R-Value and U-Value: What Value Are They to You?

If you’ve never purchased or repaired windows, you might not know what you need when that time comes. It seems like there are countless window types to sort through. Comparing window ratings is a good starting point, but even then it’s not crystal clear. R-value vs U-value? Is one more important than the other? And what is a good score? Here we break down R-value vs U-value and help you figure out where to go from there. After all, a straightforward problem like warming your home should have a straightforward solution.

Building envelope air flow diagram: windows will have worse R-value & U-value than walls & roofs.

What Is R-Value?

R-value is a score given to a barrier – such as a wall, roof, or window – based on its ability to resist the flow of heat through it. Think back to elementary school when we learned about heat conductors and insulators. Can you recall copper as one of the best examples of a heat conductor? Heat passes through it easily. So copper would make a very bad wall. For your home’s “shell” (building envelope), you want something that resists heat transfer. The R-value (the ‘R’ stands for “resistance”) will tell you how well different barriers will resist that transfer.

Frozen copper pipe. Better resistance to heat transfer means a better R-value for a surface.

In the US, you will see R-values ranging from 0 to 66. Higher numbers mean better resistance to heat transfer. While R-values can be assigned to any surface of a building envelope – roof, ceiling, floor, wall, window – it is important to think about the role a surface plays in your home.

Walls are well-insulated and meant to protect against the outside elements. Windows should insulate, but also be able to open and let fresh air in. Windows should also be transparent for visibility and sunlight. For this reason, comparing the R-value of an insulated wall to the R-value of a window is not a good teller of insulation qualities. Compare the R-value of walls to that of other walls; compare the R-value of windows to the R-value of other types of windows.

What Is the Best R-Value for Windows?

A single-pane window will have an R-value of 1. A standard double-pane window will have an R-value of 2. While a single-pane window with an R-value of 1 does not offer great insulation, you get a much better picture of its performance by comparing it to a double-pane window than to an insulated wall.

Some specialty windows that are highly insulated can reach an R-value of 5. This is great insulation, but the downside is the cost. Replacing multiple windows with these specialty windows will cost you thousands of dollars.

It is no secret that windows are the weakest spots in a building envelope when it comes to insulation. No windows will be able to insulate as well as the walls and roof of your home do. The aim is to increase their R-value and U-value as much as possible.

Performance Data Single-pane Window Double-pane Window Indow insert + Single-pane
Thermal Insulation (R-Value) R-1.0 R-2.0 R-1.87 94% of Standard Double-pane

What is U-Value?

U-value, or U-factor, is a score given to a surface based on how quickly it allows heat to pass through it. U-value can be thought of as the opposite of R-value. If R-value tells us of a surface’s thermal resistance, then U-value is a surface’s thermal transmittance. ‘R’ stands for resistance. For U-value, think of ‘U’ in circUlation, a word that has a similar meaning to “transmittance.”

But make no mistake: when it comes to winter insulation, a good U-value is given to a surface that transmits heat as slowly as possible. A good U-value does not indicate a surface quickly allowing heat to pass through it.

U-value scores typically range from 0.1 to 1. The lower the U-value, the better. A single-pane window has a U-value of about 1. This means it allows heat to pass through it quickly. A standard double-pane window has a U-value of about 0.5.

R-Value vs U-Value

While we say that R-value and U-value are opposites of each other, there are some differences that we should discuss. Traditionally, U-value was used for windows and R-value was used for the other surfaces of a building.

Sun through window. A difference in R-value vs U-value: U-value counts radiative & conductive heat.

R-value tells us how well a surface resists heat transfer. U-value tells us how quickly or slowly a surface allows heat transfer. Radiative heat from the sun has a much greater effect on windows than it does insulated roofs and walls, so that is taken into account with U-value. As a result, you can expect a window’s U-value to change based on time of year and time of day.

Calculating for R-value and U-value is easy. Dividing 1 by the R-value will give you the U-value:

R-value = 1/U-value

Dividing 1 by the U-value will give you the R-value:

U-value = 1/R-value

How Can You Improve Your Windows’ R-value and U-value?

If you have an historic home with original windows, replacement is not an option. It can hurt the historic value and many historic districts do not allow window replacements. Maybe you don’t have the ability to take on an expensive window replacement project. You can improve your windows’ R-value and U-value without replacement.

Window & Indow insert U-Value diagram: heat & cold air travel more slowly through acrylic than glass

Indow window inserts can be placed inside your window frames to improve the insulation of your window openings. When paired with single-pane windows, Indow inserts can improve the R-value to 1.87, or 94% of a double-pane window! With a double-pane window, the R-value will be improved even more. When paired with a single-pane window, Indow inserts will improve the U-value from 1 to 0.53. That is a huge improvement in insulation without the cost, time, or effort of a replacement project.

To see more performance data of Indow inserts and to learn about the various insert grades Indow offers, download our Product Data Sheet.

The Product Data Sheet also includes useful information on fit and egress compatibility with your windows.

Customer Reviews


4.49 Average 1479 Reviews logo


Don't be afraid of buying an older home with single pane windows. This company has you covered. Their inserts are super high quality and fit well thanks to the laser measuring kit they send. Highly recommended!

Posted 5 hours ago

Katie M

We purchased three Indows to reduce drafts and noise in our bedroom, which faces the street. The improvement was immediate. Our house was built in 1958 and although we have exterior storm windows we didn't want to replace the windows outright due to cost and architectural consistency. These were an excellent solution.

Posted 1 day ago

Matthew L

Indow inserts were easy to measure, order and install. They've provided great insulation for old single-pane and stained glass windows in an antique home. We can significantly feel how much better the home holds heat.

Posted 4 days ago

Ellis W

I ordered two Indow inserts for my home. They are doing a great job, entirely happy with them and the ordering process.

Posted 2 weeks ago

Elizabeth W

I can't say enough good things about this product and company. Our house is 101 years old and the sidelites next to our front door are single pane ancient wavy glass which means when it gets very cold (Chicago temps) we can have an inch of frost covering the INSIDE of our windows. I was desperately searching for a solution so our front vestibule was not so cold and a friend recommended Indow. The process could not have been simpler and the temperature difference it has made is off the charts.

Posted 2 weeks ago

Austin T

I have several historic single pane gridwork mullion windows. They're beautiful but have almost no thermal protection. Indows proved to be the perfect solution to preserve these windows while getting the thermal comfort we need. They were easy to install and work perfectly!

Posted 4 weeks ago

Kathryn M

Amazingly easy to install even the safety chain holders. I. Noticed an immediate sound level difference and the room atmosphere became more solid feeling and pleasant. They also gave an insulating feature to the window drafts are non existent! The house is 50ft off a State highway that is heavily traveled in warmer months I'm confident the indows will handle it. I'm very pleased!

Posted 1 month ago


We ordered the acoustic inserts to help with road noise, and we've been sleeping so much better with them!

Posted 1 month ago

Sienna K

We used the windows inserts in our skylight. They are great! They the horrible drafts and noise out and were cost efficient. My only comment would be to provide better measurement and installation instructions for skylights specifically. It took us a lot of guessing and research to measure and install because skylight directions were not provided and the windows install guide doesn't always apply to skylights. Other than that though great product and I would buy again!

Posted 1 month ago

ted t

Our home was built in the late 1800s and we have spent a lot of money over the last 20+years restoring it.We did not want to replace the windows with modern windows because of the cost and originality. But the single pane windows come with a price also. the main reason we purchased the Indow window inserts is because of the condensation on the inside of the windows.We would literally have a take a towel and wipe them down every morning in the winter. There would be a puddle on the sill,and wood and water dont get along to well. SO we looked into the best solution to our problem and found Indow. Was reluctant to spend that much money but am super glad we did!! They have been installed now for over two months and our condensation problem has been virtually eliminated.!!! AND our furnace does not run as much as it did before. Over all it was money well spent!! The process was a little scary at first but they have a simple formula. And our contact person,Reed was awesome. Answered any questions i had and was always available.

Posted 1 month ago


Used to block interior side of a garden window in the the kitchen. Without insert, cold air literally poured into the kitchen making a winter jacket almost mandatory when preparing meals or washing dishes

Posted 1 month ago

Rob S

Highly recommend! The team (Brian) took time to help me figure out the best solution. They were made and delivered faster than expected. It was very rewarding to install them because I could feel the cold air get squeezed out as they went flush against the single pane of glass. Solid quality and great fit.

Posted 1 month ago

Alexa L

My inserts fit beautifully into my 125 year old windows - many of which were 5 feet tall. Easy to unwrap and install. Great product and even better service! I liked the process of measuring the windows with a laser measuring tool which was easy to use and precise. The product came in good condition after shipping across country and was well marked. The customer support was excellent.

Posted 1 month ago

Sarah D

Received our Indow Windows and can tell they are blocking a lot of the noise we could hear. Right now the weather is mild, but hoping to notice a big difference when the summer heats up. Our house is in a historic district and we cannot change our windows. They are very drafty and thin glass. We only ordered two, but we are in the process of ordering for four more. Love that we don’t need to drill holes or anything and they just pop right in.

Posted 2 months ago

Allan F

Every thing arrived in perfect condition, thanks to the very secure packaging. Windows fit as expected, although quite tight. After installation we noticed that the cold drafts were gone and the room felt a lot warmer. We are very satisfied with the product and the result.

Posted 2 months ago