The Crestone Eagle, October 2006:

Insulation for passive solar buildings: Keeping the heat in
by Paul Shippee

House insulation is a rather mundane subject. It is also generally understood as the most effective means of energy conservation. Therefore, your first green dollars spent to “save home heat” are best spent on additional insulation before you even consider solar heating. Let’s take a look here at a special application of insulation.

Even though the subject of insulation is not glamorous—it works silently, invisibly, and is not expensive—it begins to get challenging and more interesting when we consider buying moveable insulation for windows, aka night insulation. Windows are often treated as a decorator opportunity, but passive solar homes have large south-facing windows designed for gathering winter sunlight. Installing moveable insulation for these windows can be a challenge, as well as an opportunity for enhancing the night-time comfort as well as the thermal performance of passive solar buildings.

In general, the recommended levels for adding insulation to passive solar buildings are not different from the established standards for well-insulated conventional buildings. A basic feature of insulation, wherever it is placed in a house, is that you only have to pay for it once—and it keeps on working for you year after year. Of course, with heating fuel you have to keep on paying more year after year into an unpredictable future. So, how much is an insulation investment worth?

Imagine a house, perhaps yours, where the large glass areas on the south side are soaking up lots of the sun’s heat energy during a typical short and cold winter day, and then the sun goes down. The house has been comfortable; it has not overheated during the day due to large enough amounts of interior thermal mass. When the sun goes down, bringing on cold winter nights that are twice as long as the days, those large glass areas are going to get cold. This can feel uncomfortable as the solar heat collected passively during the day migrates back out through the cold windows.

This excessive heat loss can spoil the comfort factor you enjoyed during the daytime. How much heat loss are we talking about? For example, during an 18-hour winter night at zero degrees, following a 6-hour typical sunny clear winter day, the night-time heat that migrates through double glass windows is about half of the daytime solar gain. That leaves only the other half to supply the night-time heat loss through the roof, the other three walls, the other windows, doors, and the cold air infiltration.

In order to get a better performance balance between house heat loss and solar gain, window coverings are a pretty good investment. By deploying night insulation on those same south windows, the heat lost on long winter nights can be cut in half, thus greatly increasing the comfort factor for the occupants. Drafts circulating from cool air moving down the windows and into the room are reduced; warmer surface areas are provided for your body to feel and radiate to.

The selection of window coverings is governed by many factors such as appearance, type, color, opacity, cost, convenience, and insulating effectiveness. In talking with Leslie Roark, a local installer of window coverings, he told me customers have generally been interested first in privacy, then comfort, then R-value, in that order. However, with heating costs rising at future unpredictable rates, this order may soon be reversed. Leslie’s company, Window Expressions, is one of three locally that advertise in the Crestone Eagle. The other two are Rock Ridgeway (It’s Curtains for Civilization) and Kai Beetch (Kai Beetch Design). The prices for custom-made window insulation coverings appear to range from $8-$16 per square foot installed.

There are many types and styles of window coverings that are effective for night insulation. One of the earliest to be developed was Beadwall™, which is styrofoam beads blown in at night between two panes of glass (3-4 inches apart) to give a very effective R10 or more. Window Quilt™ is another type, with reflective mylar embedded inside fiber batting between two decorative cloth coverings which roll up into a valence. They have edge seals provided by plastic tracks adhered to the window frame or trim. Warm Window™, a Roman shade folding style with magnetic edge seals, is another type that is available. These two might be best for old houses with leaky windows because of the positive edge seals. Another popular, attractive, versatile, and effective style is the Duette™, a honeycomb shade with unique cellular construction that provides good insulation, but no edge seal. Some of these, including styrofoam panels placed against windows, can be home-made to fit one’s needs, taste and budget.

The resistance to heat flow, or R-value, for these window coverings ranges from R2.5-R4.3 for most varieties. Given that the R-value for a plain double glass window is equal to R1.8 at best, comfort can be improved and the nighttime heat loss in winter cut nearly in half by installing quality insulating window coverings. The bottom seal is most important because it stops cold air falling down the cold glass and into the room.

Some improvements for homes that save fossil fuel energy may qualify for Federal renewable energy tax credits. A tax credit, for example, means 30% of the money spent on solar and conservation improvements for your home can be subtracted from your income tax bill. Hidden subsidies for oil and gas have traditionally made these fuels artificially cheap to buy. A solar/conservation tax credit is designed to level the playing field for saving home energy. To learn more about passive solar home design, the new tax credits, and other conservation options visit and view the informative articles posted there.

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