This test method details the calibration and testing procedures and necessary additional temperature instrumentation required in applying Test Methods C 236, C 976, or C 1363 to measure the thermal transmittance of fenestration systems mounted vertically in the thermal chamber.

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 E 1423 and Section 5.3. However, this procedure can be used with other conditions for research purposes or product development.

It should be recognized that the only true experimental measurement is the thermal transmittance, U_S, value determined in Section 7. The “standardized” thermal transmittance value, U_ST, obtained by either the Calibration Transfer Standard (CTS) or area weighting (AW) methods described in Section 8 include adjustments to the U_S value that are made because the current computer calculation methods (NFRC 100-97) for determining the thermal transmittance are not capable of applying the actual surface heat transfer coefficients that exist on the test specimen while testing at standardized conditions. The current computer calculation methods assume that uniform standardized surface heat transfer coefficients exist on the indoor and outdoor fenestration product surfaces, which is not the case. Until such a time that the computer calculation methods are upgraded to have the actual surface heat transfer coefficients applied to the actual fenestration product geometry, the modification of the true tested thermal transmittance value, U_S, to a standardized value U_ST, is necessary for rating and comparison (measured with calculated) purposes.

It should be noted that the standardized surface heat transfer coefficients, h_h and h_s, as calibrated prior to testing a fenestration product using an appropriately sized Calibration Transfer Standard (CTS) may 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 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, t₁ and t₂, in order to estimate the actual test specimen surface heat transfer coefficients. It should be recognized that this assumption will not be accurate for all fenestration products, especially for high thermal transmittance products where the surface heat transfer coefficients are a major portion of the overall thermal resistance and also for fenestration products with significant surface projections (for example, skylights, roof windows, garden windows) where the surface heat transfer coefficients are quite different from the standardized values.

In these situations, an attempt should be made 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.

Guidelines for determining which standardizing procedure to follow are given in 8.2.

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 E 1423 or NFRC 100-97.

Note 3—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 may be expected from field installations due to not accounting for solar radiation, air leakage effects, and the thermal bridge effects that may occur due to the specific design and construction of the fenestration system opening. Since there is 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. This situation allows the selection of a relatively high thermal resistance surround panel which places the focus of the test on the fenestration system thermal performance alone. Therefore, it should be recognized that the thermal transmittance results obtained from this test method are for ideal laboratory conditions in a highly insulative surround panel, and should only be used for fenestration product comparisons and as input to thermal performance analyses which also include solar, air leakage, and thermal ridge effects due to the surrounding building structure. To determine air leakage for windows and doors, refer to Test Methods E 283 and E 783.

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 either Test Methods C 236, C 976, or C 1363 for determination of fenestration system thermal transmittance.

Note 1—This test method allows the testing of projecting fenestration products (that is, garden windows, skylights, and roof windows) installed vertically in a surround panel. Current research on skylights, roof windows, and projecting products hopefully will provide additional information that can be added to the next version of this test method so that skylight and roof windows can be tested horizontally or at some angle typical of a sloping roof.

1.2 This test method refers to the thermal transmittance, U, and the corresponding thermal resistance, R, of a fenestration system installed vertically in the absence of solar and air leakage effects.

Note 2—The methods described in this document may also be adapted for use in determining the thermal transmittance of sections of building wall, and roof and floor assemblies containing thermal anomalies, which are smaller than the hot box metering area.

1.3 This test method describes how to determine a fenestration product's (also called test specimen) thermal transmittance, U_S, at well-defined environmental conditions. The thermal transmittance, which is sometimes called the air-to-air U-factor, is also a reported test result from Test Methods C 236, C 976, and C 1363. If only the thermal transmittance is reported using this test method, the test report must also include a detailed description of the environmental conditions in the thermal chamber during the test as outlined in 10.3.

1.4 For rating purposes, this test method also describes how to calculate a standardized thermal transmittance, U_ST, which can be used to compare test results from laboratories with different weather side wind directions and thermal chamber configurations, and can also be used to directly compare to calculated results from current computer programs for determining the thermal transmittance of fenestration products. Although this test method specifies two methods of calculating the standardized thermal transmittance, only the standardized thermal transmittance result from one method is reported for each test. One standardized thermal transmittance calculation procedure is the Calibration Transfer Standard (CTS) method and another is the area weighting (AW) method (see 4.3 and Section 8 for further descriptions of these two methods). The area weighting method requires that the surface temperatures on both sides of the test specimen be directly measured as specified in Practice E 1423 in order to determine the surface heat transfer coefficients on the fenestration product during the test. The CTS method does not use the measured surface temperatures on the test specimen and instead utilizes the calculation of equivalent surface temperatures from calibration data to determine the test specimen surface heat transfer coefficients. The area weighting (AW) method shall be used whenever the thermal transmittance, U_S, is greater than 3.4 W/(m²•K) {0.6 Btu/(hr•Ft ²•°F)}, or when the ratio of test specimen projected surface area to wetted (that is, total heat transfer or developed) surface area on either side of the test specimen is less than 0.80. Otherwise the CTS method shall be used to standardize the thermal transmittance results.

1.5 A discussion of the terminology and underlying assumptions for measuring the thermal transmittance are included.

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

1.7 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.