When choosing insulation for a commercial roofing system, R-value is one of the most important performance measurements to understand. It indicates how effectively an insulation material resists heat flow—the higher the R-value, the greater the insulation’s resistance to heat transfer. For polyisocyanurate, commonly called polyiso, you may also see the term LTTR, or Long-Term Thermal Resistance, on product data sheets and roofing specifications.
Understanding the difference between LTTR and other R-value measurements helps building owners, architects, contractors, and facility managers make informed decisions about commercial roof insulation.
What Is LTTR?
LTTR stands for Long-Term Thermal Resistance. It is a scientifically supported, consensus-based method used to estimate the thermal resistance of closed-cell insulation containing captive blowing agents after aging. For polyiso roof insulation, the LTTR value represents the predicted thermal resistance at approximately five years after manufacture.
Importantly, the five-year predicted value is intended to closely represent the time-weighted average thermal resistance over the first 15 years of the product’s service life. This makes LTTR particularly useful when evaluating the expected long-term thermal performance of polyiso rather than relying solely on an initial, unaged R-value.
Polyiso insulation uses captive blowing agents within its cellular structure. Over time, changes within the foam can affect thermal resistance. LTTR testing accounts for this aging behavior by using accelerated laboratory procedures rather than simply measuring the insulation immediately after manufacturing.
How Is Polyiso R-Value Officially Rated?
In the United States, the primary product specification governing faced rigid polyisocyanurate insulation board is ASTM C1289, titled Standard Specification for Faced Rigid Cellular Polyisocyanurate Thermal Insulation Board. ASTM C1289 establishes requirements for measuring and reporting thermal resistance and recognizes specific methods for determining LTTR.
Under ASTM C1289, two recognized LTTR test methods are:
- ASTM C1303/C1303M — Standard Test Method for Predicting Long-Term Thermal Resistance of Closed-Cell Foam Insulation
- CAN/ULC-S770 — Standard Test Method for Determination of Long-Term Thermal Resistance of Closed-Cell Thermal Insulating Foams
Both methods use an accelerated aging technique commonly referred to as “slicing and scaling.” This allows laboratory testing to predict the insulation’s thermal resistance after five years without waiting five years for the material to naturally age.
For certain U.S. polyiso products, R-value can also be determined after a conditioning period. ASTM C1289 permits full-thickness samples to undergo a 180-day conditioning period at 73°F and 50% relative humidity, with an accelerated 90-day conditioning procedure also available under specified conditions. Thermal resistance is then measured under steady-state conditions, typically at a mean temperature of 75°F.
Why Does LTTR Matter for Commercial Roofing?
Commercial roofs are exposed to changing temperatures, moisture, solar radiation, and other environmental conditions throughout their service life. Therefore, an insulation value that only represents its initial thermal performance may not provide the most useful basis for evaluating long-term performance.
LTTR provides a standardized way to compare the long-term thermal resistance of qualifying closed-cell foam insulation products. For this reason, polyiso manufacturers publish LTTR values for permeable-faced polyiso roof insulation sold in the United States.
For example, the published LTTR value of a particular thickness should be used when determining the insulation contribution toward a roof assembly’s required thermal performance, rather than assuming that every polyiso board has the same R-value per inch.
What Is PIMA QualityMark?
The Polyisocyanurate Insulation Manufacturers Association (PIMA) operates the voluntary QualityMark program, which provides independent third-party certification of LTTR values for participating polyiso roof insulation manufacturers.
Under the program, insulation samples are independently selected and tested by approved laboratories. This additional verification is intended to provide greater confidence in published LTTR values for designers, contractors, building owners, and code officials.
The Bottom Line
LTTR is the standardized long-term thermal resistance metric used to evaluate polyiso insulation performance after aging. In the United States, polyiso roof insulation is governed by ASTM C1289, with LTTR determined using recognized methods such as ASTM C1303 or CAN/ULC-S770.
When comparing commercial roofing insulation options, look at the manufacturer’s published LTTR value for the exact product and thickness rather than relying on a generic R-value-per-inch assumption. Product-specific data sheets provide the most appropriate information for roof design, energy-performance calculations, and code compliance.
For a commercial roofing project in Florida, selecting the correct insulation thickness and verified thermal performance can help improve energy efficiency while supporting the long-term performance of the roof assembly. Shieldline Roofing can help building owners and facility managers evaluate commercial roof insulation and restoration options based on the specific requirements of their building.
Related Questions
- Does polyiso lose R-value in heat, cold, or both?
- What Is the Difference Between R-Value and U-Factor for Roofs?
- What happens if polyiso gets wet during installation?
- How is R-value calculated for a complete assembly including deck and membrane?
- When is EPS preferred over polyiso in Florida applications?
- Are R-value requirements different in North Florida versus South Florida?
- What is the payback difference between R-20 and R-30 in Florida?
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