Standard: ASTM C518 – Standard Test Method for Steady-State Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus

Scope: This test is used on a wide range of materials.

Applicable Products: Insulation, construction materials, window materials

Test Procedure: This test uses a Heat Flow Meter Apparatus (HFM). The Heat Flow Meter consists of a cold plate and a hot plate which incorporate heat flux transducers for measuring heat flow. The material to be measured is placed between the two plates which are controlled to different temperatures to create a heat flow from hot to cold plate, which is measured by the transducers. The device is calibrated against materials of known thermal conductivity and can be used to measure flat materials in a wide range of thickness and conductivity. Tests are commonly performed with a mean temperature between 35ºF and 110ºF, with the most common being 75ºF. A temperature difference of 40ºF – 50ºF is commonly used.

End Result: The thermal conductivity (k-Value) and thermal resistance (R-Value) are determined. These values can be used to determine energy losses through a material. This test can be used to satisfy R-Value regulations for insulation materials, such as the US Federal Trade Commission’s “R-Value Rule” (10 CFR 460).

Special Notes: This method can often be used as an alternative to ASTM C177, which determines the same properties using a different device known as a Guarded Hot Plate Apparatus (GHP).


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