Have you decided to replace the old windows in your home due to a lack of energy efficiency that's costing you in utility bills? Energy efficiency has become one of the most common reasons for purchasing replacement windows. Finding the most energy-efficient model starts with frame selection and deciding on whether the glass should have multiple panes, a gas filling, and a low-e coating.
But if you narrow your replacement options down to a category or two, you will still need a way to handily compare similar models. There are a couple of different energy-efficiency metrics that can help you decide on the best home-window replacement model. These metrics tend to appear on the same label on the boxes or units so that comparisons can be made easily.
The U-Factor metric refers to how well your chosen window can protect the interior heat from escaping outside or the exterior heat from escaping inside. A low U-Factor means that the window is better at preventing that transfer and is therefore more energy efficient.
What ranges should you look for in the U-Factor numbers? Different frame styles and glass styles will determine the average factor but, as mentioned, you want to find the lowest number possible among your like models of windows.
Generally, wood and vinyl window frames have similar U-Factors while aluminum frames have a higher or less efficient number. Those numbers improve with extra panes of glass, gas within those panes of glass, and a low-e coating. The most efficient wood and vinyl windows have a U-Factor of around 0.22 while a base-model aluminum frame tops out around 0.53.
Solar Heat Gain Coefficient Metric
The Solar Heat Gain Coefficient metric, also called the SHGC metric, refers to how effectively the window can block the heat generated by sunlight. The metric shows a ratio between the amount of solar radiation that hits the window and how much the window reflects off or blocks from coming in.
Note that you don't necessarily want to pick the SHGC window with the lowest number and call it a day. A window that is extremely good at blocking out summer sun to keep your indoors cool could block out too much sunlight for the winter months when you want some added heat. Consider your climate and heating and cooling needs when looking at the SHGC metrics. If all else fails, go with a number toward the middle of the range.