Q I want to increase the R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of the single-pane windows in my 44-year-old house in New Hampshire. I am considering using movable window insulation. Is this insulation good for limiting excess heat gainIncrease in the amount of heat in a space, including heat transferred from outside (in the form of solar radiation) and heat generated within by people, lights, mechanical systems, and other sources. See heat loss. in summer? The windows also fogTo fog a room or building is to use a fog machine during a blower door test, revealing locations of air leaks where the fog escapes. The fogging material is usually a glycol-based solution, completely non-toxic. up easily. Will inner storm windows prevent the fogging and provide more R-value? Can you suggest other alternatives?
-Patrick Clary, Dover, N. H.

A Stan Griskivich, a window-insulation specialist in Yarmouth, Maine, responds: Movable window insulation would certainly help reduce winter heat loss, but the payback period could be long, depending on the type of insulation, heating system and BtuBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. cost. If you build thermal shutters, the payback period will be shortened. Curtain shops may also carry a line of insulating curtains.

Movable window insulation requires that you take an active role opening and closing the shade or shutter during the heating season. Most people lose interest in this process after the first season (or sooner) and stop using the thermal shades or the shutters.

You should not use movable window insulation to reduce summer heat gain. Dark-colored insulating shutters or shades kept closed during a summer day, especially on the south or west side of the house, will have the opposite effect. Your windows and house will become solar collectors, which store and radiate heat. Shading the summer sun is best done with roof overhangs, exterior awnings or deciduous trees.

Adding inner storm windows can solve both problems. But first you must caulk and weatherstrip the existing windows. Cold-air infiltration can account for almost as much heat loss as the conductive loss through single glazingWhen referring to windows or doors, the transparent or translucent layer that transmits light. High-performance glazing may include multiple layers of glass or plastic, low-e coatings, and low-conductivity gas fill.. Once you have sealed the windows, you could install inner storm windows. The key to preventing fogging is to get a tight seal around both windows so that there's a dead-air space between them. Fogging occurs when warm, high-moisture interior air circulates in the space between the windows and then condenses on the surface of the cool window.

A well-sealed inner storm window should give an R-value of 2. You also should remove the window casing and sill and make sure the exposed areas are properly insulated because they're often overlooked.

However, unless you want to save the original windows for historical reasons or otherwise, the best course of action, even though it may be drastic and expensive, is to replace your old windows with new windows using high-performance insulating glass. Most high-performance glass is double-pane glazing with special coatings on the glass, and the space between panes is filled with argonInert (chemically stable) gas, which, because of its low thermal conductivity, is often used as gas fill between the panes of energy-efficient windows. gas to achieve an average R-value of 4. This R-value may not sound like much, but it is 400% better than your single-pane windows (typically rated at R-1) and twice as good as standard double-pane insulating glass at R-2. Once installed, a high-performance window saves energy 24 hours a day, year-round, and you won't have to do a single thing.

From the Q&A archives of Fine Homebuilding magazine Issue 97, September 1995.