Water absorption by volume is one of the most decisive quality indicators for foam glass (cellular glass) thermal insulation, directly governing long-term thermal performance, freeze–thaw durability and resistance to moisture-driven degradation in wet, buried or cryogenic service environments. This article explains the principle, sampling, procedures and calculation methods for volume water absorption testing in accordance with internationally recognized standards such as ASTM C240, ASTM C552, EN 12087, ISO 2896 and the GB/T 5486 series, together with typical acceptance criteria and practical applications.
Technical Overview
Foam glass, also known as cellular glass, is an inorganic rigid thermal insulation material manufactured by mixing finely crushed glass with a carbon-based foaming agent and firing the mixture at approximately 750–900 °C. During firing, the expanding gas generates millions of hermetically sealed, non-interconnecting cells within a glass skeleton. Because glass itself is impermeable and non-hygroscopic, water can only enter the few cells that are opened when the product is cut, which is why foam glass exhibits extremely low water uptake—typically 0.5 % by volume or less.
Water absorption by volume is defined as the volume of water absorbed by a dried test specimen after immersion under standardized conditions, expressed as a percentage of the geometric volume of the dry specimen. The parameter is critical for design and specification: absorbed water dramatically increases thermal conductivity (water has a thermal conductivity of about 0.6 W/(m·K), roughly ten times that of dry foam glass), adds dead weight, promotes freeze–thaw damage, accelerates corrosion under insulation on pipework, and can degrade the long-term dimensional stability of the insulation system.
Testing is performed using recognized methods published by standards organizations such as ASTM International (ASTM C240 test methods and ASTM C552 specification for cellular glass thermal insulation), CEN (EN 12087 for long-term water absorption by immersion, with EN 13167 governing factory-made cellular glass products), ISO (ISO 2896 for rigid cellular plastics), and the GB/T 5486 series used in China for inorganic rigid thermal insulation products. Accredited third-party laboratories routinely apply these methods for type testing, factory production control and compliance verification.
Testing Samples
Boards and Blocks
Flat specimens are cut from finished boards or blocks at full product thickness. Common specimen formats include approximately 100 mm × 100 mm pieces referenced in the GB/T 5486 series, 150 mm × 150 mm plates as used in ISO 2896, and 200 mm × 200 mm specimens prescribed by EN 12087. A minimum of three to five specimens is recommended for each production batch or test report. Cut faces must be flat, square and free from cracks, chipping and loose glass dust, since damaged edges can artificially increase the measured uptake.
Pipe Sections and Half-Shells
For preformed cellular glass pipe insulation, arc-shaped segments are cut along the axis of the pipe section and tested at the manufactured wall thickness. The specimen volume is calculated from the measured inside and outside diameters and the axial length, for example as π/4 × (OD² − ID²) × L. Curved surfaces must be measured at several positions to reduce geometric error, because the calculation of volume water absorption depends directly on the accuracy of the specimen volume.
Specimen Conditioning and Pre-Treatment
Before immersion, all specimens are dried in a ventilated oven at 105 °C ± 5 °C until constant mass is reached, indicated by two successive weighings at least one hour apart differing by less than 0.1 %. The dried specimens are cooled to ambient laboratory temperature in a desiccator and then weighed to obtain the dry mass m₁, using a balance with a readability of 0.01 g or better.
Testing Items
Depending on the specification and intended application, the following determinations are typically carried out:
- Short-term water absorption by volume after full immersion, generally 24 hours, used for routine quality control and product release testing.
- Long-term water absorption by full immersion over 28 days, used for type testing and applications exposed to permanently wet conditions.
- Long-term water absorption by partial immersion, which evaluates capillary uptake at the exposed face and is relevant for inverted roofs, below-grade walls and ground-contact applications.
- Companion determinations supporting the calculation, including dimensional measurement of specimen volume, dry density, and periodic intermediate weighings at 24 h, 48 h, 7 d and 28 d to monitor saturation behavior.
Testing Methods
Principle and Governing Equation
The test compares the mass of a dry specimen with its mass after a defined period of immersion. The mass gain, converted to water volume using the density of water, is divided by the geometric volume of the dry specimen. The water absorption by volume is calculated as: WA(v) = [(m₂ − m₁) / (ρw × V)] × 100 %, where m₁ is the dry mass, m₂ is the mass after immersion and surface drying, ρw is the density of water (approximately 1000 kg/m³) and V is the geometric volume of the specimen.
Apparatus and Test Conditions
Required equipment includes an immersion tank of sufficient depth, a submersion cage or weighting device that keeps specimens at least 50 mm below the water surface, distilled or deionized water maintained at 23 °C ± 2 °C (or the temperature prescribed by the governing standard), a forced-ventilation drying oven, a calibrated balance, calipers with 0.05 mm to 0.1 mm resolution, an inclined draining rack and a lint-free damp cloth for removing free surface water. ASTM C240 specifies immersion of dried cellular glass specimens under a fixed head of water for a prescribed short period, while EN 12087 and ISO 2896 define the 24-hour short-term and 28-day long-term full immersion regimes.
Step-by-Step Procedure
- Measure the length, width and thickness of each specimen at several positions and compute the geometric volume V.
- Dry the specimen at 105 °C ± 5 °C to constant mass, cool it in a desiccator and record the dry mass m₁.
- Immerse the specimen fully in the water bath, ensuring at least 50 mm of water above its uppermost surface, and start timing once it is completely submerged.
- After the specified period (24 h for short-term methods, up to 28 days for long-term methods), remove the specimen and allow it to drain briefly on the inclined rack.
- Blot all faces and cut edges with a damp cloth within a fixed, consistent time to remove free surface water without drawing water out of the open cells.
- Weigh the specimen immediately to obtain m₂; for long-term tests, repeat the weighing sequence at the prescribed intervals, always returning the specimen to the bath between weighings.
- Calculate the water absorption by volume for each specimen and report individual values, the arithmetic mean and the maximum value.
Calculation, Reporting and Acceptance Criteria
Results are rounded to 0.1 % by volume and reported together with the immersion duration, water temperature, drying and conditioning details, specimen dimensions and orientation. Under ASTM C552, cellular glass thermal insulation is typically required to exhibit a maximum water absorption of 0.5 % by volume when tested in accordance with ASTM C240, and comparable thresholds are applied in Chinese product specifications. Values above the limit generally indicate interconnected porosity, internal fissures, incomplete cell closure or specimen preparation artifacts. In long-term immersion testing, a well-manufactured cellular glass shows a rapid initial uptake limited to cut surface cells followed by a plateau, whereas continuing uptake over time reveals transport pathways through the material.
Applications
Volume water absorption data for foam glass insulation are used across many engineering and regulatory contexts:
- Inverted roofs, plaza decks and green roof systems, where the insulation is permanently exposed to water and must retain its thermal resistance.
- Below-grade walls, foundations and under-slab insulation in constant contact with soil moisture and groundwater.
- District heating and chilled water pipe networks, where low uptake prevents thermal drift and mitigates corrosion under insulation.
- LNG and cryogenic tank foundations, load-bearing insulation and cold storage floors, where absorbed moisture would cause freeze–thaw degradation and ice lensing.
- Marine, offshore and chemical plant environments with sustained humidity and water splash exposure.
- Product certification, factory production control and type testing under EN 13167, ASTM C552 and the relevant GB product standards, as well as supporting documentation for green building and energy code compliance prepared by accredited third-party laboratories.
FAQ
What does water absorption by volume mean for foam glass insulation?
It is the percentage of the specimen's geometric volume that becomes occupied by water after immersion under standardized conditions. A value of 0.5 % means that only half of one percent of the block's volume is water after the prescribed immersion, confirming that the cell structure is essentially closed and the insulation will not take on moisture in service.
Why is foam glass able to achieve such low water absorption compared with organic foams?
Because the material consists of glass-walled cells that are sealed off from one another during the foaming process. Water can only lodge in the thin layer of cells exposed by sawing, whereas open or partially open cell structures in many polymeric foams allow deeper capillary penetration over time.
Which standards govern the test?
Commonly applied methods include ASTM C240 with the acceptance limits of ASTM C552, EN 12087 for long-term water absorption by immersion, ISO 2896 for rigid cellular materials and the GB/T 5486 series for inorganic rigid insulation products. The governing standard should always be stated in the test report because specimen size, immersion period and water temperature differ between methods.
How long does the test take?
A short-term full immersion test requires about 24 hours of immersion plus drying and conditioning time, so results are typically available within two to three days. Long-term immersion testing requires 28 days of immersion and additional time for intermediate weighings and final evaluation.
What can cause falsely high results?
Cracked or chipped edges, insufficient drying before the initial weighing, residual free water on the surface at final weighing, inaccurate volume measurement, and immersion temperatures or depths outside the specified range are the most common sources of error. Consistent blotting technique and calibrated measuring equipment are essential for repeatable results.
How are the results used in thermal design?
Designers rely on low verified water absorption to justify using the declared thermal conductivity over the service life of the construction, to select insulation for wet or buried applications where other materials would lose performance, and to demonstrate compliance with product standards and building energy codes.