Water Absorption Test for Thermal Insulation Materials: Standards, Procedures, and Applications

Glass wool, rock (mineral) wool, and stone wool products absorb water primarily through capillary wicking within their fibrous network. Short-term partial immersion testing per EN 1609 and humidity sorption testing per ASTM C1104 are the dominant methods for this family, because unrestricted full immersion of very low-density fibrous products can trap water mechanically rather than reflect true material behavior.

Rigid Inorganic and Natural Insulation Materials

Cellular (foam) glass, calcium silicate, expanded perlite board, vermiculite, microporous silica, cork, and wood-fiber boards are also evaluated for water uptake. These materials are commonly tested by the long-term full immersion and diffusion methods of EN 12087 and EN 12088, and the measured values feed directly into product classification under the harmonized European product standards for building insulation.

Testing Items

Depending on the material type and end use, the water absorption determination experiment may include the following test items:

• Short-term water absorption by partial immersion, expressed in kg/m², per EN 1609 and ISO 29767.
• Long-term water absorption by full immersion, expressed as a percentage by volume, per EN 12087.
• Long-term water absorption by vapor diffusion, expressed as a percentage by volume, per EN 12088.
• Water absorption of rigid cellular plastics by immersion, per ISO 2896, GB/T 8810, and ASTM D2842.
• Water sorption of mineral fiber insulation under elevated temperature and high humidity, per ASTM C1104.
• Water absorption of core materials for structural sandwich panels, per ASTM C272.

For faced, laminated, or coated products, testing can be performed on the product as supplied, and the influence of the facer on water uptake is documented in the test report.

Testing Methods

Specimen Preparation and Conditioning

Specimens are cut from representative production samples, free of visible defects, with edges straight and perpendicular. Board products for partial immersion are typically prepared at 200 mm by 200 mm with full product thickness, while cellular plastics are cut to the smaller rectangular blocks specified in ISO 2896, GB/T 8810, or ASTM D2842. A sufficient number of specimens, usually at least three, is tested to characterize variability. Before immersion, specimens are conditioned to constant mass, commonly at 23 ± 2 °C and 50 ± 5 % relative humidity, or dried at 105 °C where the standard and material allow, and the initial mass m1 and specimen dimensions are recorded.

Short-Term Partial Immersion Method

In the partial immersion procedure of EN 1609, the specimen is placed with its exposed face positioned 10 mm below the water surface and kept there for 24 hours at a controlled water temperature. The specimen is then removed, drained in a defined posture for a standardized period of about 10 minutes so that adhering surface water is eliminated without extracting water from the pores, and reweighed to obtain m2. Water absorption Wp is calculated per unit area: Wp = (m2 - m1) / A, where A is the exposed area in square meters. Typical classification limits for many mineral wool products are on the order of 1.0 kg/m² or less.

Long-Term Full Immersion Method

The full immersion method of EN 12087 submerges the specimen completely, held approximately 50 mm below the water surface by a perforated plate or grid, for a standard duration of 28 days. For low-density materials that tend to float, a floating variant with defined restraint is applied. The specimen is drained and reweighed at the end of the exposure, and the result WS is expressed as water absorption in percent by volume, which is the classification parameter used in the EN 13162 to EN 13171 series of product standards.

Immersion Method for Cellular Plastics

Following ISO 2896, GB/T 8810, and ASTM D2842, rigid cellular plastic specimens are submerged under about 50 mm of water, restrained by a grid or sinker, for 96 hours at 23 ± 2 °C. After immersion, surface water is removed by shaking and blotting with a lint-free cloth, and the specimen is weighed. ASTM D2842 additionally applies a cut-cell correction, either by testing companion specimens with sealed surfaces or through geometric correction factors, so that water entering cells opened by sawing does not overstate the true material absorption.

Calculation and Expression of Results

The primary expressions used in reporting are as follows:

• Water absorption by volume: Wv = (m2 - m1) / V × 100, in percent, where V is the specimen volume.
• Water absorption by mass: Wm = (m2 - m1) / m1 × 100, in percent.
• Water absorption per unit area (partial immersion): Wp = (m2 - m1) / A, in kg/m².

The test report should state the standard applied, specimen dimensions and number, conditioning, immersion depth, water temperature, duration, individual and mean results, and any deviations such as facer retention or cut-cell correction. Key experimental factors that influence reproducibility include drainage time and posture, water temperature, specimen geometry, surface skin integrity, and whether cut surfaces are sealed.

Applications

Water absorption data from this experiment are used throughout the insulation industry for compliance, design, and quality assurance:

• Product certification and conformity marking of building insulation under EN, GB, and ISO-based regulatory frameworks, verified by accredited third-party laboratories.
• Material selection for inverted roofs, plinth and perimeter insulation, and below-grade applications where products are permanently exposed to liquid water.
• Specification of pipe and vessel insulation for chilled-water lines, refrigeration, and cryogenic services, where water ingress causes icing, energy loss, and corrosion under insulation.
• Marine, offshore, and exterior facade systems, where cyclic rain, condensation, and splash demand validated moisture resistance.
• Factory quality control and research and development, including evaluation of facings, coatings, blowing-agent systems, and hydrophobic additives.
• Input for hygrothermal building simulation models that predict long-term moisture behavior of envelope assemblies.

FAQ

Why is water absorption often expressed per unit volume rather than per unit mass?

For closed-cell and low-density materials, percentage-by-mass values can be misleadingly large or small depending on density, while the volume percentage directly reflects how much of the insulating pore space has been replaced by water, which is the parameter that correlates with thermal performance loss and product classification.

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

Short-term partial immersion (EN 1609, 24 hours, 10 mm depth) simulates brief rain or splash exposure and yields kg/m² values; long-term full immersion (EN 12087, 28 days, complete submersion) simulates persistent water contact and yields volume percentage values, providing a more demanding assessment of moisture resistance.

How does absorbed water affect insulation performance?

Water has a thermal conductivity of about 0.6 W/(m·K) compared with roughly 0.026 W/(m·K) for still air, so even a few percent of water by volume can raise the effective thermal conductivity substantially, in addition to increasing weight and the risk of freeze damage and corrosion.

Can faced or laminated insulation products be tested?

Yes. Specimens are generally tested as supplied, with the facer intact, because the facing is an integral part of the product's moisture protection. The test report must clearly document whether facings were retained, since removal would change the exposed surface and the result.

How long does the experiment take?

Short-term tests require about 24 hours of immersion plus conditioning and weighing, typically a few days overall; full immersion tests require 28 days of exposure plus conditioning, so several weeks should be scheduled for certification purposes.