Determination of the Hydrophobicity Rate of Thermal Insulation Materials

Hydrophobicity rate testing measures the ability of thermal insulation materials to resist liquid water penetration, a property that directly protects thermal performance, energy efficiency, and long-term durability in buildings and industrial systems. This article explains the determination of the hydrophobicity rate of insulation materials such as mineral wool and perlite products, covering test principles, apparatus, procedures, calculations, applicable standards such as GB/T 10299, and practical guidance for manufacturers, engineers, and accredited third-party laboratories.

Technical Overview

The hydrophobicity rate (also known as the water repellency ratio) of a thermal insulation material is defined as the percentage of water, delivered onto the specimen surface under standardized spray conditions, that is shed by the material rather than absorbed. It is calculated from the ratio of absorbed water mass to the total mass of water applied during the test. A high hydrophobicity rate indicates that the material effectively repels liquid water and will retain its insulating performance even when exposed to rain, condensation, splashing, or moisture-laden environments.

Moisture control is one of the most critical requirements for thermal insulation. Water conducts heat approximately 20 to 25 times better than still air, so even a small amount of absorbed moisture can sharply increase thermal conductivity and destroy the energy-saving function of the insulation layer. In addition, water ingress promotes corrosion of underlying substrates, freeze-thaw damage, mold growth, dimensional instability, and loss of mechanical strength. The hydrophobicity rate is therefore a key acceptance index for water-repellent grades of insulation products.

• Moisture dramatically increases thermal conductivity and degrades energy performance
• Retained water accelerates corrosion under insulation (CUI) on pipes and vessels
• Wet insulation is prone to sagging, freeze-thaw damage, and microbial growth
• Hydrophobic treatment extends service life and maintains stable long-term performance

The primary test method for this property in the Chinese standards system is GB/T 10299-2011, the test method for the hydrophobicity of thermal insulation materials. Related international methods, such as EN 1609, EN 13472, and ISO 29767, characterize water uptake behavior through short-term partial immersion and are often evaluated alongside hydrophobicity to provide a complete picture of moisture resistance. Hydrophobic grades of mineral wool products are typically required to achieve a hydrophobicity rate of at least 98 percent.

Testing Samples

Fibrous Insulation Materials

Rock (stone) wool, slag wool, and glass wool products are the most common specimens for hydrophobicity rate determination. These fibrous materials naturally absorb and wick water unless they are treated during manufacture with hydrophobic agents such as mineral oils or silicone-based binders. Boards, blankets, felts, and pipe sections of both standard and hydrophobic grades can be tested to verify treatment quality and batch consistency.

Granular and Inorganic Board Products

Expanded perlite products, vermiculite boards, calcium silicate insulation, and similar inorganic materials are also evaluated for water repellency. These products rely on hydrophobic impregnation or coating to prevent liquid water from penetrating their porous structures, making the hydrophobicity rate an essential quality control parameter.

Foam and Specialty Products

Phenolic foam, open-cell foams, ceramic fiber boards, and certain coated or faced insulation products can be assessed with the spray method where the hydrophobic surface behavior is of interest. Closed-cell materials such as EPS and rigid polyurethane are more commonly characterized by immersion-based water absorption tests, although spray testing can still be used to compare surface water repellency of facings and coatings.

Specimen Preparation and Conditioning

Specimens are typically cut to 300 mm by 150 mm with the thickness representative of the supplied product, and surfaces must be intact and free of crushed edges or loose fibers that could distort water behavior. Mineral-based and fibrous specimens are dried at 105 ± 5 °C to constant mass, then cooled to ambient temperature in a desiccator before weighing. At least three specimens are normally tested, and the density and thickness of each specimen are recorded because these parameters strongly influence water uptake.

Testing Items

The core test item is the hydrophobicity rate, expressed as a percentage. In a comprehensive moisture-resistance evaluation program, accredited third-party laboratories frequently combine it with the following related determinations:

• Hydrophobicity rate under standardized water spray (GB/T 10299)
• Short-term water absorption by partial immersion (EN 1609, EN 13472, ISO 29767)
• Long-term water absorption by full immersion (EN 12087, EN 13469)
• Water vapor transmission rate and moisture buffering behavior
• Thermal conductivity in the dry and conditioned states
• Density, thickness, and dimensional stability for result interpretation

Testing Methods

Test Principle and Apparatus

The spray method determines how much liquid water a specimen absorbs when a controlled water spray is directed onto its exposed face. The apparatus consists of a calibrated spray system with nozzles that deliver water at a specified flow rate and distribution, a water supply with a means of measuring the total mass of water delivered, and an inclined specimen holder that allows non-absorbed water to drain from the lower edge and run into a collection vessel. A balance with a resolution of 0.01 g or better is used to determine specimen masses before and after exposure.

Test Procedure

The general sequence of the test is as follows:

• Condition and dry the specimen to constant mass, then cool it in a desiccator
• Weigh the dried specimen and record the initial mass
• Measure the mass of water in the supply reservoir or verify the calibrated delivery rate
• Mount the specimen face-up on the inclined holder and position it at the standardized distance from the spray system
• Apply the water spray at the specified intensity for the standard-specified duration
• Terminate the spray, allow the standard drainage period, and blot surface water from the specimen face and edges as required
• Weigh the specimen again and record the mass of water remaining in the reservoir and collector

Calculation and Expression of Results

The hydrophobicity rate is calculated using the following relationship:

H = (1 − (m1 − m0) / mw) × 100%

• H = hydrophobicity rate, in percent
• m0 = mass of the dried specimen before the test, in grams
• m1 = mass of the specimen after spraying and drainage, in grams
• mw = total mass of water delivered onto the specimen during the spray period, in grams

The result for each specimen is calculated individually, and the reported hydrophobicity rate is the arithmetic mean of all valid specimens, normally rounded to 0.1 percent. The test report should include the individual values, specimen density and thickness, drying conditions, and any deviations from the standard procedure. Consistent mass-balance checking between delivered, absorbed, and collected water helps validate each run.

Quality Control and Influencing Factors

Several factors must be controlled to obtain reproducible results:

• Dosage and uniformity of hydrophobic oil or binder applied during manufacture
• Contamination by surfactants, detergents, or machining fluids, which can destroy water repellency
• Thermal or humidity aging that degrades the hydrophobic agent over time
• Fiber diameter, orientation, and density, which affect capillary uptake pathways
• Specimen edge condition, since cut edges absorb differently from product faces
• Water quality, spray calibration, and nozzle cleanliness, which affect delivery accuracy

Applications

Hydrophobicity rate determination is used throughout the insulation industry for product development, certification, and quality assurance:

• External wall insulation systems, curtain walls, and cavity insulation exposed to wind-driven rain
• Roof and terrace insulation where intermittent wetting is unavoidable
• Industrial piping, vessels, and equipment insulation, particularly in corrosion under insulation (CUI) prevention programs
• HVAC ductwork, chilled water lines, and refrigeration systems subject to condensation
• Power generation, petrochemical, and marine installations, including LNG and cryogenic service supports
• Factory quality control of hydrophobic agent dosing and incoming inspection of insulation batches
• Type testing and certification of products against national and international specification standards

FAQ

What does the hydrophobicity rate actually indicate?

It indicates the percentage of water applied under standardized spray conditions that the material repels rather than absorbs. A value of 98 percent means that only 2 percent of the sprayed water mass penetrated the specimen, confirming effective water-repellent treatment.

Which standard governs the test?

GB/T 10299-2011 is the principal method for determining the hydrophobicity of thermal insulation materials. Complementary moisture properties are assessed using EN 1609, EN 13472, and ISO 29767 for partial immersion water absorption, depending on the product form and market requirements.

What is the typical acceptance criterion?

Hydrophobic grades of rock wool and glass wool products are commonly required to achieve a hydrophobicity rate of no less than 98 percent, with premium grades often exceeding 99 percent. Specific limits should always be confirmed against the applicable product specification.

How does hydrophobicity testing differ from water absorption testing?

Hydrophobicity testing evaluates surface repellency under dynamic spray exposure and yields a percentage of shed water, while immersion-based absorption tests measure the mass of water taken up per unit area after partial or full immersion. The two approaches complement each other in assessing overall moisture resistance.

How long does testing take?

The spray exposure itself is short, but drying to constant mass, conditioning, and replicate testing mean that accredited third-party laboratories typically report results within a few working days of specimen receipt.

Does hydrophobic treatment affect thermal performance?

Correctly applied hydrophobic agents do not significantly change dry thermal conductivity. Their benefit appears in service, where they prevent moisture uptake that would otherwise sharply increase conductivity and degrade long-term insulation efficiency.