Overview
ISO 8990:1994 - Thermal insulation - Determination of steady‑state thermal transmission properties - Calibrated and guarded hot box - defines the principles, measurement techniques and minimum requirements for laboratory determination of the steady‑state thermal transmission properties (thermal transmittance and thermal resistance) of building components and similar industrial components. The standard covers both the guarded hot box (GHB) and calibrated hot box (CHB) approaches, is primarily intended for large or inhomogeneous specimens, and describes apparatus design concepts, instrumentation, calibration and data reporting. It does not prescribe a single apparatus design and excludes tests involving mass transfer through the specimen.
Key topics and requirements
- Scope and principle: Reproduce conventional boundary conditions with a hot and a cold chamber; measure steady‑state surface and air temperatures and the hot‑side power input to derive thermal transmittance (U) and resistance (R).
- Two methods:
- Guarded hot box - uses a metering box surrounded by a guard box to minimize lateral heat flow and allow direct equality between meter input and specimen heat flow when perfectly balanced.
- Calibrated hot box - surrounded by a temperature‑controlled space; wall losses are measured and corrected using calibration specimens.
- Apparatus elements: metering box, guard box, specimen frame, cold‑side chamber, temperature sensors, heat flowmeters/thermopiles, and instrumentation for steady‑state control and measurement.
- Calibration and performance: Use calibration specimens covering the intended thickness and resistance range to determine box wall losses, flanking losses and imbalance resolution. The standard notes that for homogeneous specimens, accuracies within ±5% are often achievable when properly calibrated.
- Limitations & error sources: sensitivity to specimen size, edge effects, non‑uniform surface coefficients, flanking/peripheral losses, and measurement imbalance. Minimum recommended metering area and guidelines for specimen mounting are provided.
- Reporting: Required data include temperatures (air, surface, radiant/environmental), heat flow/power input, calibration corrections and test conditions.
Applications and users
- Practical uses: performance verification of walls, roofs, floors, façade elements, industrial panels and large assemblies; compliance testing, design validation, research and model verification.
- Primary users: independent test laboratories, building envelope manufacturers, product certification bodies, research institutes and regulatory authorities requiring steady‑state thermal transmission data for large or inhomogeneous specimens.
Related standards
- Normative reference: ISO 7345:1987 (Thermal insulation - Physical quantities and definitions) is cited by ISO 8990:1994.
- Annexes in ISO 8990 include guidance on heat transfer at surfaces and environmental temperatures (Annex A) and a bibliography (Annex B).
Keywords: ISO 8990:1994, thermal insulation test, guarded hot box, calibrated hot box, steady‑state thermal transmission, thermal transmittance, laboratory measurements.
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