OTTV calculation under local building ordinance requires input parameters for the thermal property of building material. The heat flow meter apparatus can measure the necessary input values for the OTTV calculation. Thermal resistance and thermal conductivity can be tested. It is used to determine material-specific conductance for specimens. Measurement can be done on roof insulation material, thin firm surface coating, etc. The test enables benchmarking well insulating material and calculation of OTTV improvement to facilitate passive design in energy-efficient envelope.

The heat flow meter test can be performed according to the following test standard:

- ASTM C518