When shopping for energy-efficient windows, one metric you will come across is the U factor. Also sometimes referred to as the U value, it represents the rate of the window’s loss of non-solar heat. It is important to understand what this figure means when you are planning a new build or updating your home with more energy-efficient windows.
Here is a closer look at low U-factor windows:
What Is U Factor?
The U factor is a rating system that was developed by the National Fenestration Rating Council, or NFRC, a nonprofit group that is in charge of administering an independent system for rating and labeling the energy efficiency of various building components. This includes not only windows but also skylights and doors.
When the US Department of Energy awards its ENERGY STAR certification to windows, U factor is one of the metrics that is taken into account. It is also used to determine eligibility for federal incentive and rebate programs.
When it comes to U factor, a lower number is better as this indicates the windows that boast the best resistance to the flood of heat and, by extension, optimal insulating qualities. Windows that are less efficient do not insulate well and will have a high U factor.
The overall combination of a window’s U factor, solar heat gain coefficient, sunlight transmittance and air leakage are all used to calculate the window’s overall energy efficiency level.
The temperature difference that is found between the inside and outside of a building creates a non-solar heat flow that causes windows to gain heat from outdoors in summer and lose heat to outside in winter. This often prompts homeowners to turn up their air conditioning or heater, which leads to higher utility bills.
In order to achieve better energy efficiency, it is therefore important to examine this quality in windows, and U factor is a way of quantifying it so that it is easier to compare different products.
How Is U Factor Determined?
When you see a U factor labeled on a window, it usually refers to the complete window assembly’s energy efficiency. This includes the glazing as well as the spacer that separates the glazing panels and the window frame.
In some cases, the center of glass U factor may also be listed. This refers to the performance rating of the glazing on its own as the window spacer will often reduce U factor at the edges of the glazing. For most types of energy-efficient windows, the U factor of the full window assembly will be higher than that found at the center of the glass.
U factors that are 0.30 or lower are highly energy efficient; you may encounter figures that are even lower in triple-pane windows or those that feature low-emittance coatings.
The ideal U factor for a home will depend on the climate zone in which it is situated. For example, the U factor should be 0.30 or lower for windows and homes in colder climates in the northern parts of the country. These areas are where finding the lowest U factor possible is of the utmost importance for minimizing heat loss.
However, in places where air conditioning is not needed often, some windows that allow solar heat gain during the day will still be energy efficient with a U factor up to 0.32. In the south and central areas of the country where both heating and cooling are often needed, the ideal U factor is 0.35 or less.
The heating costs of an individual home can serve as a guide for whether a lower U factor will be more beneficial. Windows with a low U factor are also useful in hotter weather when keeping heat out is a concern, although it is important to keep in mind that having a low solar heat gain coefficient is even more important in these situations.
In the mixed climates found in the Midwestern part of the country where both heating and cooling are used, look for a U factor that is 0.32 or less. For homes with heating bills that are particularly high, switching to a window with a lower U factor can lead to significant improvements in the home’s overall energy efficiency.
Finally, in the southern part of the country where the weather is typically very hot and air conditioning is used frequently, a U factor of 0.60 or less is acceptable. While it is still true that lower U factors are better, this metric is less important than the solar heat gain coefficient in this type of climate.
Contact the Northern Virginia Window Replacement Specialists
Many homeowners are pleasantly surprised when they see just how much their utility bills go down when they switch to energy-efficient windows.
For more information about choosing the right windows for the Northern Virginia climate or to learn more about residential window replacement and installation, contact the experienced professionals at Adelphia Exteriors.