Window U Factor Guide
Our guide to window U factor can help you understand this energy efficient term and what it means for your home windows. However, 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 (so feel free to use the links to explore). Having a basic understanding of these terms can go a long way to making better decisions in terms of buying replacement windows. And hopefully deliver better return on investment for your hard earned money!
Article By John M. - Site Editor - Publish Date: October 25, 2015
Last Update: December 18, 2025
We would really appreciate if you would consider filling out our free form. This allows us to provide you with free consumer info on replacement windows and connects you with 3 quality local price quotes.
We are not incentivized by ANY companies. Send us your bid summaries and we will help you get the BEST DEAL for your particular situation. That’s our personal guarantee to you!
U Factor 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.
U Value Windows And Ratings
However, the standard range for windows u factor is much narrower than 0.15 to 1.20. Honestly, I've never seen a U factor less than 0.17 or higher than .40. So this would be the much better range when considering replacement windows. Some really high performing vinyl and fiberglass windows will fall into the sub 0.20 range. Anything under 0.25 though is pretty darn good.
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. And lots of ways to tweak this up or down based on upgrades, adding triple pane, etc.
Ideal Window U Factor
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.
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.
Sample Window U-Factor
Alside Excalibur » .35
Andersen 200 Series » .32
Atrium 8500 Series » .34
Simonton Reflections 5500 Series » .30
Amsco Artisan Series » .30
Champion Vinyl Windows » .29
Ply Pem Premium Series » .29
Amerimax Aristocrat Window » .27
Sunrise Restorations Series » .26
Okna 800 DX Series » .24
Below are our consumer questions and answers on window u-value and u factor. Dig in to all aspects of this concept and see how other homeowners have grappled with how this term and metric has affected their road towards finding the best replacement windows and patio doors. If you have a question on this concept (or any other) please get in touch using the email toward the top.)
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, 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.
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.
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.
My wife and I live in the Southern California region close to Thousand Oaks. We want to replace our 18 aluminum single pane windows. We're wiffle-waffling between Anlin Del Mar series and the Amerimax ProMax8 series. From what we can tell, both of these are the company's top level windows. Also, they both carry double lifetime warranties -- both have argon gas between glass, foam spacers in frame, include sound reduction packages, and have excellent window U factors and SHGC numbers.
David, yes both of these windows are very good. For me, I would pick the Anlin Del Mar over the Amerimax ProMax8 series for a couple of reasons. One, Anlin is California based and if something goes wrong I think it's an easier fix there. Two, I like Anlin a bit more than Amerimax -- not a ton, but this is my brand favorite over Amerimax.
Window U Factor Q And A
Editor John - Site Editor
ProVia Endure U Factor
Toby - Homeowner - July, 2025
Website Editor's Answer
Editor John - Site Editor - July, 2025
Recommended List of Windows
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
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!
John - Site Editor - June, 2025
Anlin Del Mar Performance
Finally, the installation companies that gave us the bids both have very good reputations and very good online reviews. carry these two retrofit windows and each have excellent reputations for professional quality installation. So that's a wash right there. So my question is this - which brand in better in your opinon? Which is the better window? What decision would you make if this was your home?
David - Homeowner - April, 2024
Website Editor's Answer
I definitely believe that their performance numbers, including U-value, are going to be very similar. I would think that the air infiltration of the Del Mar might be slightly lower than the ProMax 8. Bottom line for me is that you should go with the Del Mar if the installation companies are equal to one another.
John - Site Editor - April, 2024
Related Topic: Window Capping Cost
* Window Design Pressure