5.1 Traditionally, HFTs have been incorporated into laboratory testing devices, such as the heat flow meter apparatus (Test Method C518), that employ controlled temperatures and heat flow paths to effect a thermal measurement. The application of heat flux transducers and temperature transducers to building components in situ can produce quantitative information about building thermal performance that reflects the existing properties of the building under actual thermal conditions. The literature contains a sample of reports on how these measurements have been used (1-8).³

5.2 The major advantage of this practice is the potential simplicity and ease of application of the sensors. To avoid spurious information, users of HFTs shall: (1) employ an appropriate S, (2) mask the sensors properly, (3) accommodate the time constants of the sensors and the building components, and (4) account for possible distortions of any heat flow paths attributable to the nature of the building construction or the location, size, and thermal resistance of the transducers.

5.3 The user of HFTs and TTs for measurements on buildings shall understand principles of heat flux in building components and have competence to accommodate the following:

5.3.1 Choose sensor sites using building plans, specifications and thermography to determine that the measurement represents the required conditions.

5.3.2 A single HFT site is not representative of a building component. The measurement at an HFT site represents the conditions at the sensing location of the HFT. Use thermography appropriately to identify average and extreme conditions and large surface areas for integration. Use multiple sensor sites to assess overall performance of a building component.

5.3.3 A given HFT calibration is not applicable for all measurements. The HFT disturbs heat flow at the measurement site in a manner unique to the surrounding materials (9, 10); this affects the conversion constant, S, to be used. The user shall take into account the conditions of measurement as outlined in 7.1.1. In extreme cases, the sensor is the most significant thermal feature at the location where it has been placed, for example, on a sheet metal component. In such a case, meaningful measurements are difficult to achieve. The user shall confirm the conversion factor, S, prior to use of the HFT to avoid calibration errors. See Section 7.

5.3.4 The user shall be prepared to accommodate non-steady-state thermal conditions in employing the measurement technique described in this practice. This requires obtaining data over long periods, perhaps several days, depending on the type of building component and on temperature changes.

5.3.5 Heat flux has a component parallel to the plane of the HFT. The user shall be able to minimize or accommodate this factor.

1.1 This practice covers a technique for using heat flux transducers (HFTs) and temperature transducers (TTs) in measurements of the in-situ dynamic or steady-state thermal behavior of opaque components of building envelopes. The applications for such data include determination of thermal resistances or of thermal time constants. However, such uses are beyond the scope of this practice (for information on determining thermal resistances, see Practice C1155).

1.2 Use infrared thermography with this technique to locate appropriate sites for HFTs and TTs (hereafter called sensors), unless subsurface conditions are known.

1.3 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.

1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.