Water absorption by mass is a critical quality and durability index that determines how thermal insulation materials behave when exposed to moisture, directly affecting thermal conductivity, weight gain, and long-term service performance. This article explains the testing principles, sample requirements, test methods, applicable standards such as EN ISO 16535, EN 1609, ASTM D570 and GB/T 5480, calculation formulas, and typical applications for mineral wool, polystyrene, polyurethane and other insulation products. Read on to understand how accredited third-party laboratories evaluate the moisture resistance of thermal insulation materials.
Technical Overview
Water absorption by mass quantifies the amount of water that a thermal insulation material takes up when immersed in water under strictly controlled conditions, expressed as a percentage of the mass of the dry specimen. Because the thermal conductivity of virtually every insulation material rises steeply as moisture displaces air from the pore structure, water absorption is one of the most important quality and durability indicators for products used in roofs, walls, foundations, and industrial installations.
The test is destructive and is performed on representative specimens cut from finished products. Depending on the selected procedure, specimens are either fully immersed in water (total immersion, simulating flooding or standing water) or brought into contact with water on one face only (partial immersion, simulating rain, splash, or ground moisture). After a defined exposure period, surface water is removed in a standardized manner, the specimen is re-weighed, and the absorbed water mass is calculated.
The principal international and national references include EN ISO 16535 (which superseded EN 12087) and EN 1609 for building insulation products, ISO 2896 and ASTM D570 for cellular plastics, ASTM C209 for cellular and massive insulation in North American practice, and the Chinese national standards GB/T 5480 for mineral wool products and GB/T 8810 for rigid cellular plastics. Harmonized European product standards (EN 13162 through EN 13171) use water absorption results as declared performance characteristics for CE marking.
Testing Samples
Typical product families submitted for water absorption by mass testing include the following categories:
Mineral Wool Products (Glass Wool and Stone Wool)
Boards, mats, felts, and pipe sections made from glass wool or rock wool have an open fibrous structure, so their water resistance depends heavily on the hydrophobic treatment of the binder. Immersion testing verifies the effectiveness and durability of the water-repellent additive system.
Expanded and Extruded Polystyrene (EPS and XPS)
These closed-cell foams are widely used in inverted roofs, below-grade insulation, and perimeter applications where prolonged water contact is expected. XPS products in particular are validated by long-term total immersion testing, while EPS boards are frequently assessed using cyclic immersion procedures.
Rigid Polyurethane and Polyisocyanurate Foams (PUR/PIR)
Sandwich panel cores, board stock, and spray-applied foams are tested to confirm the integrity of the closed-cell structure. Where composite facings are present, specimens are usually tested as delivered, since facings significantly influence water ingress.
Flexible Elastomeric Foam
Closed-cell rubber foam used for HVAC ductwork and chilled-water piping is tested to ensure that the material maintains low water uptake in condensing environments.
Inorganic and Specialty Insulation
Calcium silicate, expanded perlite, vermiculite, microporous boards, and aerogel blankets are evaluated to confirm suitability for humid industrial, marine, and high-temperature service conditions.
Testing Items
Core and ancillary test items typically offered under a water absorption testing program include:
- Short-term water absorption by partial immersion (24 hours), per EN 1609 and EN ISO 16535 partial immersion procedures
- Long-term water absorption by total immersion (28 days), per EN ISO 16535 (formerly EN 12087)
- Long-term water absorption by partial immersion for ground-contact applications
- Cyclic immersion testing for moisture-resistant molded boards such as EPS
- Water absorption of rigid cellular plastics per ISO 2896, ASTM D570, and GB/T 8810
- Water absorption of mineral wool products per GB/T 5480
- Conversion of mass-based results to water absorption per unit area (kg/m²) for product classification purposes
- Supporting determinations such as dry mass, dimensions, density, and moisture content of as-received samples
Testing Methods
Specimen Preparation and Conditioning
Specimens are cut from representative positions of the finished product, normally at full product thickness, with common specimen dimensions of 200 mm × 200 mm for European procedures and a minimum of three to five replicates per test. Loose fibers and dust are removed, and specimens are conditioned to constant mass at a controlled atmosphere (typically 23 °C) or dried to constant mass where the standard requires an oven-dry reference. The dry mass (m1), dimensions, and density of each specimen are recorded before immersion.
Total Immersion Method — Long-Term Water Absorption
Specimens are completely submerged in water at a controlled temperature of approximately 23 °C for the standard long-term period, normally 28 days. Floating products are held below the water surface by a perforated device that does not compress the specimen. After the immersion period, each specimen is removed, allowed to drain vertically for a standardized short period of about two minutes so that surface water is shed, and immediately weighed to obtain the wet mass (m2). This procedure corresponds to EN ISO 16535 total immersion and reflects conditions such as flooding, buried insulation, or inverted roof assemblies.
Partial Immersion Method — Short-Term Water Absorption
Specimens are placed with only the bottom face in contact with a shallow depth of water for 24 hours, simulating rain exposure, splash, or capillary ground moisture, in accordance with EN 1609 and the partial immersion procedures of EN ISO 16535. After exposure, specimens drain vertically for approximately ten minutes before weighing. Results are reported as water absorbed per unit area (kg/m²), which is the basis for classification levels such as WSt, WSp, WL(P), and WL(T) declared under the harmonized European product standards.
Cyclic and Material-Specific Procedures
For cellular plastics, ASTM D570 specifies 24-hour immersion at 23 °C on rectangular specimens with surface water removed by blotting before weighing, while ISO 2896 and its Chinese counterpart GB/T 8810 typically employ a longer immersion of about 96 hours on specified slabs kept submerged by a holding device. ASTM C209 addresses water absorption of cellular and massive insulation in North American practice. Cyclic immersion procedures, involving repeated immersion and drainage sequences, are applied to certain moisture-resistant boards to assess behavior under fluctuating wetting conditions.
Calculation and Expression of Results
Water absorption by mass is calculated as the percentage increase in specimen mass:
Wm = [(m2 − m1) / m1] × 100 %
where m1 is the dry mass and m2 is the mass after immersion and standardized drainage. The individual values and the mean are reported, and where required the result is converted to water absorption per unit area using the exposed specimen area. Because low-density open-cell materials can absorb several hundred percent by mass, product specifications frequently rely on the area-based value (kg/m²) for classification and comparison across thicknesses.
Applications
Water absorption by mass testing supports a wide range of engineering, regulatory, and commercial purposes:
- Compliance demonstration and declaration of performance for building insulation under harmonized standards and CE marking requirements
- Factory production control and batch release testing for insulation manufacturers
- Qualification of inverted roof, green roof, below-grade, and foundation insulation subject to sustained wetting
- Verification of hydrophobic treatment effectiveness in mineral wool production
- Material selection for district heating pipe systems, cold storage, chilled pipework, and LNG facilities
- Marine, offshore, and industrial installations where condensation and water exposure are unavoidable
- Research and development of closed-cell structures, facings, coatings, and water-repellent additives
- Failure analysis and condition assessment of insulation removed from service
- Third-party verification for buyers, contractors, insurers, and regulatory authorities
FAQ
What is water absorption by mass and why is it important?
It is the mass of water absorbed by an insulation specimen under standardized immersion conditions, expressed as a percentage of the dry specimen mass. It is important because even small amounts of moisture measurably increase thermal conductivity, add structural load, promote freeze-thaw damage, corrosion, and microbial growth, and reduce the service life of the insulated assembly.
Which standard should be used for my product?
The choice depends on the material and target market. European building products are generally tested to EN ISO 16535 or EN 1609; cellular plastics to ISO 2896, ASTM D570, or GB/T 8810; mineral wool in the Chinese market to GB/T 5480; and cellular or massive insulation for North American projects to ASTM C209. Accredited third-party laboratories can confirm the correct method against the governing product specification.
How long does the test take?
Short-term partial immersion requires 24 hours of exposure plus conditioning and weighing, with typical turnaround of a few working days. Long-term total immersion requires 28 days of exposure plus conditioning, so a complete report usually requires five to six weeks.
What is the difference between mass-based and area-based results?
Mass-based water absorption (percentage) relates absorbed water to dry specimen mass, while area-based results (kg/m²) relate absorbed water to the exposed surface area. Open-cell mineral wools can show very high mass percentages while remaining compliant, so harmonized product standards classify performance using kg/m² levels.
How many specimens are required and how are they taken?
Most standards require at least three replicate specimens, cut at full product thickness from representative areas of the product and away from damaged edges. Faced or laminated products are normally tested as delivered unless the specification directs removal of the facing.
Which factors influence the test result?
Key factors include cell structure (open versus closed cell), density, the effectiveness of hydrophobic additives, the presence and integrity of facings, specimen edge effects, water temperature, and consistency in drainage time. Strict adherence to the standardized procedure is therefore essential for reproducible results.
How can manufacturers reduce water absorption?
Common measures include optimizing closed-cell content, applying water-repellent additives during production, using impermeable facings or skins, protecting cut edges, and maintaining rigorous factory quality control verified through regular immersion testing by accredited third-party laboratories.