Hydrophobicity rate analysis is a critical quality-control test that quantifies how effectively thermal insulation materials such as rock wool, glass wool, and polymer foams repel water and preserve their thermal performance in humid or wet service environments. This article explains the hydrophobicity testing principle, the governing standards such as GB/T 10299-2011, EN 1609, and ASTM D570, specimen preparation, calculation methods, and typical applications across construction and industry. It also provides practical guidance for engineers, specifiers, and quality teams selecting insulation for moisture-exposed installations.
Technical Overview
The hydrophobicity rate (also called the water repellency rate) is a key performance index for thermal insulation materials. It expresses, as a percentage, the proportion of water deposited onto the surface of a specimen that is repelled rather than absorbed into the material matrix. A high hydrophobicity rate indicates that water beads up and drains off the insulation instead of wicking into pores, fibers, or cell structures.
Maintaining a high hydrophobicity rate is essential because liquid water has a thermal conductivity of approximately 0.6 W/(m·K), roughly 20 times that of still air. Even small amounts of absorbed moisture can significantly degrade the effective thermal conductivity of insulation, increase structural loads, promote freeze–thaw damage, accelerate corrosion under insulation (CUI) on pipes and vessels, and encourage mold growth on adjacent building components.
In fibrous products such as rock wool and glass wool, hydrophobicity is engineered by dosing silicone-based water-repellent agents (silanes and siloxanes) into the fiberization or binder system. Closed-cell organic foams are inherently water resistant, while cut or damaged faces, joints, and aging can still allow water uptake through wicking and capillary action. Chinese product specifications for hydrophobic mineral wool products, such as GB/T 11835 and GB/T 25975, and the corresponding European standards in the EN 13162 family, commonly require a hydrophobicity rate of 98 percent or higher, with premium grades achieving 99 percent or more.
- Moisture ingress can raise the apparent thermal conductivity of insulation by tens of percent.
- Water retention adds dead weight and may cause sagging or settlement of blankets and boards.
- Trapped moisture drives corrosion of steel substrates and supports microbial growth.
- Hydrophobic treatment must be verified batch by batch because binder and additive dosing varies in production.
Testing Samples
Mineral Wool Products (Rock Wool and Glass Wool)
Rock wool boards, blankets, and pipe sections conforming to GB/T 11835, GB/T 25975, EN 13162, ASTM C612, or JIS A 9504, together with glass wool products per GB/T 13350, ASTM C553, or JIS A 9505, are the most frequently tested materials. Their open, fibrous structure makes them the primary target of hydrophobic treatment, so the hydrophobicity rate is a routine release test and type test for these products.
Organic Foam Insulation (PUR/PIR, Phenolic, EPS, XPS)
Rigid polyurethane, polyisocyanurate, phenolic foam, expanded and extruded polystyrene boards are naturally water repellent in their closed-cell core, but facings, cut edges, and aging can affect surface wetting behavior. These materials are more commonly evaluated through immersion-based water absorption tests in combination with surface wettability checks.
Inorganic and Advanced Insulation Materials
Cellular glass (EN 13167, ASTM C552), expanded perlite boards (EN 13169, ASTM C728), vermiculite, calcium silicate, and silica aerogel blankets are also assessed for water-related behavior. For these materials, hydrophobicity testing is often supplemented with long-term immersion or humidity-aging tests, since service conditions differ from short surface-spray exposure.
Specimen Preparation and Conditioning
For the spray method, representative flat specimens of approximately 300 mm × 150 mm are cut at the product's original thickness using tools that do not crush the surface or pull out fibers. At least three specimens should be tested per sample. Mineral wool specimens are typically dried to constant mass at 105 °C ± 5 °C, while polymer foams are conditioned at 23 °C ± 2 °C and 50% ± 5% relative humidity. The face intended to be exposed to water in service should be marked as the test face, and cut edges are left unsealed unless the product specification states otherwise.
Testing Items
A complete moisture-performance evaluation of insulation materials typically includes the following test items, selected according to the relevant product standard and the intended application:
- Hydrophobicity rate H (%), determined by the spray method per GB/T 10299-2011.
- Short-term water absorption by partial immersion Wp (kg/m²), per EN 1609.
- Long-term water absorption by full immersion WL, per EN 12087, ISO 2896, or ASTM D570.
- Water absorption of preformed pipe insulation, per ASTM C1514.
- Water sorption of mineral fiber insulation after hot–humid aging, per ASTM C1104.
- Water vapor transmission rate, per ASTM E96 or ISO 12572, to characterize moisture buffering and drying behavior.
- Surface wettability and static contact angle, per ASTM D5946 or ISO 15989, as a complementary indicator.
- Thermal conductivity in the dry state and after moisture exposure, per GB/T 10294, GB/T 10295, ASTM C518, ISO 8301/8302, or EN 12667, to quantify the performance penalty of wetting.
Testing Methods
Hydrophobicity Rate by the Spray Method (GB/T 10299-2011)
The spray method is the reference procedure for the hydrophobicity rate. The apparatus consists of a calibrated spray nozzle, a specimen holder, and a water metering system. A defined quantity of water, approximately 1 L, is sprayed over the inclined test face of the conditioned specimen at a standardized spray height and controlled flow rate, simulating exposure to driving rain or condensation runoff.
- Dry or condition the specimen, then weigh it to obtain the initial mass m1 using a balance with 0.01 g resolution.
- Mount the specimen with the designated face upward on the inclined holder beneath the nozzle.
- Spray the specified volume of water uniformly over the specimen surface.
- Allow free water to drain and gently blot adhering droplets from the surface without pressing water into the material.
- Reweigh the specimen to obtain the final mass m2 and calculate the hydrophobicity rate.
The hydrophobicity rate is calculated as: H = (1 − (m2 − m1) / (ρw × V)) × 100%, where m1 and m2 are the specimen masses in grams before and after spraying, ρw is the density of water (approximately 1.0 g/mL), and V is the volume of water sprayed in milliliters. For example, if 1000 mL of water is sprayed and the specimen gains 1.5 g, the hydrophobicity rate is (1 − 1.5/1000) × 100% = 99.85%. Results are reported as the average of the specimens, normally rounded to 0.1%.
Key influence factors that laboratories must control include the uniformity of the spray pattern, water temperature, completeness of pre-drying, loss of fibers or dust during handling, the presence of faced or bonded surfaces, and consistency in blotting technique. Specimens should be transported and stored so that surfaces remain uncontaminated by oils, dust, or compaction.
Short-Term Water Absorption by Partial Immersion (EN 1609)
In this method, a 200 mm × 200 mm specimen is placed in a tray and immersed in water to a depth of about 10 mm above its lower face for 24 hours. After draining and removal of surface water, the mass gain per unit area Wp = (m2 − m1) / A is reported in kg/m². This test evaluates capillary suction through the underside, which is representative of occasional rain exposure or condensation on the cold face, and typical classification limits for mineral wool products are on the order of 1 kg/m² or less.
Water Absorption by Full Immersion (EN 12087, ISO 2896, ASTM D570)
Full immersion methods submerge the whole specimen, typically for 24 hours or for extended durations in long-term variants, and report water uptake as a percentage by volume or by mass. EN 12087 covers building insulation products, ISO 2896 covers rigid cellular plastics, and ASTM D570 covers plastics generally. These methods are more severe than the spray test and are used to characterize materials intended for intermittent or permanent contact with liquid water.
Surface Wettability and Contact Angle
Static contact angle measurement with a sessile water droplet provides a rapid, sensitive indication of surface treatment quality. Surfaces with contact angles above 90 degrees are classified as hydrophobic, while values above 150 degrees indicate superhydrophobic behavior. A simple qualitative check, observing whether applied droplets bead and roll off or spread and darken the surface, is widely used for incoming inspection, although it cannot replace the standardized spray method for certification.
Quality Control and Reporting
Testing should be performed with calibrated balances, verified spray assemblies, and documented environmental conditions. For certification, type testing, and dispute resolution, accredited third-party laboratories operating under ISO/IEC 17025 quality systems provide traceable results. Test reports should identify the product and batch, specimen orientation and test face, conditioning procedure, water volume sprayed, individual and average hydrophobicity rates, and the applicable acceptance criterion.
Applications
Hydrophobicity rate analysis supports material selection, product qualification, and quality assurance across the building and industrial sectors:
- External thermal insulation composite systems (ETICS), ventilated facades, and curtain-wall insulation, where rock wool boards face rain and condensation during construction and service.
- Flat and pitched roof insulation, where intermittent wetting is possible before and after membrane installation.
- HVAC and chilled-water pipe insulation, where condensation on cold surfaces would otherwise saturate the insulation and destroy its thermal resistance.
- Petrochemical and power plants, where hydrophobic mineral wool pipe sections and vessel lagging help mitigate corrosion under insulation beneath cladding.
- Marine, offshore, and tunnel applications with high humidity and salt-laden atmospheres.
- Factory quality control of hydrophobic agent dosing, binder formulation, and production consistency for mineral wool lines.
- Research and development of new water-repellent treatments, facings, and aerogel or hybrid insulation composites.
- Failure diagnostics on aged or water-damaged insulation removed from service, supporting repair decisions and warranty assessment.
FAQ
What is the hydrophobicity rate and how is it calculated?
The hydrophobicity rate is the percentage of the water sprayed onto a specimen that is repelled rather than absorbed. Using the GB/T 10299-2011 spray method, it is calculated as H = (1 − (m2 − m1) / (ρw × V)) × 100%, based on the specimen mass gain relative to the volume of water applied.
What is the typical acceptance criterion?
Most Chinese and European specifications for hydrophobic mineral wool products require a hydrophobicity rate of at least 98%, and many manufacturers target 99% or higher. Project specifications may impose stricter limits, or alternatively specify short-term water absorption limits per EN 1609.
How does hydrophobicity rate differ from water absorption testing?
The hydrophobicity rate measures rejection of surface-sprayed water under a short, standardized exposure, whereas immersion tests such as EN 12087, ISO 2896, and ASTM D570 measure total water uptake under sustained contact, including capillary uptake through cut edges. The two families of tests are complementary and are often reported together.
Does a high hydrophobicity rate mean the insulation is waterproof?
No. Hydrophobic treatment prevents wetting of fibers or surfaces, but mineral wool remains an open, vapor-permeable structure, and no insulation product substitutes for a waterproofing membrane. Systems exposed to standing water or hydrostatic pressure require appropriate membranes, cladding, or closed-cell materials with verified long-term immersion performance.
Does hydrophobic treatment affect fire performance or thermal conductivity?
The silicone-based agents are applied in very small dosages, so their effect on fire classification and dry thermal conductivity is negligible. The purpose of the treatment is to preserve the dry-state thermal conductivity when the material encounters moisture.
How often should hydrophobicity be tested?
Hydrophobicity rate is typically verified as a type test during product certification, at defined intervals during continuous production, and on a batch basis whenever raw materials, binder formulations, or process conditions change. Independent verification by accredited third-party laboratories is recommended for certification and for high-risk projects such as petrochemical or marine installations.
