It’s intimidating and annoying when you need to replace your windows and all of your research keeps bringing up technical jargon like R-values, U-values, and SHGCs, whatever those are. What do these things tell you, as the homeowner? Why should you care about them? I’ll tell you exactly why you should be interested in them.
The R-value of a window measures the thermal resistance of its material (the glass). It measures how effectively barriers, such as windows, resist heat flow, therefore keeping warm air from passing through it. R-values are not the only unit of measurement important for rating a window’s thermal efficiency. U-values are also important.
To better understand the usefulness of the R-factor and its relationship with U-factors, read below.
What is the R-Factor of Windows?
The R-factor, or R-value, of barriers, such as windows, doors, insulation, walls, etc., measures the resistance of the barrier to the flow of heat. It’s how well a window resists heat flow. In other words, how well a window insulates. So, the way R-values work is, the higher the R-value, the more resistant the barrier is to heat flow, and therefore the better insulator it is for the home.
We call these units of measurement R-values because we’re measuring the glass’ resistance to heat. Why do R-values matter? Because they measure the effectiveness of the glass’s resistance to heat conductivity, these values tell contractors which windows are the most energy-efficient and will therefore save homeowners as much money in heating and cooling as possible. As a homeowner, you can imagine that is pretty useful information for you too as you start to watch the electric and gas bill amounts decrease bit by bit.
What is the Difference Between Window R and U Values?
Whenever you find glass or barrier performance charts, in addition to seeing the R-values, you’ll likely find U-values. U-values, or U-factors, refer to the whole window’s ability to insulate, not just the glass, like the R-value measures. After all, when a window is made, air can often sneak its way in or out through the grid, the track that the window slides on, and/or the edges of the frame. The U-value measures the efficiency of all of the materials used for the window, the frame, and the glass that was used.
Along with the R and U-values, manufacturers will include something called the Solar Heat Gain Coefficient (SHGC). Glass windows are usually double-paned. While the R and U-values measure how well the interior pane in the windows keeps heat and cold inside the home, the SHGC measures how well the exterior pane keeps outdoor heat and cold out.
All of these units tell a manufacturer and a contractor the thermal efficiency of the glass so that they can find the best windows for your home and budget.
What is the Best R-Value for Windows?
R-values will normally lie somewhere between the range of 0.9 and 3.0, and like I was telling you earlier, the higher the R-value is, the more effective the window is, so you want as high an R-value as you can get. R-values can certainly reach higher levels, even as high as 8.065, but this is rare. It’s also important to remember that when one value is high, another is sacrificed. It’s just the balance of things.
What is the Best U-Value for Windows?
Unlike R-values, desired U-factors will vary based on geographical location. The Department of Energy’s “Energy STAR” joint program with the Environmental Protection Agency divides the United States into four climate zones:
- Northern Climate Zone
- North Central Climate Zone
- South Central Climate Zone
- And Southern Climate Zone
Each quadrant will naturally desire different levels of transmittance in their windows. While Maine, in the northern climate, would want less transmittance in order to keep as much light and heat from the sun inside as possible, Texas would want more transmittance so that as the sun sends heat through the windows, it can be sent out almost as quickly and keep the air conditioning from working too hard.
U-values will usually fall between the range of 0.20 and 1.20, where the lower the U-value is, the better because a lower U-factor means there is a lower thermal transmittance, where the warm or cool air comes through the window from the outside to the inside and creating a shift in the temperature difference. When the U-factor is low, that means there is less and less heat escaping.
How to Calculate R and U-Values
As a homeowner, you won’t have to do these calculations when you’re replacing your old windows, because the values will be on the window labels for you. However, if you would like to calculate your own windows’ efficiency I’ll let you know how you can calculate both the R-value and U-value of a window.
Normally, U-values are calculated by measuring the heat flow per unit area, time, and degrees F difference. In other words, U = [Btu/(ft²•°F•hr)]. Meanwhile, R-values are calculated inversely, with [(ft²•°F•hr)/Btu]. Want to know a secret? Once you’ve calculated one value, like U, R would be 1/U, and vice versa.
There’s also a convenient calculator available here.
As you calculate these values, you might end up with a less-than-desired result. There are four factors that will influence what R and U values you get:
- The number of glass layers the windows have
- The space between each layer of glass
- What glaze was used
- How well and how tightly the windows were fitted
What Windows Have Great R-Values?
Now that you know what values to look out for the next time you look for windows, where are you going to get your windows from? For my part, I recommend taking a look at Amerimax Windows.
Besides including stunning finishes like rain, glue chip, and satin etched, just to name a few but their glass and grid options include an argon gas option and super spacer option that will increase thermal efficiency, regardless of which one you choose.
When argon gas is added to windows in between the glass layers in production, the gas will help insulate the house further by acting as a moving, thin barrier that minimizes heat transfer from the inside to the outside.
Naturally, windows that are going to have high R-values and low U-values like you want will be labeled as Low-E windows. Low Emissivity glass is coated with layers of metallic oxide. This way, natural light comes in through the windows but the UV rays and infrared are deflected. In doing so, the metallic coats will act like a reflective mirror with heat and light waves of specific wavelengths and frequencies to come in and out, keeping the house warm during the winter but cool during the summer.
As you can see, replacement windows with good R and U-factors and SHGCs are a worthwhile investment to consider. From my experience as the owner of a professional home-improvement company, Amerimax Windows have consistently had some of the best thermally efficient windows, which is why I like to use them so much. Are you interested in switching out your tired windows to experience the difference? If you’re located in the Northern Colorado or Southern Wyoming areas, feel free to give us a call today at (970) 663-2868 or visit the window services page and fill out the contact form to make an appointment to discuss the finer details of your replacement window home improvement project.