Question: What is the “R-Value” of Glass?
Can you tell me how to calculate the “R-Value” of glass based on its “U-Value”? I have an issue with a building inspector in my town based on energy codes, and my wall area must achieve a certain overall “R-Value” for me to get a certificate of occupancy permit. I have “R-Values” for everything but the glass. The glass company that put the glass in can only tell me the “U-Value” which they said is 0.29. According to the label on the glass when it was installed, the glass is an insulated unit of Solarban 60 with Low-E. Does any of this make sense to you, and if so, how do I get an “R-Value” for this glass?
This whole “U-Value” vs. “R-Value” thing can be a little confusing. For many years now, most glass manufacturers have provided “U-Values” only. However, the conversion, or glass “R-Value” calculator if you will, is pretty simple. You see, “U-Values” and “R-Values” are the reciprocals of each other. Remember those old high school math classes? We were taught that to get any number’s reciprocal, you simply divide 1 by that number. So the reciprocal of your glass unit’s “U-Value of 0.29 can be determined by dividing the number 1 by 0.29 which would give us 3.4482 or roughly 3.5 and that would be the “R-Value” for your glass. If you had a product’s “R-Value” and wanted to know its “U-Value” the process would get reversed … divide the number 1 by its “R-Value” and you would have its “U-Value.” I really hope this helps and thank you for reaching out!
It’s a question regularly asked of us at Glass.com. What is the difference between R-Value and U-Value? Which one is more important when dealing with glass installations? The answer is that they both are actually dealing with the same issue but doing so differently. Before we go any further with this, let’s get a basic understanding of what these values are and how they differ.
R-values are calculations based on a formula that measures the amount of heat flow that a given material (insulation, for example) allows. The “R” represents “resistance” to heat transfer. Thus, the higher the R-Value rating, the better that material or assembly resists heat transfer. If you were buying an insulating material to be used in a wall or attic, for instance, the higher the R-value, the better that material will be at keeping you warm in the winter and cooler in the summer.
Similarly, U-Values measure the transfer of heat through a material. The formula for this approach does not directly provide a resistance factor or resistance value but merely a rate of heat transfer based on a formula that provides an actual amount of transfer (stated in BTUs). So in the U-Value approach, the lower the number, the better.
Now one might ask why there are two approaches for the same thing. Why can’t everything be measured or rated the same way? The simple answer might be that it could be done this way. From an engineering perspective dealing with BTUs is necessary for determining heating and cooling requirements for a building. For instance, furnaces and air conditioning equipment are engineered to provide the output levels required to deal with the anticipated levels of heat or cold anticipated in a given climate zone. Merely knowing the resistance rating of a material is good but precise measures are needed to determine what equipment will be required. This explanation is somewhat simplistic but stay with us for just a bit longer.
In doing a little research for this report, building code requirements were reviewed for a given part of the country. Code reviewed referenced what is known as “Energy Star” standards. The Energy Star program is administered by the federal government’s Environmental Protection Agency (EPA) with energy evaluations and recommendations being provided for everything from refrigerators to windows and other building products. In this Energy Star report, the following requirements were discovered:
- Attics had an R-Value of 49;
- Wall requirements of R-21;
- Window requirements with a U-Value of U-0.30.
Why the switch from “R” to “U”? I think, for one thing, the typical consumer can relate to an R-Value rating system approach far easier than U-Values. Still, there is a little more to it than just that.
Now, let’s dig just a bit deeper into this whole “R” vs. “U” thing. As we explore, we quickly discover a direct relationship and, therefore, a simple way to convert “R” to “U” or vice-versa. Specifically, the reciprocal of a material’s R-Value equals that material’s U-Value. The same holds true going the other way. If we want to find the “U” value for a product that we have the R-Value for, we divide that number into the number 1, and we get the U-Value. For example, an R-Value of 21 (our wall requirement) will yield a U-Value of .0476 when the number 1 is divided by 21. Remember, the reciprocal of any value is that value is divided into the number 1. Therefore, R-Value multiplied by U-Value always equals 1.
So we take our R-Value of 21 and divide one by that number, and we get a U-Value of .0476. Now, in the above-noted code requirements, we had a window U-value requirement of 0.30. If we do our reciprocal calculation using a U-Value of .030, we get an R-Value for the window of 3.3333. Quite a difference and nowhere close to the wall requirements. Now, our engineers would go to work and decide what was needed to raise the insulation value of the non-window areas of our building to get the wall average to come out to what would pass building code requirements. Then, our mechanical engineers could calculate what was needed in heating and cooling equipment to handle what the outcome of our final construction process was going to yield.
It can all be a little confusing, but for the most part, code requirements will dictate the minimum performance requirements for a building’s components. In window replacement or retrofit instances, remember that the higher the “R”, the better the performance, and the lower the “U,” the better.
Hopefully, you now have a better idea of how to decipher the R-Values and U-Values of replacement glass. When you’re ready to start your search for energy-efficient windows or doors, look to Glass.com. We can connect you with a professional in your area to help with your glass replacement project.