Water Absorption Test Methods for Thermal Insulation Materials

Water absorption is one of the most important quality and durability indicators for thermal insulation materials, because absorbed moisture dramatically increases thermal conductivity and accelerates product degradation. This article provides a comprehensive technical overview of water absorption test methods for insulation products, covering sample preparation, testing items, standard procedures such as ISO 2896, ASTM D2842, EN 12087, EN 1609, and EN 12088, calculation of results, typical applications, and frequently asked questions.

Technical Overview

Thermal insulation materials achieve their low thermal conductivity by trapping still air or a blowing gas within a cellular, fibrous, or granular structure. Still air has a thermal conductivity of approximately 0.026 W/(m·K), while liquid water conducts heat at approximately 0.6 W/(m·K), roughly twenty-five times higher. As a result, even a small amount of absorbed water can measurably increase the apparent thermal conductivity of an insulation product, reduce its thermal resistance, and compromise the energy performance of the entire insulated system.

Water absorption is defined as the quantity of water taken up by a test specimen when it is exposed to liquid water under controlled conditions of temperature, immersion depth, and duration. The result is normally expressed as a percentage of the dry mass, as a percentage of the specimen volume, or as the absorbed mass per unit area in kilograms per square meter. Because the exposure condition strongly influences the outcome, standardized test methods must be followed to obtain comparable and reproducible results.

The most widely used reference methods include:

• ISO 2896 — Rigid cellular plastics, determination of water absorption by immersion
• ASTM D2842 — Water absorption of rigid cellular plastics, with volume correction for cut cells
• EN 12087 — Long-term water absorption of building insulation by immersion (total, partial, and floating variants)
• EN 1609 — Short-term water absorption by partial immersion
• EN 12088 — Long-term water absorption by diffusion
• ASTM C272 — Water absorption of core materials for structural sandwich panels
• GB/T 8810 — Determination of water absorption for rigid cellular plastics, technically aligned with ISO 2896

Beyond thermal performance, absorbed water can cause freeze-thaw damage, corrosion of metal facings and substrates, dimensional change, loss of compressive strength, and microbial growth. For these reasons, water absorption limits are declared in the European product standards EN 13162 through EN 13167 and are routinely verified by manufacturers and accredited third-party laboratories as part of factory production control, product certification, and declaration of performance.

Testing Samples

Reliable results begin with representative sampling and careful specimen preparation. Specimens are normally cut from finished products, kept free of mechanical damage, and conditioned to constant mass in a standard atmosphere of 23 ± 2 °C and 50 ± 5 % relative humidity. ISO 2896 specifies conditioning for seven days, while EN methods generally require conditioning until constant mass. The number of specimens is typically three to five depending on the standard, and the specimen thickness is usually the full product thickness. Skins or facings that form an integral part of the product are retained where the standard requires it.

Rigid Cellular Plastics

Expanded polystyrene (EPS), extruded polystyrene (XPS), polyurethane (PUR), polyisocyanurate (PIR), and phenolic foam are tested as square slabs, commonly 150 mm × 150 mm. Edges are cut cleanly and squarely, and molded or extruded skins are kept intact on the faces unless the standard specifies otherwise. Open cut edges expose cut cells and must be taken into account when interpreting results, particularly with ASTM D2842 and its volume correction procedure.

Mineral Wool and Other Fibrous Products

Glass wool and rock wool boards, mats, and pipe sections are sampled as square plates, commonly 200 mm × 200 mm, cut without compressing or tearing the fiber structure. Specimens should represent the nominal density and fiber orientation of the product, and loose particles are removed before weighing. Pipe sections may be tested as representative arc segments or as flattened slabs according to the product standard.

Flexible Elastomeric Foams

Closed-cell elastomeric foam sheets and tubes used in HVAC and refrigeration insulation are tested as flat slabs or as tube sections. Because these products are frequently supplied with factory-applied skins or self-adhesive facings, the laboratory must document whether the facing was retained or removed, since this decision has a decisive effect on the result.

Composite and Specialty Insulation

Calcium silicate, expanded cork, aerogel blankets, microporous panels, and faced composite boards are prepared in accordance with their product standards. Composite products are tested in their delivered configuration unless the relevant specification calls for removal of facings, and any deviation must be reported together with the results.

Testing Items

Depending on the product family and end use, laboratories typically determine one or more of the following water absorption characteristics:

• Short-term water absorption by partial immersion, expressed in kg/m² (EN 1609)
• Long-term water absorption by total immersion, expressed in % by volume (EN 12087)
• Long-term water absorption by partial immersion, expressed in kg/m² (EN 12087)
• Long-term water absorption under an additional 1 m water head, for inverted roof and below-grade applications (EN 12087)
• Long-term water absorption by diffusion, expressed in kg/m² (EN 12088)
• Water absorption of rigid cellular plastics after 96 h immersion, expressed in % by volume or % by mass (ISO 2896, ASTM D2842, GB/T 8810)
• Water absorption of sandwich panel core materials after 24 h immersion, expressed in % by mass (ASTM C272)

Testing Methods

All immersion-based methods share the same principle: the conditioned specimen is weighed dry, exposed to water under precisely defined geometry and duration, drained and surface-dried in a standardized manner, weighed wet, and the absorbed water is calculated from the mass difference. Water temperature is controlled at 23 °C with a tolerance of ±2 °C to ±5 °C depending on the standard. The following subsections summarize the principal procedures.

Full Immersion Method for Rigid Cellular Plastics (ISO 2896 / ASTM D2842 / GB/T 8810)

• Condition the specimens and determine the initial dry mass m1 to the nearest 0.01 g or 0.1 % of the mass.
• Immerse the specimens with their upper face 50 mm below the water surface, using a sinker or grid to hold buoyant materials in position.
• Maintain the water at 23 ± 2 °C for 96 h ± 4 h (4 days).
• Remove the specimen, incline it to drain for approximately 10 minutes, and remove adhering surface water in a reproducible way without drawing water out of the cells.
• Weigh the wet specimen to obtain m2 and calculate the result.

ASTM D2842 additionally determines a volume correction factor from companion specimens of two different sizes, compensating for the extra water taken up by cells cut open at the specimen edges so that the corrected result better represents the behavior of a large product surface.

Long-Term Immersion (EN 12087)

• Total immersion: the specimen is kept fully submerged, typically with its upper face 50 mm below the water surface, for 28 days; the result WL(T) is expressed in % by volume.
• Water head variant: for inverted roofs and below-grade applications, the immersion is performed under an additional 1 m water head to simulate hydrostatic pressure.
• Partial immersion: the lower face of the specimen contacts a shallow water layer for 28 days, and the result WL(P) is expressed in kg/m².
• Floating method: for buoyant products that cannot easily be restrained, the specimen is floated with a defined face in contact with the water surface.

Short-Term Partial Immersion (EN 1609)

• The specimen is placed with its test face held approximately 10 mm below the water surface for 24 h ± 0.5 h.
• After draining for a defined short period, the specimen is weighed and the absorbed mass is divided by the exposed area.
• The result WS in kg/m² characterizes the behavior of the product during brief wetting events such as rain exposure on site, and it is a key declared property for mineral wool in EN 13162.

Diffusion Method (EN 12088)

• One face of the specimen is exposed to a warm, nearly saturated atmosphere while the opposite face is held at a lower temperature, creating a vapor pressure gradient that drives water vapor into the product.
• Vapor condenses inside the specimen, and the accumulated mass is determined after a defined exposure period of up to 28 days depending on the declared class.
• The result in kg/m² reflects long-term moisture accumulation by vapor diffusion rather than direct liquid contact, which is decisive for cold-store and inverted-roof applications.

Calculation and Expression of Results

• Percentage by mass: Wm = (m2 − m1) / m1 × 100
• Percentage by volume: Wv = (m2 − m1) / V × 100, where the absorbed mass in grams is divided by the specimen volume in cubic centimeters
• Mass per unit area: W = (m2 − m1) / A, expressed in kg/m²
• In these formulas, m1 and m2 are the dry and wet masses, V is the specimen volume, and A is the exposed area; results are reported as individual values and the mean, together with the standard used, specimen orientation, conditioning details, and any corrections applied.

Several practical factors influence the measured value and must be controlled or reported: the presence of intact skins versus cut edges, water temperature, immersion depth and any hydrostatic head, drainage time, and whether buoyant specimens were restrained. Comparing results obtained by different methods or on differently sized specimens is not valid without stating the test conditions.

Applications

Water absorption data are used throughout the insulation industry for product development, qualification, and specification:

• Inverted and protected-membrane roofs, where XPS boards remain permanently in contact with water and long-term absorption under a 1 m head must remain very low.
• Below-grade walls, foundations, and perimeter insulation exposed to soil moisture and hydrostatic pressure.
• Cold stores, refrigerated piping, and cryogenic equipment, where vapor diffusion and internal condensation govern moisture pickup.
• Directly buried district heating and cooling networks insulated with rigid polyurethane foam.
• Marine and offshore insulation exposed to humid and splash environments.
• Declaration of performance and CE marking under EN 13162 to EN 13167, where water absorption classes are declared.
• Factory production control, incoming inspection, and supplier qualification by manufacturers and contractors.
• Input data for hygrothermal simulation and long-term durability assessment of building assemblies.

Accredited third-party laboratories routinely perform these tests for certification bodies, specifiers, and manufacturers, ensuring that declared values are traceable to recognized international standards.

FAQ

Why is water absorption so important for insulation performance?

Water conducts heat approximately twenty-five times better than still air, so absorbed moisture raises thermal conductivity and lowers thermal resistance. Moisture also drives freeze-thaw degradation, corrosion, dimensional change, and microbial growth, all of which shorten service life.

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

Short-term tests such as EN 1609 expose the product for 24 hours by partial immersion and simulate transient wetting such as rain during installation. Long-term tests such as EN 12087 use 28 days of full immersion, and EN 12088 uses sustained vapor diffusion exposure, simulating permanent contact with water or continuous moisture drive; they therefore yield higher, more conservative values.

Which standard applies to my product?

Rigid cellular plastics such as XPS, EPS, PUR, PIR, and phenolic foam are commonly tested to ISO 2896, ASTM D2842, or GB/T 8810. Building insulation placed on the European market is tested to EN 1609, EN 12087, or EN 12088 depending on the product standard. Sandwich panel cores follow ASTM C272. When in doubt, select the method referenced by the product certification scheme or the project specification.

How are the results expressed and can they be converted?

Results are expressed as percent by mass, percent by volume, or kg/m² of exposed area. For a material of known dry density, volume-based and mass-based percentages can be converted arithmetically. However, values obtained by different methods, such as total immersion, partial immersion, and diffusion, are not interchangeable and must always be quoted together with the test method.

What are typical acceptance values?

Typical declared requirements include long-term total immersion values of 0.7 %, 2.0 %, or 5.0 % by volume for XPS in EN 13164, short-term partial immersion values of no more than 5 kg/m² for water-repellent mineral wool in EN 13162, and low single-digit percentages by volume for closed-cell elastomeric and phenolic foams after 96 h immersion. Acceptance limits always depend on the product standard and the application, so the declared class should be verified case by case.

How long does testing take?

Short-term tests require about one to two days of exposure plus conditioning, ISO 2896 immersion takes four days, EN 12087 requires 28 days of immersion, and diffusion tests may extend up to 28 days. Including sampling, conditioning, and reporting, accredited third-party laboratories typically quote one to six weeks depending on the method.