R-value vs. LTTR

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RValueVsLTTRWith the movement to be more “green” and environmentally conscious, the rules are quickly changing when it comes to the building envelope. From the walls to the floors, ceilings to parking lots, it is much desired to reduce the carbon footprint one building at a time. So, when it comes to commercial roofing, there have been a lot of changes since Chicago adopted the International Code Councils (ICC) proposal back in 2009 that was later adopted by the state of Illinois. One of those changes that impacted the commercial roofs was the required R-value for the system itself. As a result, building owners, property managers and facility directors have been paying much more attention to R-values.

Prior to 2009, it was common for a commercial roof to be installed with an R-value of no more than 12 (2 inches). Now, an R-value of 25 (over 4-inches with the new testing results) is required, with discussion to move to R-30 within the next few years! But that is an article all to itself. The purpose here is to educate on the difference between the products R-value, or measure of the products thermal resistance (ability to retain the buildings temperature and resist external temperatures), versus the products LTTR or long term thermal resistance. Sometimes these terms are use interchangeably. However, they actually mean something different as the R-value refers to the materials current thermal resistance while the LTTR references the R-value of the product over a designated period of time.

There are two interesting facts to point out here:

  1. The LTTR for roof insulation in calculated over a 5-year period. The interesting fact being that the average commercial roof life is 17.4 years as per the CRCA (Chicago Roofing Council Association).
  2. The testing for this LTTR is done in ideal temperatures (70 degrees) in lab conditions down in Tennessee. It has been discovered that the R-value changes based on the temperatures. In an area such as Chicago with extreme heat and equally extreme cold, the R-value would be reduced in both instances.

So what does this all mean? Well, it means to be aware that when referencing the R-value of a new product (in this case, roof insulation), it is referring to the thermal resistance from the factory. However, the LTTR is providing what that adjusted figure will be over a designated period of time. While it seems absurd that the LTTR for roof insulation is not recorded over a longer period of time, say 15 or 20-years, it still will be a different value as to indicate the thermal protection you will have down the road. In addition, your R-value will be significantly reduced in severe weather conditions, like those we are enduring now.

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