Long-Term Water Absorption Testing of Thermal Insulation Materials: Standards, Methods and Applicati

Other Insulation Products

Cellular glass, calcium silicate, wood wool, wood fibre, expanded cork, perlite boards, aerogel blankets and faced or laminated composite boards can also be evaluated. For faced products, the vapor barrier variants of the immersion methods may be applied so that water uptake occurs only through the surface intended to be exposed to liquid water in service.

Specimen Preparation and Conditioning

Specimens are cut to the dimensions required by the selected method, commonly 150 mm × 150 mm for partial immersion and 100 mm × 100 mm for full immersion, retaining the full product thickness. A sufficient number of specimens, typically at least three to four per method, is taken to represent the product. Before testing, specimens are dried to constant mass in a ventilated oven, generally at 105 °C ± 5 °C, or at a reduced temperature such as 60 °C where the material could be damaged by higher temperatures, and their dimensions and initial mass are recorded.

Testing Items

The core testing items offered under a long-term water absorption program include the following:

• Long-term water absorption by partial immersion over 28 days, expressed in kilograms per square meter (kg/m²) of the exposed surface.
• Long-term water absorption by full immersion over 28 days, expressed as a percentage by volume (% v/v).
• Verification of declared classification levels, such as the WS class for mineral wool and WL(T) levels for EPS, XPS, PUR/PIR and phenolic foam products.
• Short-term water absorption by partial immersion over 24 hours in accordance with EN 1609, for comparison and quality control purposes.
• Water absorption of rigid cellular plastics in accordance with ISO 2896 or GB/T 8810, and of sandwich panel core materials in accordance with ASTM C272.
• Related moisture performance assessments, such as water vapour sorption of mineral fiber insulation per ASTM C1104, moisture content determination, and freeze-thaw durability screening.

Test reports present individual and mean results, the method variant applied, specimen orientation, conditioning details and compliance with the declared classification level.

Testing Methods

Partial Immersion Method (Simulating One-Sided Water Exposure)

The partial immersion method reproduces service conditions in which insulation is in contact with water on one face only, for example boards laid over a waterproofing membrane on an inverted roof or insulation in contact with groundwater. Specimens are placed in a test tank and water is added to the level prescribed by the standard, typically maintained 10 mm below the top edge of the specimens, and kept constant for the full 28-day test duration. In the vapour barrier variant, a barrier is applied to the surfaces that are not intended to contact liquid water so that uptake occurs only through the immersed face. At the end of the exposure period, specimens are removed, allowed to drain for a defined period, commonly 10 minutes with the specimen inclined, surface water is removed with a damp cloth, and the final mass is recorded.

Full Immersion Method (Simulating Complete Submersion)

The full immersion method evaluates products that may become completely surrounded by water, such as buried insulation, insulation in wet ground or flooded constructions. Specimens are kept fully submerged, typically held at least 10 mm below the water surface with a grid or restraint, for 28 days in water maintained at the temperature specified by the standard, commonly around 23 °C. After immersion, surface water is blotted off with a damp cloth and the specimens are weighed immediately. This method is the basis of the WL(T) classification levels used in the harmonized product standards for foamed insulation materials.

Complementary Short-Term Methods

EN 1609 determines water absorption after only 24 hours of partial immersion and provides a rapid indicator for production control. ISO 2896 and its Chinese equivalent GB/T 8810 address water absorption of rigid cellular plastics under shorter immersion, while ASTM C272 covers core materials for structural sandwich panels and ASTM D570 covers solid plastics. These short-term results cannot replace the 28-day long-term value but are useful for comparative screening.

Calculation and Expression of Results

For partial immersion, the long-term water absorption is calculated from the mass gain divided by the exposed plan area of the specimen and is reported in kg/m². For full immersion, the mass gain is divided by the specimen volume, taking the density of water as 1,000 kg/m³, and is reported as a percentage by volume. The mean of the individual specimen results is compared with the declared class, for example WS requiring no more than 5 kg/m² for mineral wool, or WL(T) levels limiting volumetric absorption for foamed plastics.

Equipment and Test Environment

Testing requires immersion tanks with means of maintaining a constant water level, a balance with readability of 0.1 g or better, a ventilated drying oven, calibrated callipers or other dimensional measuring equipment, thermometers for water and ambient temperature control, and vapour barrier materials where the barrier variants are applied. The test environment and water temperature are held within the tolerances specified by the applicable standard throughout the 28-day exposure.

Applications

Long-term water absorption data are used at every stage of the insulation value chain, from product development to regulatory compliance and field failure analysis. Key applications include:

• Declaration of Performance and CE marking of insulation products under the harmonized European product standards, where long-term water absorption is a mandated or voluntary declared characteristic.
• Qualification of XPS and other boards for inverted roofs, protected membrane roofs, green roofs and plaza decks, where insulation remains permanently exposed to water.
• Design verification of below-grade wall insulation, perimeter and plinth insulation, floor slabs on grade and foundation applications in contact with soil moisture and groundwater.
• Civil engineering uses such as frost-heave protection under roads and railways, insulation of cold store and freezer floors, and ground engineering in permafrost regions.
• Quality control during manufacturing, benchmarking of competing materials, and research and development of new insulation formulations and facings.
• Input data for hygrothermal performance simulations and condensation risk assessments, and evidence in technical due diligence or failure investigations.

Because absorbed moisture can raise the thermal conductivity of insulation substantially, long-term water absorption results are often combined with thermal conductivity measurements on moist specimens to demonstrate the durability of the declared thermal performance.

FAQ

What is the difference between short-term and long-term water absorption testing?

Short-term testing, such as EN 1609, measures uptake after 24 hours of partial immersion and is mainly used for rapid quality control. Long-term testing per EN 12087 or EN ISO 16535 measures uptake after 28 days, which better represents permanent or repeated water exposure in service and is the basis of the WS and WL(T) classifications used in product standards.

How are the results expressed?

Partial immersion results are reported in kg/m² of exposed surface, while full immersion results are reported as a percentage by volume. These values are compared directly with the declared classification level in the applicable product standard.

How long does the complete test take?

The immersion period itself is 28 days. Including specimen cutting, drying to constant mass, weighing and reporting, a typical program requires approximately five to seven weeks from sample receipt.

Which method should be selected for a given product?

Partial immersion is appropriate where water contacts one face, such as inverted roofs and below-grade applications, and is the standard route for the mineral wool WS class. Full immersion is appropriate where the product may be completely submerged or buried, and underpins the WL(T) classes for foamed plastics. The product standard and the intended end use determine the method.

Why is water absorption so important for thermal performance?

Water has a much higher thermal conductivity than air, so replacing air in the insulation structure with water sharply increases the overall thermal conductivity. Even a small volumetric moisture content can measurably reduce the energy efficiency of the construction and, with freeze-thaw cycling, can also damage the material structure.

Where should the testing be performed?

Testing should be commissioned from accredited third-party laboratories with competence in thermal insulation testing and accreditation to ISO/IEC 17025, ensuring that results are traceable, reproducible and accepted for regulatory submissions and product certification.