Skip to main content
U.S. flag

An official website of the United States government

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure .gov websites use HTTPS
A lock ( ) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.

Line Heat-Source Guarded Hot Plate

The 1-meter guarded hot-plate apparatus measures thermal conductivity of building insulation. This facility provides for absolute measurement of thermal resistance of thick and low-density test specimens used as transfer standards. These standards are used to calibrate heat-flow-meter apparatus (ASTM C 518)or verify guarded-hot-plate apparatus (ASTM C177). This facility is the only one of its kind in the world that will permit low-density thick insulation to be measured with an expanded uncertainty of less 1%.

[History of the Guarded-Hot-Plate Apparatus at NIST]


Laboratory services for thermal resistance measurements (and related thermal properties) are provided for thermal insulation (and building materials) having thermal conductivities of 0.02 W·m -¹·K-¹ to 0. 15 W·m-¹·K-¹. In general, the highest accuracy is obtained for homogeneous specimens.; The preferred size for the test specimen is 1016 mm in diameter; the minimum size, 610 mm square. Customers can supply their own material for specimens, or request NIST to select specimens from an in-house inventory of fibrous-glass material. All tests are performed at an ambient atmospheric pressure of approximately 100 ± 20 kPa (site pressure at Gaithersburg, MD). Services at ambient pressures outside these limits or with other gases are not provided. A dry-air purge is available to reduce the relative humidity to less than 15 percent. Additional information on the apparatus is available in the reference, below. Laboratory capabilities are summarized below.

Thermal Conductivity (W·m-1K-1) Temperature (K) Temperature Difference (K) Thickness (mm) 

0.02 to 0.05


20 to 30

13 to 225

0.05 to 0.15



13 to 25

Usage Information

Access Information

This apparatus is available for use by those outside NIST, but it must be operated by BFRL staff. Collaborative programs may be arranged on cost reimbursable basis. Scheduling of services is assigned on a first-come, first-serve basis, determined by receipt of the Order Form and Purchase Order. Services are best when arranged in advance, since turnaround time varies from several

Created February 9, 2010, Updated January 5, 2017