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Energy Efficient Replacement Window Information
Let's talk about energy efficient replacement windows and discuss the main determinants. We'll go through six important terms and ratings, which include air infiltration, U-value, design pressure, solar heat gain coefficient, condensation resistance, and visible transmittance. Essentially, the numbers indicate how well a replacement window unit is designed and built, as well as how effectively it keeps out air, heat/cold, moisture, etc. These numbers are also how we determine what the best replacement windows are for consumers.
I've listed these six factors of energy efficient replacement windows in order from most important to less important. Of course, if you live in a very cold climate, for example, condensation resistance might be more important to you than design pressure. Conversely, if you live on the coast where impact windows are a must, then design pressure might be your biggest factor. Nonetheless, less explore these six factors and I'll provide specfic examples for each.
My Take: Finding the most energy efficient replacement windows is a key element to maximizing your return on investment for this costly home improvement purchase. Understanding these terms and making sure you purchase energy efficient replacement windows will save you money in the long run and make your home cozy all winter (and summer) long.
(Always Get 3 to 4 Local Price Quotes)
Dane - Site Editor - Publish Date: February 20, 2015
Last Update: January 3, 2026
Page Sections
- Energy Efficient Replacement Windows Introduction
- What Is Window Air Infiltration (AI)
- Explain U-Value And Window U-factor Ratings
- What Is Window Design Pressure Rating (DP)
- Window Solar Heat Gain Coefficient (SHGC)
- Window Visible Transmittance Ratings (VT)
- What Is Condensation Resistance (CR)
- Energy Efficient Replacement Windows Q And A
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Window Air Infiltration Rating
Air infiltration or AI is a measure of how much air leaks through the window unit. For me, this is the most important factor when it comes to finding an energy efficient replacement window. Air infiltration ratings range between 0.01 and 1.0. The lower the AI rating, the less air enters through the unit and the better the window unit. There are some ways to fudge these numbers and some companies do do that, but for the most part, replacement windows with low air infiltration rates are excellent buys.
Window Air Leakage Ratings
Okna 800 Series - 0.01 AI (outstanding)
Sunrise Standard Series - 0.06 AI (impressive)
Soft-Lite Bainbridge Series - 0.09 AI (decent)
Vinylmax Hyde Park - 0.17 AI (not very impressive)
Energy Efficient Replacement Windows :: U-Value
U-Value (also known as U-factor) is a measure of how much heat loss a window gives up. U-factor ranges from 0.10 and 1.20. The lower the U-factor, the better your windows energy efficiency. plays a very important role in your window selection and the U-Factor rating that each window provides tells you what the heat loss will be.
U-Value Windows Ratings
Soft-Lite Elements Series - 0.19 U-value (outstanding)
Sunrise Restorations Sunrise - 0.27 U-value (impressive)
ProVia Integra Series - 0.30 U-value (average)
Silverline V3 Series - 0.37 U-value (not impressive)
Energy Efficient Replacement Windows :: DP
Design pressure is the load, or pounds per square foot, that a window can withstand. Many windows are tested by increasing the pressure on the window until it fails. The pressure has to reach 1.5 times the design pressure for ten seconds without causing the unit to undergo permanent damage. A standard DP is 30, which would mean that that window has to be able to withstand 45 pdf.
The required DP rating for a product will vary based on several factors, including geographic location, building height, and window location on the building itself. Homes on the coast, for example, may require a much higher DP (like a DP60 in some Florida coastal towns) than for homes that are located inland.
Window Design Pressure Ratings
Simonton Stormbreaker Plus Series - 65 DP Rating (outstanding)
ProVia Endure Series - 50 DP Rating (impressive)
Window Nation Envision Series - 40 DP Rating (good)
FiberFrame Series - 30 DP Rating (average)
Energy Efficient Replacement Windows :: SHGC
Solar Heat Gain Coefficient measures the heat that enters through the window glass and frame (mainly through the glass). SHGC window ratings range from 0 and 1.0. The proer SHGC level will depend on where you live - so it's not a set number at all. If you live in a hot climates, you would want window glass that reflects more heat and has a lower SHGC. Conversely, if you live in a cold climate, you would want to let more heat enter through the glass and therefore has a higher SGHC. The main way SHGC is modulated is through low-e coatings on the window glass.
Solar Heat Gain Coefficient Ratings
Apex Insignia2 Series - 0.24 SHGC
Krestmark 100 Series - 0.23 SHGC
MI 1650 Series - 0.19 SHGC
Vista Panorama Series - 0.16 SHGC
Visible Transmittance (VT)
Visible Transmittance is the measure of how much light a window lets in. VT ranges from 0 and 1. The higher the visible transmittance rating, the more natural light will be allowed to flow into the room. Clear glass will have a high VT as it lets in lots of light, but is not very energy efficient. A high-end low-e3 glass will have a low VT. There is a balance here that consumers will have to take into consideration -- how much heat do you want to stay out of the house and how much light do you want to allow in.
Visible Transmittance Ratings
Amsco Studio Series - 0.57 VT Rating
Feldco Sophia Series - 0.51 VT Rating
Paradigm 8300 Series - 0.47 VT Rating
Earthwise Insulator Series 700i - 0.41 VT Rating
Energy Efficient Replacement Windows :: CR
Condensation Resistance is a measure of how resistance window glass is to forming condensation.Condensation forms on any surface where the temperature is lower than the dew point of the air. During cold weather, the coldest surface in most rooms is the window. That's why you would get condensation or water droplets on the inside of the window glass. Condensation resistance numbers range anywhere from 30 to 80. The higher the CR, the better it is at resisting condensation, which can be important for homes in cold weather where condensation happens frequently in winter.
Condensation Resistance Ratings
Hayfield Series - 61 CR Rating (impressive)
Andersen 200 Series - 59 CR Rating (very good)
Champion Series - 55 CR Rating (good)
Great Lakes ecoSmart Series - 42 CR (average)
Energy Efficient Replacement Windows Q And A
Below are consumer questions and answers on all things related to the topic at hand. If you have additional questions regarding energy efficient windows, shoot me an email and I'll get back to you as soon as possible.
Dane - Site Editor
Recommended Window Ratings
Hi Dane, enjoying your site and your videos on YouTube. Could I get your list of recommended windows for Raleigh, NC? If you are taking other questions, I have 2
1) There are some videos on YouTube from Houston Window Experts. Perhaps because of the windows he carries, he is a big proponent of Cardinal Glass. I’ve seen you speak highly of OKNA and it looks like they use Guardian Glass. Is there any difference between the Cardinal vs Guardian that I should consider?
2) Will I feel the difference new energy efficient windows give? The front of my house faces South, SouthEast. There are no trees blocking the view. It was built in 1998 with vinyl windows that seem ok; middle of the road house for pricing based on size. I had to have tinting put on the side soon after we moved in But the heat that I can feel in both summer and winter is a lot. The glass on the back of my hand is quite hot. BTW, I looking at triple low-E to get the heat gain as much under control as possible.
Greg - Homeowner - from 2025
[Site Editor's Answer]
Greg, both of these types of glass are very good. They are essentially the big two glass companies in the U.S. If there is a difference, I don't know what it is - either one is great.
You should definitely feel the difference if your glass is clear glass, which it sounds like it is, based on how hot it gets and when it was made. Talk to each of the window companies that comes out about keeping that SHGC low on the windows that face south or gets lots of sun. You may want to do a "low-e2" - most of the companies will have a name for the glass package, but basically it will probably be a step up from the single low-e. Triple pane might not be necessary, but explore the option and price point with the sales people.
Get five bids and by the end of the process I think you will hopefully have a good idea what the best glass option is for your windows and how they face, how much shade they get etc.
Dane - Site Editor - from 2025
PG Rating vs. DP Rating
Hi Dane, how much stock would you put into the PG rating (allegedly incorporates more than just the DP rating)? Is it a good measurement of the overall build quality?
Understand the energy efficiency is separate using the U-factor & SHGC.
Thank you much.
Chris - Homeowner - from 2022
[Website Editor Answers]
Chris, I put quite a bit of stock in PG rating, along with air infiltration AI, and U-factor. Those three can tell you a lot about a window -- but I always marry then with overall brand quality year after year...
Dane - Website Editor - from 2022
Important Energy Ratings Stats
Tim, I'm told by the Okna rep, U, Air Infiltration and DP are most important when picking windows - is this true? If I keep digging, I might be able to learn enough to get a job in the window realm..
U Values - energy rating
SHGC Values - solar heat gain coefficient (not sure what that means)
Air Infiltration Values - along with design Pressure value - how it handles air flow via frame/sash
DP Values
CR Values - condensation rating
Katie - Homeowner - from 2020
[Website Editor Answers]
Katie, you are well on your way to becoming an energy efficient replacement windows expert! To fill in the blanks a bit, DP stands for design pressure, which measures how much wind pressure a window can stand. However, unless you are buying impact windows, this isn't a measure or rating that you need to worry too much about. (If you are looking for impact windows, then it's the most important of ratings to look at.)
Solar heat gain coefficient is how much solar heat enters through the window. If you live in a very cold climate, then you want to allow in more heat to help warm your home. Conversely, if you live in a very hot climate, then you want a much lower SHGC to keep your home cooler. So location and climate is the big factor here.
I'm of the opinion that a quality window and door company should have their performance numbers down pat and should be able to tell you what those are and why they are ideal for where you live. While the consumer ought to know the terms, the local company who installs the windows should be the people who guide you toward a window that makes sense for your home and that offers you excellent energy efficiency for the money that you are shelling out.
Tim - Site Editor - from 2020
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