Long-term thermal resistance (LTTR) testing quantifies how the insulating performance of extruded polystyrene (XPS) boards used in cold storage facilities changes over decades of service due to gas diffusion within the closed-cell structure. This article explains the test samples, evaluated parameters, standardized methods based on EN 13164, ASTM C1303, ISO 11561, and GB/T 10294, and how accredited third-party laboratories apply the results to freezer floors, cold-room walls, and refrigerated envelope design. Understanding LTTR helps specifiers select board thicknesses and grades that will still meet energy and condensation requirements at the end of the design service life.
Technical Overview
Extruded polystyrene (XPS) foam board is a preferred insulation material for cold storage construction because of its closed-cell structure, low water absorption, and high compressive strength, which suit freezer floors, walls, and ceilings exposed to sustained temperature gradients and mechanical loads. Immediately after manufacture, the closed cells of XPS contain blowing-agent gases whose thermal conductivity is considerably lower than that of air. Over months and years, air diffuses slowly into the cells while the blowing agent diffuses out, so the effective thermal conductivity of the board rises and its thermal resistance (R-value) gradually declines. This phenomenon is known as thermal drift or aging.
Long-term thermal resistance testing measures this drift under controlled, accelerated laboratory conditions and predicts the thermal resistance the product will deliver after a defined reference service life. This matters especially for cold stores, which are typically designed for 30 to 50 years of continuous operation, because a board selected only on its fresh-production R-value may underperform later in life, increasing refrigeration energy consumption and the risk of condensation and frost at the warm face of the envelope. Since natural aging of a 50 mm to 100 mm thick XPS board can take decades, standardized acceleration techniques, namely thin slicing and elevated-temperature storage, are combined with periodic thermal conductivity measurement to extrapolate long-term performance.
- EN 13164 – Product specification for factory-made extruded polystyrene foam (XPS) insulation, including long-term thermal resistance determination.
- ASTM C1303/C1303M – Predicting the long-term thermal resistance of closed-cell foam insulation by the slicing and scaling method.
- ISO 11561 – Accelerated laboratory ageing methods for determining the long-term change of thermal resistance of closed-cell plastics.
- ISO 8301, ISO 8302, ASTM C518, ASTM C177, EN 12667, EN 12939, GB/T 10294, GB/T 10295, and JIS A 1412 – Thermal conductivity and thermal resistance measurement by heat flow meter and guarded hot plate apparatus.
Testing Samples
Full-Thickness Production Boards
Samples are cut from regular production boards at the thicknesses declared for the cold storage project, commonly 50 mm for freezer floors with heavy rack and forklift loads, and 75 mm to 150 mm for walls and ceilings. Specimens retain their natural extruded skins and are sized to match the measurement apparatus, typically 300 mm × 300 mm or 600 mm × 600 mm plates. These samples provide the initial thermal conductivity, the aged value after accelerated storage, and the thickness reference used in the scaling calculations.
Thin-Sliced Specimens for Accelerated Aging
For the slicing and scaling method, thin slices are cut parallel to the board faces from representative positions across the thickness, at a minimum of two slice thicknesses, typically 5 mm to 10 mm. Because gas diffusion through a thinner specimen reaches the same aging state far faster, a 10 mm slice ages roughly (d/10)² times faster than a board of thickness d in millimeters. Multiple slice thicknesses are used to verify that the diffusion behavior follows the assumed scaling law before extrapolation to the full product is accepted.
Specimen Conditioning and Preparation
All specimens are conditioned at 23 °C ± 2 °C and 50% ± 5% relative humidity until mass stabilization before baseline measurement. Sawn surfaces are dressed flat and parallel, and thickness is measured at multiple points under a specified contact pressure, since thickness error translates directly into thermal resistance error. Specimen density, skin condition, and cutting direction are recorded because slice orientation relative to the extrusion direction can influence measured diffusion rates.
Testing Items
The test program for cold storage XPS board long-term thermal resistance typically covers the following parameters, executed by accredited third-party laboratories under the applicable product and method standards:
- Initial thermal conductivity and thermal resistance of freshly manufactured board at the standard mean temperature of 10 °C, with optional measurements at 0 °C, −10 °C, or lower mean temperatures to represent cold store service conditions.
- Thermal conductivity of thin slices measured at successive aging intervals, plotted against the square root of elapsed aging time.
- Thermal resistance of full-thickness boards after accelerated aging by elevated-temperature storage.
- Predicted long-term thermal resistance (LTTR) at the reference service life, for example 15, 20, or 25 years, and the derived aging or drift factor relative to the initial value.
- Declared thermal conductivity for design use, determined on a statistical basis as defined by the product standard.
- Supporting characterization where specified: apparent density, closed-cell content, compressive creep behavior under sustained floor loads, and long-term water absorption by diffusion or immersion, all of which influence in-service thermal performance of cold storage insulation.
Testing Methods
Thermal Conductivity and Thermal Resistance Measurement
Baseline and interval measurements are performed using a heat flow meter apparatus in accordance with ISO 8301, ASTM C518, EN 12667, GB/T 10295, or JIS A 1412-2, or a guarded hot plate apparatus in accordance with ISO 8302, ASTM C177, EN 12939, GB/T 10294, or JIS A 1412-1. The apparatus is calibrated against certified reference materials traceable to national metrology institutes, and measurements are repeated under defined plate temperature differences. Declared values for building applications are determined at a 10 °C mean temperature; for cold store design, additional measurements at sub-zero mean temperatures capture the reduction in gas conductivity that occurs as blowing agents condense at low temperature. Temperature conversions between mean temperatures can be verified using the conversion relationships given in ISO 10456.
Slicing and Scaling Method
In the procedure of ASTM C1303/C1303M and ISO 11561, thin slices are aged at controlled temperature and their thermal resistance is measured at defined intervals. The aging curve, plotted against the square root of time, is extrapolated, and the time axis is scaled to the full board thickness using the quadratic diffusion relationship, in which the time for a full-thickness board to reach a given aging state equals the slice aging time multiplied by the square of the thickness ratio. Agreement between slices of different thicknesses confirms Fickian diffusion behavior and validates the extrapolation. The output is the predicted thermal resistance of the full product at the selected reference age, which in North American practice is commonly reported as a 15-year value.
Elevated-Temperature Accelerated Aging Method
For products placed on the market under EN 13164, long-term thermal resistance is determined by storing full-thickness boards at an elevated temperature, typically 70 °C, for a prescribed period that increases with product thickness. The elevated temperature accelerates gas diffusion by well over an order of magnitude, so a storage period of weeks corresponds to years of natural aging at ambient conditions. After the storage period, specimens are cooled, reconditioned, and measured in the heat flow meter or guarded hot plate apparatus to obtain the aged thermal resistance from which the declared value is derived. Complementary slice aging at the same elevated temperature may be used to shorten the program further.
Data Evaluation and Reporting
The laboratory report documents specimen identification, thickness, density, conditioning, calibration traceability, measurement temperatures, and the complete aging history. Statistical treatment follows the product standard, with declared values established at a defined confidence level and fractile. The extrapolated LTTR, the aging factor, and any comparison between the slicing method and the storage method are reported so that the specifier can verify that the aged thermal resistance remains adequate for the refrigeration load calculation, dew-point control, and energy compliance of the cold store.
Applications
Long-term thermal resistance data for XPS boards are applied throughout the cold chain wherever sustained low temperatures and long service lives make aged performance the governing design value:
- Freezer and chilled warehouse floors, where boards with compressive strengths of 300 kPa to 500 kPa and higher are installed beneath concrete slabs and must retain their R-value for decades under static and dynamic loads.
- Cold room walls and ceilings, partition walls between chambers of different temperatures, and insulated doors and air-lock vestibals in food distribution centers.
- Food and beverage processing plants, blast freezers, ice plants, and refrigerated docking areas subject to thermal cycling and moisture exposure.
- Pharmaceutical and vaccine cold storage, where temperature stability requirements demand conservative, aged insulation values in the envelope calculation.
- Procurement and quality control: LTTR reports are used to compare suppliers, verify performance after blowing-agent changes such as transitions to HFO, CO2, or water-blown formulations, and demonstrate conformity with national product standards such as GB/T 21558 and with cold store design codes.
FAQ
Why does the R-value of XPS board decrease over time?
The closed cells of new XPS contain low-conductivity blowing-agent gases. Over time, air diffuses inward and the blowing agent diffuses outward, raising the effective cell gas conductivity and therefore the thermal conductivity of the board. This drift is largest in the first years and continues at a decreasing rate until the cell gas approaches equilibrium with atmospheric air.
What is the difference between the initial R-value and the long-term thermal resistance?
The initial R-value is measured on freshly manufactured board and represents best-case performance. The long-term thermal resistance is the predicted value after the defined reference service life, obtained by accelerated aging and extrapolation. Design of cold store envelopes should be based on the aged value, because the facility will operate for decades and refrigeration energy costs accumulate over the entire life.
Which reference service life is used in LTTR testing?
Common reference ages are 15 years in North American practice under ASTM C1303/C1303M and values consistent with the declared-value framework of EN 13164 in Europe. For cold stores designed for 30 to 50 years, specifiers may request extrapolations to longer ages or apply the aged thermal conductivity as a conservative design value for the full life.
Can the boards be tested at cold storage temperatures?
Yes. Although declared values are normally determined at a 10 °C mean temperature, accredited laboratories can measure thermal conductivity at lower mean temperatures such as 0 °C or −10 °C. At these temperatures part of the cell gas condenses, which lowers thermal conductivity, so low-temperature data provide a more accurate input for freezer envelope calculations, while the aging drift determined at standard conditions is still applied.
How long does a complete LTTR test program take?
Depending on board thickness, the chosen method, and the number of measurement intervals, a full program typically runs from several weeks to several months. The slicing and scaling method is generally the fastest, while full-thickness elevated-temperature storage takes longer because thicker boards require proportionally extended aging periods.
Is long-term thermal resistance testing mandatory for cold storage projects?
Requirements depend on the governing product standard and project specification. For XPS, which is a closed-cell foam subject to aging, most specifications and certification schemes require aged or declared thermal values rather than initial values, and owners of cold storage assets commonly require LTTR evidence from an accredited third-party laboratory as part of procurement to protect lifetime energy performance.
How do cold storage conditions affect the aging process?
Low service temperatures slow gas diffusion compared with ambient aging, which is favorable, and condensation of blowing agents at sub-zero temperatures further reduces conductivity. However, sustained vapor drive from the warm side toward the cold side means that any moisture uptake degrades performance, so the low water absorption of XPS, confirmed alongside LTTR testing, is a key part of the qualification package for cold store insulation.