We have had a recent cold snap and this puts the inefficiency of single-pane windows on display.
Arriving at the house, built 1973 with all its original windows, the temperature was 15F. It had gotten down to about 7F during the night.
This is what the windows looked like from outdoors.
Every single window was icy.
Single-pane windows contain 1/8" thick glass and have an R-value (insulation resistance) of between .5 and 1.
Typical wall insulation, by comparison, is R-13.
In addition, since these are older windows, they do not fit tightly and so there are gaps all around and between the moving sashes. Moving air can be felt at every window.
This is inefficiency on parade!
Heat moves toward cold. In the winter heat is escaping each window opening. In the summer heat is trying to enter each window opening.
A house's energy efficiency breaks down to percentages - approximately 30% walls, 30% windows/doors, 30% attic insulation and upper-level lights, and 10% wall openings (like receptacles and switches). That 10% increases if there is a fireplace. A crawl space changes all that too. And the direction the house faces.
If you add up the area of all the windows and doors in this house the area adds up to about the equivalent of three exterior bedroom walls with R-1 or less! That represents huge energy losses!
The house is vacant so the heat was not turned up very high. As we increased the heat the ice inside the windows began to melt. This created puddles here and there, which rested on wood surfaces, and on wood windows water is not a good thing!
Adding storm windows can increase the window insulation to between R-3 and R-4. Storm windows are a cheaper alternative to replacement windows.
Newer double-pane windows with a 1/4" gap between the panes increases the insulation to about R-2. If there is a 3/4" gap between the panes the R-value is bumped to between R-2 and R-3. A good low-E coating on the inside of the two panes increases the R-value to between R-3 and R-4 and adds heat protection, depending on the quality of the window. Really good replacement windows boast R-values of R-6 to R-9, but they don't come cheap!
If/when you buy replacement windows, look for the U-factor ratings. If the number 1 is divided by the U-factor you arrive at the R-value. So a lower U-factor is the higher the R-value. In Virginia the Energy Star standard for new windows is a U-factor of .32 or less. The low-E coating provides sun/UV/heat protection, called the Solar Heat Gain Coefficient (SHGC).
My recommendation: one of the most common upgrades to older homes is to replace the windows. This not only gives the house more energy efficiency (despite what the commercials say, cost break evens are variable, if ever obtainable) but the new windows work! They don't stick or act like guillotines! Cheaper than new windows, if the old ones still work well, is the installation of storm windows. They add great efficiency more cheaply, and work too!
Jay Markanich Real Estate Inspections, LLC
Based in Bristow, serving all of Northern Virginia.
Office (703) 330-6388 Cell (703) 585-7560