By Paul Shippee

House insulation is a rather mundane subject, well-enough understood by most folks these days. It is one of the most important and effective means of energy conservation so that your first dollars spent to “save home heat” are best spent on additional insulation (up to a point of diminishing returns).

Although the subject of insulation is not glamorous – it works silently, invisibly, and is not expensive – it begins to get interesting when we consider moveable insulation for windows, or night insulation, as a big enhancement for the comfort and performance of passive solar buildings which have large south-facing windows. One outstanding feature of insulation in general, 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. With heating fuel you have to keep on paying more year after year into an unpredictable future.

Municipal building codes have long since adopted energy-conserving standards for insulation levels in new construction so that R30 or R40 in the roof, R19 in the walls, good perimeter insulation for slabs and crawl spaces, and tight construction all around are common in newer buildings. But as houses are made tighter by sealing up all cracks around windows and construction joints to eliminate cold air infiltration in the winter, the interior air quality is jeopardized. House air may become stale and stuffy inviting you to open windows to get fresh air. Then more energy is needed for heating. An energy-conserving remedy is to install a device called an air-to-air heat exchanger that warms the incoming fresh air by extracting heat from the outgoing air stream as it is pushed outside by a small fan.

Recommended insulation levels for passive solar buildings are not different from the standards for well-insulated conventional buildings. However, since a passive solar system utilizes the house itself as a solar collector, there are now large glass areas on the south side. 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 because of large-enough amounts of interior thermal mass. But 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 to the body in our climate as some of the solar heat collected 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 nighttime heat that migrates through double glass windows is about half of the daytime solar gain. That leaves only the other half to supply the heat loss through the roof, the other three walls, the other windows, doors, and the cold air infiltration.

In order to get a better balance between house heat loss and solar gain, window coverings are a good investment. By deploying night insulation on those same south windows, the heat lost on long winter nights can easily be cut in half, thus greatly increasing the comfort factor for the occupants. That is, 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 (aka moveable insulation or night insulation) is governed by many factors such as appearance, type, cost, convenience, and insulating effectiveness. In talking with Leslie Roark, a local installer of window coverings, he told me that customers have generally been interested first in privacy, then in comfort and 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 with fiber batting between two decorative cloth coverings which roll up into a valence and has 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. Some of these varieties, including Styrofoam panels placed against windows, can be home-made to fit one’s needs, taste and budget. All of the above brand names are registered commercial trademarks.

The resistance to heat flow through window areas, or R-value, for these window coverings that are intended to insulate, range from R2.5 to R4.3 for most varieties. Given that the R-value for a plain double glass window is equal to R1.8 at best, you can see that  comfort can be improved and the nighttime heat loss in winter cut nearly in half by installing quality insulating window coverings. What does quality mean? Good quality means 1. what are the layers of material made of and, 2. how effective are the edge, and especially the bottom, seals. The bottom seal is the most important because it stops the cold air falling down the cold glass and into the room. Second in importance is the edge seal along the sides in preventing the cold air near the window glass from pouring into the room.