Q:

I want to increase the R-value 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 gain in summer? The windows also fog up easily. Will inner storm windows prevent the fogging and provide more R-value? Can you suggest other alternatives?

Patrick Clary , Dover, NH

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 Btu 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 after the first season (or sooner) and stop using the thermal shades or shutters.

You should not use movable window insulation to reduce summer heat gain. With a dark-color insulating shutter or shade closed during the day, especially on the south or west side of the house, you have created a solar collector, which stores and radiates 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 glazing. 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 airspace between them. Fogging occurs when warm, high-moisture interior air circulates in the space between the windows and condenses on the cool window surface.

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 (HP) insulating glass. Most HP glass is double-pane glazing with special coatings on the glass,  and the space between panes is filled with argon gas to achieve an average R-value of 4. This 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, an HP window saves energy 24 hours a day, year-round, without your having to do anything.

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