Dew point testing is the key method for verifying the sealing quality and moisture control of insulating glass units (IGUs) used in curtain wall systems. This technical article explains the testing principle, applicable standards such as GB/T 11944, ASTM E546/E576 and EN 1279, specimen requirements, test procedures, acceptance criteria, and typical applications in facade quality control. Read on to learn how accredited third-party laboratories evaluate IGU dew point performance and diagnose fogging failures in building envelopes.
Technical Overview
An insulating glass unit (IGU) consists of two or more glass lites separated by a spacer bar containing a desiccant, with the cavity hermetically sealed by a primary seal (typically butyl) and a secondary seal (silicone, polysulfide, or polyurethane). The cavity is filled with dry air or an inert gas such as argon. As long as the edge seal remains intact and the desiccant retains sufficient adsorption capacity, the moisture content inside the cavity stays extremely low and the internal surfaces remain clear.
The dew point is the temperature at which water vapor in the cavity gas begins to condense on the interior glass surfaces. A freshly manufactured, well-sealed IGU typically has a cavity dew point far below −60 °C. As moisture slowly permeates through the edge seal over time—or rapidly if the seal is defective—the desiccant saturates and the dew point rises. When the dew point approaches or exceeds the lowest surface temperature experienced by the unit, permanent condensation, fogging, and staining occur inside the cavity, which cannot be cleaned and requires replacement.
For curtain wall applications, IGUs are subjected to wind loads, thermal cycling, UV radiation, and long-term weathering, making edge-seal durability critical. Dew point testing is therefore a core acceptance and quality-control item in relevant standards and project specifications:
- GB/T 11944 (Insulating glass) specifies a maximum dew point of −40 °C for qualified units.
- ASTM E546 and ASTM E576 specify frost/dew point test methods for sealed insulating glass units in the laboratory and in the vertical (installed) position.
- EN 1279 series (Glass in building — Insulating glass units) controls moisture penetration through desiccant moisture-content measurement and factory production control, complementing dew point verification.
Testing Samples
Laboratory Test Specimens
For type testing and product certification, dedicated test pieces are fabricated with the same glass, spacer, desiccant, sealants, and edge-seal construction as the actual curtain wall units. Under GB/T 11944, dew point verification is commonly performed on 20 pieces measuring 510 mm × 360 mm, cut from or produced alongside production units. Specimens must be representative of the current production line and within the validity period after manufacture.
Full-Size Project Units
For incoming inspection and project acceptance, full-size curtain wall IGUs can be sampled directly from delivered lots. Sampling plans are usually defined by the project specification, the curtain wall standard (for example GB/T 21086), and the agreed inspection level.
In-Situ Field Units
Installed curtain wall glazing can be tested on site using portable dew point or frost point instruments placed on the interior side of the outer lite (or the interior lite, depending on instrument configuration). Field testing is widely used for failure diagnosis of fogging complaints and for periodic condition assessment of existing facades.
Conditioning
Specimens should be conditioned in the laboratory, typically at 23 ± 2 °C, for a sufficient period (commonly at least 24 hours) before testing, so that the cavity gas reaches thermal equilibrium. Surfaces must be clean and dry, and direct sunlight or strong radiant heat sources should be avoided during measurement.
Testing Items
Depending on the purpose of the evaluation, dew point testing may be carried out as a single measurement or as part of a durability test sequence:
- Initial dew point: measured on units in the as-delivered or as-manufactured state, with acceptance at or below −40 °C per GB/T 11944.
- Dew point after high-temperature/high-humidity aging: measured following moisture-aging cycles to evaluate long-term seal performance.
- Dew point after climate cycling: measured after high-low temperature and humidity cycling to assess resistance to thermal fatigue.
- Frost point per ASTM E546/E576: reported in degrees Celsius with pass/fail criteria defined by the referencing specification (such as ASTM E2188/E2190).
- Supporting evaluations under EN 1279: moisture content of the desiccant and moisture penetration index (EN 1279-2) and gas leakage rate (EN 1279-3) may be tested together to give a complete picture of edge-seal quality.
Testing Methods
GB/T 11944 Dew Point Method
The test uses a dew point meter consisting of a metal block (typically copper) with an embedded thermometer, cooled by a refrigerant such as a dry-ice and ethanol mixture to below −60 °C. The cooled block is placed in full contact with the glass surface at the center of the specimen, usually for about 3 minutes, chilling the glass locally. Immediately after the block is removed, the observer inspects the cavity-facing surface through the opposite lite under suitable lighting. If no condensation or frost appears on the interior surface when the contact surface reaches −40 °C, the dew point of the cavity gas is below −40 °C and the unit passes. All specimens in the sample set must satisfy the requirement.
ASTM E546 / ASTM E576 Frost Point Methods
ASTM E546 determines the frost/dew point of sealed IGU specimens in a controlled laboratory position, generally using specimens approximately 300 mm × 300 mm or agreed full-size units. A chilled test surface is held against the unit and the interior cavity surface is observed for the formation of frost or dew; the lowest temperature at which frost just fails to form is reported as the frost point. ASTM E576 applies the same principle to units tested in the vertical position, which makes it suitable for on-site evaluation of glazed curtain wall units. Results are reported together with the ambient temperature and relative humidity.
EN 1279 Moisture Penetration Approach
The EN 1279 system does not rely solely on a direct dew point reading. EN 1279-2 evaluates moisture penetration by comparing the moisture content of the desiccant sampled from aged and unaged units after sustained humidity exposure, calculating a moisture penetration index. EN 1279-6 addresses factory production control, under which dew point verification of system IGUs is performed. In practice, dew point testing and desiccant moisture analysis are frequently combined by accredited third-party laboratories for a robust assessment.
Factors Influencing Test Results
Several conditions must be controlled to obtain reliable dew point data:
- Specimen temperature equilibrium and avoidance of solar gain before testing.
- Clean, dry glass surfaces and full contact between the cooling block and the glass.
- Sufficient contact time, especially for thick, coated, or laminated lites that slow heat transfer from the cavity surface.
- Correct refrigerant temperature verification with a calibrated thermometer or thermocouple.
- Proper lighting and viewing angle when observing condensation on low-emissivity coated surfaces.
- Safe handling of dry ice and cold solvents, with appropriate personal protective equipment and ventilation.
Applications
Dew point testing of curtain wall insulating glass supports the entire life cycle of facade glazing, from manufacturing to in-service diagnosis:
- Factory quality control by IGU fabricators, verifying desiccant charging, butyl primary seal continuity, and secondary seal workmanship.
- Product certification and type testing against GB/T 11944, EN 1279, and ASTM performance specifications.
- Incoming material inspection for curtain wall contractors and general contractors before glazing installation.
- Project acceptance testing for high-rise commercial buildings, airports, hotels, and public facilities with unitized or stick curtain wall systems.
- Failure diagnosis of fogged or stained units in existing buildings, distinguishing cavity condensation from external condensation or glass surface contamination.
- Dispute resolution and expert testimony, where accredited third-party laboratories provide traceable test reports.
- Research and development of warm-edge spacers, high-capacity desiccants, silicone sealants, and gas-filling processes.
FAQ
What is the acceptance criterion for dew point of curtain wall insulating glass?
Under GB/T 11944, the dew point inside the cavity of every tested specimen must be at or below −40 °C. This ensures that internal condensation will not occur under normal service temperatures in most climates.
Why is the limit set at −40 °C?
The cavity-facing surfaces of an IGU in service can cool significantly below outdoor air temperature due to night-sky radiation and low ambient conditions. A margin of −40 °C guarantees that even in severe climates the moisture remaining in the cavity stays in the vapor phase, keeping the unit clear.
Why does the dew point of an IGU rise over time?
Moisture slowly diffuses through the edge seal system, especially under thermal cycling, UV exposure, and standing water in glazing pockets. Once the desiccant approaches saturation, it can no longer adsorb additional moisture and the cavity dew point climbs, eventually causing fogging. Poor workmanship, damaged butyl primary seals, or incompatible sealants accelerate this process.
Can dew point be tested on installed curtain wall units?
Yes. Portable frost point instruments based on ASTM E576 allow measurement of glazed units in the vertical, as-installed position, provided the interior or exterior surface is accessible, the unit is shielded from direct sunlight, and the construction (thick, laminated, or coated lites) is accounted for in the cooling time.
What should be done if a unit fails the dew point test?
A failing result indicates moisture-laden cavity gas and likely edge-seal or desiccant deficiency. Affected units are typically rejected or, if already installed, scheduled for replacement, since internal fogging cannot be repaired. The fabricator should investigate sealant application, desiccant handling, and production environment humidity.
How is dew point testing related to argon gas content testing?
Both properties depend on edge-seal integrity. Dew point reflects moisture tightness, while gas content and gas leakage rate (EN 1279-3) reflect gas retention. A unit with a failing dew point frequently also shows reduced argon concentration and degraded thermal performance, so the two tests are often performed together by accredited third-party laboratories.