Evaluation of Water Absorption Test Results for Thermal Insulation Materials

Water absorption is one of the most decisive quality indicators for thermal insulation materials, because even small amounts of absorbed moisture sharply increase thermal conductivity and accelerate long-term deterioration of the insulation system. This article provides a professional framework for evaluating water absorption test results on mineral wool, EPS, XPS, and PUR/PIR foams, covering EN 12087, EN 12088, EN 1609, ISO 2896, ASTM C272, and GB/T 8810 test methods, calculation formulas, product classification levels, and the key factors that influence reported values. Whether you are reviewing a laboratory report for building approval, factory production control, or failure analysis, the evaluation criteria below will help you interpret the numbers correctly.

Technical Overview

Water absorption quantifies the capacity of an insulation material to take up liquid water under controlled exposure conditions such as partial immersion, full immersion, or prolonged exposure to water vapor. Depending on the governing standard, the result is expressed as a percentage by volume, a percentage by mass, or as water uptake per unit exposed area in kg/m². Because water has a thermal conductivity of approximately 0.6 W/(m·K), roughly twenty times that of stationary air, absorbed moisture can dramatically degrade the insulating performance of even a high-quality product.

The consequences of excessive water uptake extend beyond thermal losses. Added water increases dead load on roofs and structures, promotes freeze-thaw damage in cold climates, supports mold growth and corrosion of adjacent metal components, reduces dimensional stability, and can destroy buoyancy in flotation applications. For these reasons, water absorption limits are mandatory declared characteristics in most product standards for factory-made insulation, including the EN 13162 to EN 13167 series for building products, and corresponding GB standards for the Chinese market.

The principal test methods are EN 1609 (short-term water absorption by partial immersion), EN 12087 (long-term water absorption by full immersion), EN 12088 (long-term water absorption by diffusion), ISO 2896 and GB/T 8810 (immersion of rigid cellular plastics with cut-cell correction), ASTM C272 (sandwich panel cores), and ASTM D570 (plastics). Type testing is typically performed by accredited third-party laboratories, and the resulting values must always be evaluated against the specific method used and the declared class of the product.

Testing Samples

Mineral Wool Products (Glass Wool and Rock Wool)

Mineral wool has a fibrous, open structure in which water is transported primarily by capillary action between fibers. Test specimens are typically 200 mm × 200 mm at the full delivered product thickness, cut so that both faces represent the product as supplied. Specimens are conditioned or dried to constant mass before the initial weighing, with the drying temperature selected to avoid damaging the organic binder system. Because fiber orientation and density distribution influence wetting behavior, several specimens taken from different positions are required for a representative evaluation.

Rigid Cellular Foams (EPS, XPS, PUR/PIR)

For expanded polystyrene, extruded polystyrene, and polyurethane or polyisocyanurate foams, water absorption is governed by the closed-cell content (determined separately according to ISO 4590) and by the integrity of molded or extruded skins. Specimens for EN methods are commonly 200 mm × 200 mm at product thickness, while ISO 2896 and GB/T 8810 immersion tests use smaller specimens, commonly 100 mm × 100 mm with a thickness of about 25 mm. Cutting the foam opens cells at the machined surfaces, so the standards prescribe defined drainage procedures and, for ISO and GB methods, a cut-cell volume correction to avoid overstating the true absorption of the material.

Other Insulation Materials

Cellular glass exhibits near-zero water absorption due to its fully closed, non-interconnected cell structure. Hygroscopic and capillary-active materials such as calcium silicate, wood fiberboard, cork, and aerogel or microporous blankets can show substantial uptake and require careful method selection. For sandwich panel cores, ASTM C272 defines specimen dimensions according to core type and density, while faced composite products are usually tested as delivered unless the product standard requires removal of facings.

Testing Items

Depending on the intended application and the governing product standard, one or more of the following determinations are performed and evaluated:

• Short-term water absorption by partial immersion (WS): 24-hour contact of the bottom face with a 10 mm water layer, result in kg/m² according to EN 1609.
• Long-term water absorption by full immersion (WL): 28-day total immersion, result in kg/m² or as a percentage by volume according to EN 12087.
• Long-term water absorption by diffusion (WD): exposure of one face to water vapor for a defined period, result as a percentage by volume according to EN 12088.
• Water absorption of rigid cellular plastics by immersion: typically 96 hours at 23 °C with cut-cell correction, according to ISO 2896 and GB/T 8810.
• Water absorption of sandwich panel cores by immersion or boiling, according to ASTM C272, including 24-hour and 96-hour and boiling variants.
• Water absorption of plastics by 24-hour immersion, long-term immersion, and saturation, according to ASTM D570.
• Supporting determinations such as bulk density, dimensions, and closed-cell content, which are required to express and interpret the results correctly.

Testing Methods

Short-Term Partial Immersion (EN 1609)

The specimen is placed horizontally with its lower face in contact with water at a depth of 10 mm for 24 hours at 23 ± 5 °C. After immersion, the specimen is removed and allowed to drain in a defined vertical position for 10 minutes. The standard provides two weighing variants: one in which adhering surface water is retained on the specimen, and one in which surface water is removed before weighing, which corresponds to the lower declared class levels. The result is calculated as Wp = (m2 - m1) / A, where m1 and m2 are the masses before and after immersion in grams and A is the exposed area in square meters, giving an uptake in kg/m². The reported value is the mean of the required number of specimens, typically three or more.

Long-Term Full Immersion (EN 12087)

EN 12087 defines alternative immersion geometries and water renewal regimes, the most commonly declared being the method in which the specimen is kept permanently immersed approximately 50 mm below the water surface for 28 days. After the exposure period, the specimen is drained for a defined period, optionally has surface water removed, and is weighed. Results are expressed either per unit area, WL = (m2 - m1) / A in kg/m², or as a percentage by volume, WL(V) = (m2 - m1) / (ρw × V) × 100, where ρw is the density of water and V is the specimen volume. This method represents a severe, near-worst-case liquid water exposure and is the basis for most product classification levels.

Long-Term Absorption by Diffusion (EN 12088)

For applications dominated by vapor drive rather than direct liquid contact, such as cold-side insulation and inverted roof assemblies, EN 12088 evaluates uptake by water vapor diffusion. The edges and one face of the specimen are sealed, for example with aluminum foil, so that vapor can enter only through the exposed face, and the assembly is exposed above a water surface under a defined climate for the specified duration, commonly 28 days. The mass gain is converted to a percentage by volume, reported as WD(V). This parameter discriminates between materials that resist vapor transport through their cell structure and those that allow progressive internal condensation.

Immersion of Cellular Plastics (ISO 2896 / GB/T 8810)

Specimens are immersed with their upper surface approximately 50 mm below the water level at 23 ± 2 °C for 96 hours, after which surface water is removed with a defined technique before weighing. The water absorption is calculated as a percentage by volume using the specimen volume corrected for the cells opened by cutting, so that preparation artifacts do not inflate the result. The mass percentage can also be reported, but it is strongly density-dependent and is not suitable for comparing different materials.

General Procedural Requirements Common to All Methods

• Determination of the initial mass m1 after conditioning or drying to constant mass, using a temperature that does not alter the polymer or binder.
• Immersion in clean water with controlled temperature, immersion depth, orientation, and duration as specified by the method.
• A defined drainage period, typically 10 minutes, and a reproducible surface-water removal technique, since these steps dominate the repeatability of the test.
• Weighing on a balance with resolution appropriate to the specimen mass, and measurement of dimensions for the calculation of volume and area under the same atmospheric conditions.

Evaluation of Test Results

Expression and Conversion of Results

• Percentage by volume: Wv = (m2 - m1) / (ρw × V) × 100. This is the preferred basis for comparing the intrinsic wetting resistance of different materials.
• Percentage by mass: Wm = (m2 - m1) / m1 × 100. This basis depends on density; a low-density foam can show a high mass percentage while its volume percentage remains modest.
• Uptake per unit area: WS = (m2 - m1) / A in kg/m². This basis is used for partial immersion results and for mineral wool declarations, because it reflects the water load per exposed face.
• Conversions between the three bases require the bulk density and assume nominally uniform uptake through the specimen; converted values should always be flagged as calculated, not measured.

Comparison with Declared Values and Class Levels

A water absorption number is meaningless without the method and the product-specific declared class against which it is evaluated. Under the EN product standards, mineral wool products (EN 13162) declare short-term absorption WS at levels such as 1 kg/m² or 5 kg/m² and long-term absorption WL at levels such as 2 kg/m², depending on the weighing variant and product type. Extruded polystyrene (EN 13164) declares long-term immersion classes WL(T) typically at 0.7, 1.5, 2.5, or 5 percent by volume, together with diffusion classes WD(V) at levels such as 1, 2, 3, or 5 percent by volume. EPS (EN 13163) and PUR/PIR (EN 13166) declare their own WL(T) and WD(V) levels, commonly up to 5 percent by volume for standard building grades. For projects specified under GB standards, such as GB/T 10801.1 and GB/T 10801.2 for EPS and XPS boards, immersion results measured per GB/T 8810 are compared with grade-dependent limits, commonly between 1 and 5 percent by volume. Compliance is judged on the mean of the specimen set, with individual outliers investigated for cutting damage or non-representative density.

Statistical and Metrological Evaluation

• Report individual specimen values together with the mean, and examine the spread; a high coefficient of variation in fibrous products usually indicates density variation or uncontrolled wetting channels rather than product failure.
• Quantify or control the dominant uncertainty sources: balance resolution, dimension and volume measurement, drainage time repeatability, water temperature, and the operator sensitivity of surface-water removal.
• Verify that specimen dimensions, density, and conditioning meet the tolerance requirements of the method before testing, because deviations propagate directly into the calculated volume percentage.
• Round results as specified by the standard, typically to 0.1 kg/m² or 0.1 percent by volume, and evaluate compliance against the declared class at the stated rounding level.

Factors to Check Before Rejecting a Product

• Cut versus molded surfaces: freshly cut faces of XPS and PUR open surface cells and can multiply the measured uptake relative to the extruded or molded skin.
• Immersion orientation: horizontal and vertical orientations trap air differently and are not directly comparable.
• Aging and storage: thermal or moisture history can alter cell structure and binder condition; facings, skins, and laminations reduce uptake and may be tested as delivered.
• Sealing quality in diffusion tests: incomplete edge sealing produces falsely high WD(V) values, and damaged edges or crushed corners in immersion tests act as preferential water paths.

Engineering Significance of the Evaluated Values

• Long-term immersion results of roughly 0.5 to 1 percent by volume indicate materials suitable for inverted roofs, below-grade walls, and direct freeze-thaw exposure.
• Results in the range of about 2 to 5 percent by volume are generally acceptable for protected or ventilated positions, provided that water-shedding layers and vapor control are designed into the assembly.
• Results above 5 percent by volume, or short-term values above 5 kg/m² for unprotected mineral wool, call for facings, drainage layers, or redesign of the detail.
• Thermal impact: even modest volumetric moisture content can raise the effective thermal conductivity of fibrous insulation by tens of percent, and design methods such as EN ISO 6946 apply moisture correction factors for inverted roofs; water absorption results are therefore an input to the energy performance calculation, not merely a pass or fail attribute.

Applications

Evaluated water absorption data support decisions across design, compliance, and quality control:

• Building envelope design: selecting WS and WL classes for ETICS, cavity walls, pitched roofs, and flat roofs according to the exposure zone and construction detail.
• Inverted roofs and plaza decks: specifying closed-cell boards with long-term immersion values at or below 0.7 percent by volume and low diffusion uptake, combined with the moisture correction design method of EN ISO 6946.
• Below-grade and foundation insulation: using 28-day immersion results together with freeze-thaw durability assessments for permanent ground contact.
• Cold chain, refrigeration, and cryogenic systems: relying on diffusion-controlled uptake values to size vapor retarders and prevent hidden condensation within the insulation layer.
• Sandwich panels and transport equipment: qualifying cores per ASTM C272 for facades, refrigerated vehicles, and marine structures, including boiling variants for severe service.
• Conformity assessment: type testing and factory production control for CE marking under the Construction Products Regulation and for GB-based certification schemes, carried out by accredited third-party laboratories.
• Failure analysis: comparing the uptake and moisture-corrected thermal conductivity of retrieved field specimens with the original declared values to diagnose in-service moisture problems.

FAQ

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

Short-term testing (EN 1609) exposes the bottom face of the specimen to a 10 mm water layer for 24 hours and simulates brief wetting such as rain during construction. Long-term testing (EN 12087) immerses the specimen fully for 28 days and represents sustained contact with liquid water, as in ground contact or water ponding. The two results answer different questions and are declared as separate characteristics in the product standards.

Why are results reported as percent by volume, percent by mass, and kg/m²?

Percent by volume best reflects how much of the insulating pore space has been replaced by water and allows comparison between materials. Percent by mass is dominated by density and is mainly useful for tracking a single product. Uptake per unit area in kg/m² describes the water load on a construction element and is used for mineral wool declarations and partial immersion results. Always confirm which basis a report uses before comparing values.

How long does a complete evaluation take?

Count the conditioning or drying period to constant mass, typically one to seven days, plus 24 hours for partial immersion or 28 days for full immersion and diffusion methods, plus weighing and reporting. A full EN-based type test package for water absorption therefore takes roughly four to five weeks.

Why does extruded polystyrene absorb far less water than expanded polystyrene?

Extrusion produces a continuous closed-cell structure with an integral skin, so liquid water can enter only through cut edges or damaged cells. Molded EPS consists of fused beads, and the inter-bead channels allow slow water migration, which raises the long-term immersion value. Closed-cell content determined per ISO 4590 correlates closely with these differences.

Can results from different standards be compared directly?

No. Duration, immersion depth, orientation, drainage rules, surface-water removal, and cut-cell corrections differ between EN, ISO, ASTM, and GB methods. A GB/T 8810 96-hour value cannot be substituted for an EN 12087 28-day class level; each result is only valid within its own method framework and its associated declared class.

Do facings and laminations affect the result?

Yes. Facings, skins, and coatings substantially reduce uptake and can hide edge ingress if specimens are not sealed or cut as prescribed. Product standards specify whether faced products are tested as delivered or with facings removed, and the test report must state which condition applied.

How do laboratory results relate to real building performance?

Laboratory immersion is a severe screening condition. In service, moisture behavior depends on the interaction of liquid absorption, vapor diffusion, climate exposure, and construction detailing. A sound evaluation therefore combines water absorption class levels with vapor transmission properties, hygrothermal simulation, and the moisture correction procedures of the applicable design standards.