If you are looking for a window upgrade, no doubt a part of your decision has to do with improving your home’s energy efficiency. And no doubt, your window search has also found you coming across information about a window’s U-value. So, what does U-value mean and what role does it play in assessing energy efficiency in windows? Let’s take a look at what U-values are, how it contributes to a window’s energy efficiency, and its importance in both residential and commercial building insulation.
U-value Meaning: What is it?
In short, U-value meaning comes down to a measure of thermal transmittance or, put simply, the amount of heat transfer through windows. A lower window U-value means that a window has less transmittance and provides better insulation. With that in mind, what higher U-value means is that the window transfers more heat through the window.
Understanding How U-Values Affect Insulation Performance
Essentially, installing a window with a high U-value means that more heat will escape from your home’s interior during the colder months and more heat will penetrate into your home’s interior during warmer months. All and all, the less heat that is transmitted through your window, indicated by a low U-value, the easier it is to maintain a comfortable interior temperature, which in turn lowers the energy costs to heat and cool your home.
This can be especially important in climates that experience extreme temperatures, where it already takes more energy to maintain a steady and comfortable indoor temperature. If you live in a climate that experiences extremely cold temperatures, for example, you are already spending more on heating costs than you would in a more moderate climate. Installing windows with a low U-value can make this effort easier by improving your home’s insulation and its energy efficiency.
As an example of how this works, let’s compare the energy loss from a standard size window with a U-value of 1.6 to that of .3 with an outside temperature of 60F. Under these conditions, a window with a U-value of 1.6 will lose about 48 Watts of energy while the window with a .3 U-value will lose only 9 Watts. It is easy to see with this comparison how much more energy it would take to heat or cool your home if 48 Watts are being lost instead of 9.
Choosing Windows with the Best U-Values
There are window energy ratings and certifications that can help you in your search for windows with low U-values. These certifications include Energy Star ratings from the U.S. Environmental Protection Agency (EPA) and the Department of Energy (DOE) who set efficiency guidelines for construction products and homes.
To be given an Energy Star rating by a window must meet certain requirements and score 75 or above on a 100-point scale. Energy Star window criteria include testing in an EPA-recognized lab and meet recognized international conformity standards for insulation and performance.
There is also the National Fenestration Rating Council (NFRC), a non-profit organization that provides consumers with ratings on windows, doors, and skylights. NFRC window rating labels certify that they have been tested and meet energy performance standards. These rating labels can help you compare windows and narrow your search.
Best windows for cold climates, like Minnesota, include windows with a U-factor of .30 or less.
Understanding Other Energy Efficiency Measures
U-value is not the only measure available to weigh the energy efficiency of a window. There is also the solar heat gain coefficient (SHGC), which measures how much solar heat enters through a window. It is measured between 0 and 1, with 0 being the least amount of heat and 1 being the most. This measurement is most helpful in warmer climates. Another measurement is a window’s R-value, which is used to measure insulation quality. When comparing R-values vs. U-values, it is important to remember that with R-values a higher number is better, which is the opposite of U-values.
Comparing U-values and other measures for various window types, such as single-pane vs. triple-pane windows, can be useful when looking for efficient window replacements. The table below shows typical U-values, SHGC and R-values for various types of windows.
Window Type U-value Range SHGC R-Value
Single-pane windows 1.0 – 1.20 .70 to .80 1.0 or lower
Double-pane windows .80 to .30 .58 to .76 1.7 to 3.0
Triple-pane windows .20 to .15 .37 to .69 4.0 to 6.0
These ranges can be affected by other factors that can raise the quality of a window’s energy efficiency.
Considering Additional Factors for Window Energy Efficiency
U-value is not the only factor to look at when deciding what windows to install to improve your home’s energy efficiency.
Glazing Options
When it comes to a window’s energy efficiency, glazing or the number of glass panels a window has matters. Single, double, and triple glazing means much what their name suggests. Single glazed windows have one panel of glass while double and triple glazed windows have two and three respectively. It is also not difficult to figure out what double-glazing benefits would be for energy efficiency over single-glazing, and triple-glazing benefits over double-glazing.
Gas Fills and Spacers
Another factor in double and triple pane windows that can contribute to improving a window’s U-value is gas fills. This process fills the space between the panes in the window with argon gas or krypton gas. This design improves the window insulation performance and contributes to better U-values.
Low-E Glass
Another factor to consider is the role low-emissivity (Low-E) coatings can have on a window’s U-value. These microscopic silver or metal coatings are applied directly to the glass in manufacturing and are invisible to the eye. They do serve to keep heat in the home during winter and keep heat out of the home during summer. These coatings can significantly improve a windows energy efficient properties.
Frame Materials
Window glass is not the only material that needs to be considered when installing energy efficient windows. Your window frame plays a part too, particularly when looking at thermal bridging in windows. This occurs when the window frame material allows too much heat transfer out of your home compared to the surrounding insulation. Different frame materials transfer heat differently and can impact your windows overall U-value. Wood and vinyl have better thermal bridging than aluminum or other metals. Fiberglass is also a good option for framing as it has a low thermal transfer rate.
Passive House Principles
Passive house design principals, employed to minimize the energy used to heat and cool a structure, have emerged as the pursuit of energy efficient construction has grown.
These principles include:
- Thick insulation
- Airtightness
- Energy-efficient, insulated windows and doors
- Balanced ventilation
- Shading strategies
Passive house windows are a key element to passive house design and are characterized by:
- High thermal insulation
- Low-e glass
- Gas-filled panes
- Thermally broken frames
Balancing Cost with Energy Savings
Not unlike most home projects, your window upgrade does come with budget considerations. With this in mind, higher-performing windows with lower U-values are more expensive to install initially than higher U-value windows. Weighing the overall cost of energy-efficiency windows however is a balance between upfront costs and the significant long-term savings in energy costs over time.
This Case Study using a window replacement project on an apartment complex shows how the cost of installing triple-pane windows with a low U-value rating proved to have strong ROI when the upgrade yielded lower energy consumption and improved indoor comfort.
There are also window energy rebate programs that may be available to help offset some of the upfront costs of installing energy efficient windows.
Using U-Value Calculators
The U-value meaning and energy efficiency quality for any given window can be determined with a window heat loss calculation. It uses a formula that takes the window’s U-value, surface area and temperature difference between the exterior and interior to figure the heat loss in watts. There are online tools to help with this calculation, but it can also be helpful to consult with a professional to determine the best windows for your energy efficiency upgrade.
Consulting Window Professionals
Understanding all you can about U-value meaning when selecting windows and how it can apply to upping the energy efficiency of your home is one thing, but applying that knowledge to your window selection and installation can be more complicated, especially when balancing upfront costs with long-term energy savings. One of the benefits of working with professional window experts, like Summit Construction, is the help you get to determine the best options for your specific needs. No two homes are exactly alike and contacting Summit Construction Group can help you find the exact right fit for your home.
