We can estimate savings of a window improvement if we know the U-values before and after. U-value is a measure of heat loss through a window. It’s just the opposite of R-value, the resistance to heat flow. For insulation, we figured the uninsulated wall had an R-value of 4. Well . . . windows can have an R-value as low as ONE. Yep, the resistance to heat movement is 25% of an uninsulated wall. So a single pane window will allow four times as much heat flow as the same area of uninsulated wall. How could we double the R-value of that window to get R-2? It’s as simple as adding a 2nd pane of glass, such as a storm window. The U-value is now 0.5 for an R-2 window. So having windows with smaller and smaller U-values is great because they have greater resistance to heat flow (higher R-value). Triple pane = U-value of 0.33 = R-3.
Panes of glass aren’t the whole story. Glass that has an invisible “low-E” coating will perform as though there is an additional pane. A single pane of glass with Low E coating becomes nearly R-2, and double pane glass with low-E is about R-3. Low-E coating is the industry standard but sadly, some builders do “cut corners” and save a few dollars by special ordering non-standard glass. (Low-E means low emission, which reflects long wave (heat) radiation while letting in short wave (light)radiation.
Using the Window Graph
How can you figure savings of a window upgrade? Select the R-value of present windows on the window graph (shown below). From the point where R-value of present windows falls on the appropriate curve move left and read the cost for your house type*. This is the cost for the area of window shown. For the window option considered, convert its U-value to its equivalent R-value by dividing “1” by the U-value. For example, for a U-0.3 window (Energy Star level), the result will be 3.3, or R-3.3. Move straight up from R-3.3 to the point on the appropriate curve and then move left to find the energy cost if you install R-3.3 windows. Subtract the cost of the window upgrade from the cost for your present windows.
Let’s say you have single pane windows with storms, which are R-2, and you are replacing them with Energy Star windows rated at R-3.3. Let’s also imagine that you live in an apartment (purple line on graph). The difference in energy costs per year would be about $10 per year, or $10 savings per year because of the replacement windows.
The window graph shows that each increase in R-value provides lower return. Pay attention to cost as the highest R-values provide less savings compared to the next lower choice. Also, be aware that you need not replace the window to have low-E glass. Exterior storm windows are available for double-hung, sliders, casements, and fixed windows. Storms with low-E are also available for interior installation.
Window frames have their own R-value, though windows have an overall U-value. The frame and glass are included in U-value.
It is important that the frame not conduct more heat than the glass, as may be the case with old frames. (In fact, new windows should have frames that stay warmer than the glass.) Old metal frames often “sweat” in the winter and drip onto drywall or trim. Rather than suffer water damage to the trim and wall, one may install an exterior storm. If you use a low-E storm over an older double pane window, you will double the R-value from R-2 to about R-4. Utility savings may not cover storm window cost, though avoiding condensation damage plus utility savings might be worth the investment.
| Key | Window Type | R-value =1/U-value | U-value |
| A | Single pane, aluminum frame | 0.9 | 1.08 |
| B | Single Pane wood | 1.0 | 0.98 |
| C | Glass block, 6 inch | 1.1 | 0.88 |
| D | Double pane/Aluminum frame, no thermal break | 1.2 | 0.81 |
| E | Double pane/Aluminum frame, with thermal break | 1.7 | 0.6 |
| F | Double pane/Wood or vinyl frame | 2.0 | 0.51 |
| G | Single pane + interior film | 2.0 | 0.5 |
| H | Single pane + ext. storm | 2.0 | 0.49 |
| I | Double pane/Wood, Argon gas | 2.2 | 0.46 |
| J | Double pane/Fiberglass or insulated vinyl | 2.3 | 0.44 |
| K | Double pane/med solar low-e | 2.5 | 0.4 |
| L | Triple pane/Wood | 2.6 | 0.39 |
| M | Single pane + low-E storm | 2.6 | 0.38 |
| N | Triple pane/one low-e | 2.9 | 0.35 |
| O | Double pane wood/vinyl low solar low-e argon | 2.9 | 0.34 |
| P | Single pane + storm + film | 3.1 | 0.32 |
| Q | Double pane wood + exterior storm | 3.1 | 0.32 |
| R | Triple pane/two low-e | 3.2 | 0.31 |
| S | Double pane wood low-e + storm | 3.4 | 0.29 |
| T | Quad pane/good spacer, two low-e, krypton gas | 4.5 | 0.22 |
| U | Rated “R-5″ | 5.0 | 0.2 |
Insulating Window Treatments
Insulating window treatments (window “quilts” or “blankets”) make sense if you want to address the cold sensation that a large window can bring – even an energy efficient window. Insulating treatments must seal tightly against the window frame or else water condensation (“sweating”) will be a problem. These special insulating “quilts,” “blanket,” and shades are typically rated at R-8 or more. They can reduce heat loss by 87% in a single glazed window and 75% in a double pane window. Even if you only closed the insulating treatment half the time, heat loss is greatly diminished in winter. In fact, since it is colder at night, and you will not benefit from daylight or heat from the sun, closing an insulating cover when it is dark outside in winter could reduce window heat cost by more than 50%. Consider an insulating cover that you will open and close as an alternative to adding layers of glass.
*House types represented in the graphs are: apartment, bungalow, four-square, three flat. The apartment graph assumes 100 sf of window area, the bungalow 180 sq ft of windows area, the four square 300 sq ft of window area, and the 3 flat, 360 sq ft of window area.