of heat can cross the window in the form of radiation. That is by way of visible and invisible (infra -red) light that passes unimpeded through the double pane glass and plastic. This mode of heat transfer is more important in the summer in hot climates, where heat comes in this way, and a coating is useful to preserve air conditioning power. Still, even in the winter in Detroit, it's can be worthwhile to add a partially reflective coat on ones glass windows. Reflective plastic coats are cheap enough and readily available, though they can be hard to apply, and are not always attractive.

Between all these window treatments, you can not expect to reduce the per-area heat loss of the window down to the level of the walls, but you can expect to reduce the window heat loss by a factor of 3 or more, and in doing this you can manage to reduce the total heat lost through to window below that exiting the rest of the wall space. This should provide a significant reduction in heat bill, and will allow you to make previously uninhabitable rooms pleasant. And so you get a favorite shade of green: you save money, warm your home, and help preserve this fair planet of ours. Enjoy.