LTTR Defined Posted on January 28, 2014 by John Myers By:
LTTR & Published R-Value
With 2014 and the changes in the Polyisocyanurate R-Values, it might be beneficial to try and better understand just how manufacturers came up with their published R-Values. Specifically, how does the PIMA designation affect those values? I had the opportunity to partake in a CSI webinar presentation by Chris Griffin, the Technical Director at Johns Manville and Chairman of PIMA. I will share some of the information he brought forward.
First, what is the definition of LTTR. “LTTR is a 15-year, time weighted average of thermal resistance, equivalent to a 5-year aged R-Value test at lab conditions”, as defined by Mr. Griffin. Rather than the actual 15-year aged R-Value, LTTR is a calculation based on the “Aging Process”, which averages the R-Value over a given period of time. In this case, 15-years. “Diffusion of gases in and out of the polyiso cells reduce the R-Value overtime. Essentially, R-Value is a moving target”, according to Mr. Griffin. The current test to determine LTTR is CAN/ULC-S770-09. Again, LTTR is not the final R-Value of the insulation at the end of its service life. It is only used for testing purposes. In comparison, Foam Control® Plus+® has a 50-year, 100% thermal warranty.
PIMA, the Polyisocyanurate Manufacturers Association, developed third party testing using the current testing standard, CAN/ULC-S770-09, to determine individual manufacturers R-Values. Membership is voluntary; however, all US manufacturer’s participate in the PIMA labeling program. The PIMA standard based on 5-year testing data, determined that all manufacturers who wish to carry the PIMA Seal must obtain a minimum LTTR of 5.6 per inch. To avoid the appearance of collusion, it is only a recommended published value. Each manufacturer is free to publish their own value; however, it must test to a minimum of 5.6/inch to carry the PIMA stamp. To date, all manufacturers are publishing 5.6 as their R-Value.
Thicknesses of Polyisocyanurate insulations are going to increase by 4-6% in order to meet the former R-Values. While the manufacturers are confident they now have the correct formula for determining the LTTR, I think the question is: if the insulation is going to stay in-service more than 15-years, what is the real R-Value? The only stable insulating medium for all rigid foam insulation is trapped air. Foam-Control® and Foam-Control®Plus+® use only trapped air when measuring R-Value; and therefore, can provide a 50-year 100% R value warranty. Instead of using published R-values, we would like to see design professionals and contractors use actual or warranted R values when designing and building structures that require heating and cooling. PIMA promotes designing around thickness rather than R-Value, while I would suggest it is easier to hit a stationary target, than a moving one. EPS rigid foam insulation, such as Foam Control® Plus+®, have a clear sustainable and consistent R-Value.