One of the goals of this web site is to help folks reduce their dependence on conventional fossil fuels for their energy needs. Possibly the most effective way to accomplish this is simply through conservation.
Along with an innovative yet practical solar design, [see Alternative Energy] the passive solar home we've lived in for the last twenty years employs many energy conserving principles. Added insulation, tight vapor barrier, air lock entry and attention to reducing air infiltration at typical points of entry are complemented with a moveable insulation system that greatly reduces heat loss through windows.
Windows, on one hand, are possibly the best forms of solar collector. They let the sun's energy enter directly into our living spaces while also providing light and visibility. And those windows that open provide a source of ventilation for the home. This function, on the other hand, often adds to the negative side of a window's energy contribution.
Windows often create a source of infiltration whereby cold outside air leaks in around the window sash. In addition, glass is a very poor insulator and provides a direct path for heat to be conducted to the outside. See conduction. The cold glass surface of a window also creates a third physical principle called convection. This happens when the air that's next to a window cools off, becomes heavier, and settles to the floor pulling other air toward the window continuing the process. We more commonly recognize these three phenomena as drafts.
To minimize the negative effect that windows have on our houses, many types of moveable window insulation have been developed. Their goal is to slow down (no insulation completely stops) heat loss when they are in place over the window. There are four principles that contribute to the effectiveness of a moveable window insulation or MWI:
- ADDED INSULATION. This is the most obvious of the criteria. It is also obvious that more insulation is better, within reason, until a point of diminishing returns is reached.
- TIGHT SEAL. In order to reap the benefit from the added insulation of this window covering it has to be tightly sealed on all sides. Just as a ski parka keeps you warmest when it's zipped up, so too must the MWI keep the cold on the window side and the heat on the house side by tightly sealing all the way around the insulating layer.
- VAPOR BARRIER. This is required to keep warm moist interior air from travelling through the MWI and condensing against the window. An effective vapor barrier in this application can greatly reduce the condensation buildup, which plagues many homeowners and sometimes causes rotting of windowsills and framing. This vapor barrier must also be tightly sealed to minimize water vapor transfer.
- EASE OF OPERATION. Many MWI systems have been developed over the years, shutters, roman shades, blinds and so forth that claimed outrageous insulating qualities (R values) but were so cumbersome that they seldom got used. The key to an effective MWI is a balance of all features that encourages the owner to use it regularly, as soon as the temperature begins to drop in the Fall.