Window U Factor Guide
Our guide to window U factor can help you understand more about this energy efficient term and what it means for your home windows. It's important to note that U-value (often used synonymously with U-factor) is only one of many metrics that industry experts use to judge the energy efficiency of a window. Other important factors include R-value, design pressure, air infiltration, solar high gain coefficient, and visible transmittance.
While we won't discuss these other terms on this page, it is helpful to have a cursory understanding of each one. Having a very basic understanding of these terms can go a long way to making better decisions in terms of buying replacement windows. However, you really don't need to know more than the basics.
Written By John M. - Site Editor
U Value Windows And Ratings
U Factor (often referred to as U value) is a measure of the amount of heat loss from a window that ranges anywhere from 0.15 to 1.20. The lower the number, the less heat escapes through the window and therefore the more energy efficient the unit. U factor, often referred to as U-value, is one of the main performance numbers for consumers to look at when buying replacement windows for your home. Other important measures include SHGC, air infiltration and design pressure rating.
Ideal Window U Factor
Ideally, a window will have a U factor of .28 or below. So, a mid range vinyl window will have a U-factor of .30. Of course, there are huge variations in U-factor based on the window frame material, glass package and any and all upgrades and adds on that a homeowner opts for.
Generally, fiberglass and vinyl windows will have the best overall performance numbers. The best way to drive down U-factor is by upgrading to a better low-e glass or glass package. Therefore, upgrading to a triple pane glass is going to be the best way to lower that U-value.
Sample Window U-Factor
The following are sample U-factors from popular window manufacturers. Check out our Energy Star program article to see the role that U-factor plays in getting this energy efficient rating.
Alside Excalibur » .35
Simonton Reflections 5500 » .30
Champion Vinyl Windows » .29
Sunrise Restorations » .26
ProVia Endure Window Ratings
John, I got a quote on 5 ProVia Endure window DLA-UV Single Strength Glass w/colonial flat grids at $4,984.18 (maybe this combination with double strength glass?). So it looks like it would be roughly $5,500 and here are the performance numbers: U-Factor .27, SHGC .18, VT .43, CR 61.
Next, the Endure with DLA Double Strength Glass w/colonial flat grids at $5,293 and here are the performance numbers: U-Factor .27, SHGC .27, VT .48, and CR .60. Finally, the Endure with Triple Pane window glass TLA Double Strength Glass w/colonial flat grids at $5,771.08 - U-Factor .21, SHGC .23, VT .38, and CR 69.
Toby - Homeowner - July, 2025
Website Editor's Answer
Toby, thanks for the question. Generally, while ProVia windows prices are not cheap, they do make a very good window and door. In 2025, this price point is about what I would expect. Therefore, if the company doing the installation has a good reputation, then I think you are good to go. Personally, I don't think you need to go triple pane unless you live in a very colld climate.
Editor John - July, 2025
Recommended List of Windows
John, if you could send me your recommended list of windows for replacement windows, I would appreciate it. So, I received comparable quotes from local installers to replace 12 windows in our home in Northern Virginia. Thank you for your time and assistance. Very Respectfully.
Company 1 proposed the Ideal Platinum 3000 series double pane with window U Factor .23, SHGC .21, VT .44 and Air Leakage < 0.3. Alternately, company 2 proposed the KHPP Genesis window with South Glass Package. The estimate did not provide further information on the windows. I have requested the additional detailed information from the company.
David - Homeowner - June, 2025
Website Editor's Answer
David, both great windows. I would go with the Ideal Platinum 3000 windows myself. I feel like its the safer bet. The one year labor warranty is just fine in 99% of all cases because the windows either have problems when shipped or when installed. Once they are in are working well, you rarely have issues until quite some time down the road.
However, a good labor warranty might indicate that the company believes in their product and wants to put their best foot forward for their customers. So maybe the KHPP windows is the choice. I would make the choice based on the company doing the work over the window as these are both great options!
Editor John - June, 2025
Related Topic: Window Capping Cost
* Window Design Pressure