Glass Detective Question – “R Value” vs. “U Value” of Glass

By Daniel Snow


Glass Detective,

Can you tell me how to calculate the “R Value” for a piece 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?

Thank you,

Dave R.




This whole “U-Value” vs. “R-Value” thing can be a little confusing. For many years now, most of the glass manufacturers have provided “U-Values” only. However, the conversion, or calculation 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 divided 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!

-Glass Detective

Leave a Reply

Your email address will not be published. Required fields are marked *