Understanding R-Value vs. LTTR in Polyiso Insulation
When evaluating insulation products, especially rigid foam boards like polyisocyanurate (polyiso), you’ll often encounter terms like R-Value and LTTR. While both are measures of thermal performance, they represent different things. Understanding the distinction is critical for architects, contractors, energy consultants, and building owners who want to make informed decisions about building envelope performance.
Let’s break it down.
QUESTION:
What Is R-Value?
ANSWER:
R-Value is a measure of thermal resistance – the higher the R-Value, the better the material resists heat flow. It’s a standardized metric used to compare the insulating properties of different materials.
In simple terms:
R-Value = Resistance to Heat Flow
For polyiso insulation, you might see R-values like R-6 or R-6.5 per inch, which makes it one of the highest-performing rigid board insulations available.
However, there’s a catch – R-Value can change over time, especially for certain types of insulation, which brings us to LTTR.
QUESTION:
What Is LTTR?
ANSWER:
LTTR stands for Long-Term Thermal Resistance. It’s a more accurate representation of the average thermal performance of an insulation product over time – typically 15 years.
Polyiso insulation is a manufactured gas-filled closed-cell structure. Initially, the blowing agents (such as pentane) provide excellent thermal resistance. Over time, air begins to replace the blowing agent in the cells, slightly reducing the insulation’s thermal performance. The concept of LTTR accounts for this natural aging process.
LTTR = Realistic R-Value over 15 Years
Rather than just using the initial R-Value, LTTR method provides a representative measure of long-term thermal resistance. That’s why building codes and standards now prefer LTTR for closed-cell foam products like polyiso.
WHY THE DISTINCTION MATTERS FOR POLYISO
Polyiso manufacturers used to report only initial R-values, (sometimes referred to as “fresh R-value”). But this could lead to overestimating real-world performance over the product’s life.
By contrast, LTTR provides a more conservative and realistic estimate, which helps specifiers and designers ensure energy-code compliance and long-term energy savings.
Here’s a comparison to illustrate (note: these are approximate and will vary by manufacturer and product type).
LTTR AND ASTM STANDARDS
The LTTR values for polyiso are determined using standardized test methods under ASTM C1289 (and related test methods) which include accelerated aging and “slicing-and-scaling” techniques.
Specifically, the “Thermal Metric” method. The Polyisocyanurate Insulation Manufacturers Association (PIMA) began reporting LTTR values through its third-party Quality-MarkCM certification program. This program uses the CAN/ULC-S770 protocol from Canada and the ASTM C1289 standard in the United States to determine the LTTR of polyiso insulation.
These standards help ensure consistent and reliable LTTR data across polyiso products and manufacturers.
KEY TAKEAWAYS
• R-Value represents the immediate (initial) thermal resistance; LTTR represents the 15-year, time-weighted average thermal resistance.
• LTTR provides a more realistic and code-compliant way to evaluate the thermal performance of polyiso insulation.
• For polyiso, LTTR values are slightly lower than initial R-values due to the natural aging process of the foam’s blowing agents.
• Always check that polyiso insulation products are tested and labeled according to relevant standards (e.g., ASTM C1289) and report LLTR values.
FINAL THOUGHTS
While both R-Value and LTTR are important, LTTR is the industry-standard metric for long-term performance in polyiso insulation. By specifying LTTR-compliant products, you’re ensuring your building’s thermal envelope meets energy codes and delivers reliable performance for years to come.
When in doubt, consult the technical data sheets provided by Hunter Panels for the LTTR values based on standardized testing.
Need help selecting the right polyiso insulation for your project? Get in touch with our team of building envelope specialists today.