ASTM C1199-12

4.1 This test method details the calibration and testing procedures and necessary additional temperature instrumentation required in applying Test Method C1363 to measure the thermal transmittance of fenestration systems mounted vertically in the thermal chamber.

4.2 The thermal transmittance of a test specimen is affected by its size and three-dimensional geometry. Care must be exercised when extrapolating to product sizes smaller or larger than the test specimen. Therefore, it is recommended that fenestration systems be tested at the recommended sizes specified in Practice E1423 or NFRC 100.

4.3 Since both temperature and surface heat transfer coefficient conditions affect results, use of recommended conditions will assist in reducing confusion caused by comparing results of tests performed under dissimilar conditions. Standardized test conditions for determining the thermal transmittance of fenestration systems are specified in Practice E1423 and Section 4.4 Similarly, it would be desirable to have a surround panel that closely duplicates the actual wall where the fenestration system would be installed. Since there are such a wide variety of fenestration system openings in North American residential, commercial and industrial buildings, it is not feasible to select a typical surround panel construction for installing the fenestration system test specimen. Furthermore, for high resistance fenestration systems installed in fenestration opening designs and constructions that have thermal bridges, the large relative amount of heat transfer through the thermal bridge will cause the relatively small amount of heat transfer through the fenestration system to have a larger than desirable error. For this reason, the Calibration Transfer Standard and test specimen are installed in a homogeneous surround panel constructed from materials having a relatively high thermal resistance. Installing the test specimen in a relatively high thermal resistance surround panel places the focus of the test on the fenestration system thermal performance alone. Therefore, it is important to recognize that the thermal transmittance results obtained from this test method are for ideal laboratory conditions, and should only be used for fenestration product comparisons unless the thermal bridge effects that have the potential to occur due to the specific design and construction of the fenestration system opening are included in the analysis.

4.5 This test method does not include procedures to determine the heat flow due to either air movement through the specimen or solar radiation effects. As a consequence, the thermal transmittance results obtained do not reflect performances that are expected from field installations. It is possible to use the results from this test method as input to annual energy performance analyses which include solar, and air leakage effects to get a better estimate of how the test specimen would perform when installed in an actual building. To determine the Solar Heat Gain Coefficient of fenestration products, refer to NFRC 200. To determine air leakage for windows and doors, refer to Test Methods E283 and E783.

4.6 It is important to recognize that the thermal transmittance, U_S, value determined in Section 4.6.1 It is important to recognize that due to radiation effects, the room side or weather side temperature (t_h and t_c, respectively), has the potential to differ from the respective room side or weather side baffle temperatures (t_b1 and t_b2, respectively). If there is a difference of more than ±1 °C (±2 °F), either on the room side or weather side, the radiation effects shall be accounted for as described in Sections 6 and 9 to maintain accuracy in the calculated surface heat transfer coefficients. Calculating the radiation exchange for highly conductive test specimens or projecting fenestration products as described in Annex A2 is not a trivial task.

4.6.2 The calculation of the standardized thermal transmittance assumes that only the surface heat transfer coefficients change from the calibrated standardized values for the conditions of the test. This assumption is possibly not valid if the surface temperature differentials for the standardized calibration conditions are different from the surface temperature differential that exists on the test specimen during the test. Currently, specifications for the Calibration Transfer Standard give it a thermal transmittance of 1.7 W/(m²·K) [0.3 Btu/(hr·ft²·°F)]. Accordingly, the calculation of the standardized thermal transmittance produces the least error when performed on test specimens with a similar thermal transmittance.

4.6.3 It is important to note that the standardized surface heat transfer coefficients, h_h and h_c, as calibrated prior to testing a fenestration product using an appropriately sized Calibration Transfer Standard (CTS) have the potential to differ from the surface heat transfer coefficients that exist during a hot box test on a specific test specimen. Fenestration systems usually have frame and sash surfaces that introduce two- and three-dimensional convective heat transfer effects which result in variable surface heat transfer coefficients, which differ from the uniform standardized values. As a result of this, the test specimen surface heat transfer coefficients will differ from those obtained with the non-framed, essentially flat Calibration Transfer Standard tested under the same conditions. In this standardizing procedure, it is assumed that the differences are small enough so that the calibration surface heat transfer coefficients can be used to calculate equivalent test specimen average surfaces temperatures, 4.6.4 In these situations, it is important to attempt to measure the test specimen surface temperature distributions and then calculate directly the test specimen average area weighted surfaces temperatures, t₁ and t₂. This area weighting (AW) method also has problems in that the placement of temperature sensors to get an accurate area weighting is not known, especially on high conductivity horizontal surfaces that act as heat transfer extended surfaces (that is, fins). In addition, the placement of many temperature sensors on the test specimen surfaces will affect the velocity fields in the vicinity of these surfaces which will affect the surface temperatures and surface heat transfer coefficients.

1.1 This test method covers requirements and guidelines and specifies calibration procedures required for the measurement of the steady-state thermal transmittance of fenestration systems installed vertically in the test chamber. This test method specifies the necessary measurements to be made using measurement systems conforming to Test Method C1363 for determination of fenestration system thermal transmittance.