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Short answer
ASTM C518 is the standard test method for steady-state thermal transmission properties measured with a heat flow meter apparatus. It measures how readily heat passes through a flat slab of insulation or similar material, and reports thermal conductivity, thermal resistivity, thermal resistance and thermal conductance. It is a comparative method: the instrument must be calibrated with reference specimens whose properties are traceable to an absolute method.
What ASTM C518 covers
The full title of the current edition is Standard Test Method for Steady-State Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus (C518-21). Its scope is steady-state heat transfer through flat slab specimens. From the measured heat flow and temperature difference, the properties are calculated using Fourier’s law of heat conduction. ASTM describes the method as a rapid means of determining steady-state thermal transmission properties, and the Pacific Northwest National Laboratory’s Building America Solution Center library likewise calls it a rapid means of doing so.
Earlier editions are on record from 1991 through 2021 (1991, 1998, 2002, 2004, 2010, 2015, 2017 and 2021, as listed by PNNL), so a test report should always state which edition it followed.
How a heat flow meter measures
According to the test laboratory Intertek, the apparatus places the specimen between a hot plate and a cold plate, with heat flux transducers recording the heat passing through. Once the specimen reaches steady state, the temperature difference across it and the measured heat flow give the thermal properties. The apparatus is calibrated against reference materials of known conductivity. That calibration step is what makes C518 a comparative (secondary) method.
ASTM states that the calibration specimens should resemble the test specimens in conductance, thickness, mean temperature and temperature gradient. ASTM states the method has been used at ambient conditions of 10 to 40 °C. Intertek describes typical test conditions as a mean temperature between 35 and 110 °F (75 °F being the most common) with a temperature difference of 40 to 50 °F.
Conditions a valid C518 result depends on
This checklist collects the conditions named in the ASTM scope and the Intertek description. It is a way to read a report, not a substitute for the standard.
- Flat slab specimen. The method addresses flat slab geometry.
- Sufficient specimen resistance. The specimen’s thermal resistance must exceed 0.10 m²·K/W.
- Controlled edge heat loss. Edge losses are managed with edge insulation, a guard heater, or both.
- Traceable calibration. The reference specimens used to calibrate the apparatus must be traceable to an absolute method.
- Similar calibration material. The reference should resemble the sample in conductance, thickness, mean temperature and temperature gradient.
- Ambient range. ASTM states the method has been used at ambient conditions of 10 to 40 °C.
ASTM C518 versus ASTM C177
Intertek notes that a heat flow meter test can often serve as an alternative to ASTM C177, the guarded hot plate method. Both determine the same family of properties; what differs is the apparatus.
| Question | ASTM C518 | ASTM C177 |
|---|---|---|
| Apparatus | Heat flow meter with heat flux transducers | Guarded hot plate |
| Properties reported | Thermal conductivity, resistivity, resistance, conductance | Same family of properties |
| Typical role | Rapid determination for insulations | Can often be substituted by a C518 test |
Where it matters for labs and supply decisions
Insulating materials are often compared by a single conductivity or R-value figure. Intertek states that the test can be used to satisfy R-value regulations such as the US Federal Trade Commission R-Value Rule (10 CFR 460). When a figure on a data sheet cites C518, these questions come straight from the standard’s own conditions:
- Which edition of C518 was used?
- What reference materials was the apparatus calibrated with, and are they traceable to an absolute method?
- At what mean temperature and temperature difference was the specimen tested?
- Was the specimen’s thermal resistance above 0.10 m²·K/W, and how were edge losses controlled?
For the general practice of reading calibration paperwork, see our guides to the calibration certificate, the calibration interval, accredited calibration services and A2LA accreditation. For the wider field, see what is materials science and what is analytical chemistry.
Common misunderstandings
C518 is not an absolute method
The heat flow meter is calibrated against reference specimens, so its accuracy depends on the quality and similarity of that calibration material.
It is not for any shape
The scope is flat slab specimens.
A conductivity figure is not meaningful without conditions
Mean temperature, temperature difference and specimen thickness all belong with the number.
FAQ
What does ASTM C518 test?
Steady-state thermal transmission properties of flat slab specimens: thermal conductivity, resistivity, resistance and conductance, measured with a heat flow meter apparatus.
Is ASTM C518 the same as a guarded hot plate test?
No. Both determine similar properties, but the guarded hot plate method is ASTM C177 and uses a different apparatus. Intertek says a heat flow meter test can often be used as an alternative.
What is the current edition?
C518-21; the PNNL library lists its publication date as September 2021.
Sources
- ASTM International, C518-21 Standard Test Method for Steady-State Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus, scope and abstract, checked 2026-10-29.
- Intertek, ASTM C518 testing, checked 2026-10-29.
- Pacific Northwest National Laboratory, Building America Solution Center, library entry for the C518 standard, checked 2026-10-29.
