By Bob Bacon
The important function that windows play in building performance: the role that windows play in affecting the comfort and quality of our lives.
Since northern New England’s building stock is the oldest in the country, a great number of our region’s windows are old, single glazed and either very loose fitting or, or they’re inoperable because they’ve been painted shut. But, precisely because these original windows are very old, they are usually very well built, very attractive, and very valuable assets, making them excellent candidates for improvement, restoration, and preservation rather than replacement.
Where do we start, and what exactly are we trying to improve? To be clear, what we’re interested in improving is the performance of the “hole in the wall” into which our ‘window’ was installed. We put holes in the walls of our buildings to provide us with light, ventilation, views, and access; then we install ‘windows’ and other devices into these holes to regulate and ‘control’ the invited transfer of light, air, vision and movement through the opening while achieving weather-tightness, security, safety, energy efficiency, and privacy…all in an a manner that is ideally attractive, flexible, durable and long-lived. This is a tall order in a cold climate such as ours but we can accomplish all of these things if we think of the solution as an ‘assembly’ of components that work together rather than as a single window ‘product.’
For most of us, the rising costs of heating fuels have made reducing heat loss our top home-improvement priority; and if we’ve already insulated our walls and ceilings as best we can we’re now faced with reducing heat loss through our door and window openings. Although a thorough energy audit can estimate the costs, fuel savings and relative payback of various home improvement options.
As a general rule, eliminating air movement through the opening with simple repair and weather stripping is almost always the most prudent and cost effective first step. Once these relatively easy and inexpensive convective heat losses have been eliminated, the next step is to improve the “R” value (or insulative value) across the opening to reduce conductive heat losses. These options generally include adding “supplemental” panels or layers of material to the opening such as ‘storm’ windows, a second layer of glass to the sash, insulated curtains and shades, and/or transparent interior insulation panels. Deciding among these options requires that we evaluate the characteristics of each option against the performance goals mentioned earlier. For quick reference, the Table “A” below compares the functional attributes of several of the most common generic options and Table “B” illustrates many of the “quality-of-life” attributes.
But, what about improving the opening’s performance by replacing the entire window with a new, double or triple pane unit with Low ‘E” glass that is fully weatherstripped and clad for low maintenance? In rare cases where the original wood sash has rotted beyond repair or restoration, full replacement may be the best option, but this improvement strategy is often not the most cost effective or performance enhancing option. Vinyl replacement windows, for example, have been very heavily marketed for their low maintenance and relatively low initial cost, but they have consistently proven to be very poor investments because thermoplastics don’t stand up well to the weather resulting in a very short useful life. Quality wood windows, both clad and unclad, have proven to be much more durable and long lived than plastic, however, wood sashes are vulnerable to rot from persistent exposure to moisture. The outside of windows on the building’s north side may not receive enough sun to fully dry the wood after rain and snow, while the inside of windows may remain wet throughout the winter because of repeated condensation on the inside of the glass. Composite materials such as fiberglass are impervious to rot and stand up well to the elements but since the walls and trim around the windows are still made of wood, fiberglass windows don’t solve the underlying problem of moisture build up from condensation.
Replacement windows, even those with dual glazing and Low E glass, are not the solution to eliminating condensation. Moisture vapor condenses into a liquid when warm moist air comes into contact with a cold surface and, since glass is a very poor insulator because of its density; it remains cold compared to a comfortable room temperature – that’s why even expensive replacement windows will ‘sweat’ in cold weather. The solution is to prevent warm, moist air from coming into contact with cold surfaces.
The densities of glass and of cold air are also the causes of uncomfortable cold air movement next to the inside of otherwise new and well-sealed windows. In cold weather glass continues to extract heat from the adjacent interior air which, as it becomes heavier, falls resulting in an uncomfortable “draft” near the new window. Since improving our comfort and reducing the maintenance caused by the stains and rot of condensation, replacement windows may not be the best solution.
In our next article we will discuss the common types of windows and discuss how to operate them to achieve maximum comfort with minimal energy use.
Bob Bacon is the Planning and Design Consultant and President of Window Improvement Masters, Orford, New Hampshire: windowimprovementmasters.com