What is Window Thermal Performance?

The Window Geeks: Master Class series is here, and we’re starting with the basics. The first topic we’ll be looking at is the one that typically comes to mind when considering efficiency in modern building design – thermal performance. 

By the end of today, you’ll be able to answer the following: 

• How is the thermal performance of a window measured under NFRC? 
• What’s the difference between U-Values and R-Values, and how are they calculated?  

What is thermal performance of a window?  

Thermal performance refers to the heat transfer between the window and its surroundings, specifically how much heat it allows to flow from the interior of a structure to the exterior and vice versa. Thermal performance in general is measured in terms of heat loss. When it comes to windows, this is usually stated as a U-value or R-value. 

What is the difference between a U-Value and R-Value? 

An R-value measures how well the window can resist heat flow, whereas a U-value measures heat loss itself. Therefore, U-Values and R-Values are reciprocals of each other. As a U-value gets smaller (indicating better insulating performance), the R-Value increases (indicating higher thermal resistance). Easy enough, right? 

How to calculate U-Value from R-Value

The U-Value is the inverse of the R-Value – 1/U or 1/R… or, to put it as plainly as possible, in order to get the U-Value you just need to divide 1 by the given R-Value. If you have an R-Value of 4, you will end up with a U-Value of 0.25. This also works in reverse. 

WARNING: pay attention to units of measurement 

The R-Value is an imperial system unit of measurement (ft2·°F·h/BTU). Before you can calculate an R-Value in the manner stated above (dividing 1 by the U-Value), you must convert the U-Value to imperial from metric to match the R-Value.

How do I convert U metric to U imperial? 

Do this by dividing the U metric number by 5.678. This will give you the U imperial number, which 1 is divided by to achieve the uniform U-Value and R-Value.  

Step 1 

U1.40 (metric)/ 5.678 = U0.25 (Imperial)  

Step 2  

1/0.25= R4 

Ta-da! With that, you have the basics of a window’s thermal performance. That’s particularly useful as next time we’ll be exploring how thermal performance is regulated and enforced.

Sign up today for the Window Geeks: Master Class series! Stay tuned to become a window and door expert and save yourself time and money going forward.