Acceptance criteria for solar reflectance index (SRI)

Related services Solar reflectance index (SRI)

We are often asked, “can the SRI of my material pass the requirement?”. This post aims to answer this question.

There are no universal acceptance criteria for SRI. In practice, there could be 3 types of acceptance criteria:

There might be other types of acceptance criteria depending on your objective (e.g. if you want to certify your products under SGBP Certification, SGBC has their own requirements).

Project-specific SRI requirements

Please check with the buyer (e.g. architect, consultant or owner) for the project-specific SRI requirements.

USGBC LEED SRI requirements

Please refer to this page for the requirements in LEED.

Singapore Green Mark requirements

Please refer to this document for the requirements in Green Mark (please search “SRI” in the PDF document).


The information listed above may be outdated. Please check with your LEED or Green Mark consultant for the latest requirements.

Additionally, 3 SRI results are presented in our test reports (low-wind, medium-wind, and high-wind, as shown below). If it is not explicitly stated in the requirement, the medium-wind SRI result shall be used in the evaluation by default.

Can daylight reflectance be tested with ASTM E903 only?

Related services Daylight reflectance

As suggested by the title of ASTM E903:

Standard test method for solar absorptance, reflectance, and transmittance of materials using integrating spheres

ASTM E903 is for the testing of solar absorptance, reflectance, and transmittance, but not daylight reflectance directly. The procedures of spectral reflectance measurement with integrating spheres are described in ASTM E903 in detail, but it is still insufficient to get the daylight reflectance results with ASTM E903 only.

Therefore, at OTM, ASTM E903 is supplemented with other test methods in daylight reflectance testing:

  • For general materials: ASTM E971 and CIE 130
  • For glass materials: NFRC 300

Please refer to our daylight reflectance page for more information.

How to get window film optical & thermal properties with different glass substrates?

Related services Glass optical & thermal properties

Typically, window film optical & thermal properties are tested with 3 – 6 mm clear or low-iron glasses as the substrate. Window film properties obtained with such high transparency glass substrates are more appropriate for product performance rating purposes.

In real buildings, window film products can be attached to all possible glass substrate types, such as tinted glasses, low-e coated glasses, laminated glasses, and double glazing units (DGUs). There are two methods to get window film optical & thermal properties with different glass substrates, as described below.

Option 1: direct physical test method

With the direct physical test method, the window film shall be attached to the actual glass substrate to be used. The whole glass system with window film is tested as usual.

This method is recommended for most applications, with a small number of glass substrate types.

Option 2: physical test + calculation method

With the physical test + calculation method, the following glasses need to be tested (based on the NFRC 304 method):

  1. Window film on a reference glass substrate (typically a 3 – 6 mm clear or low iron glass)
  2. The reference glass substrate (without window film)
  3. Other glass substrates

With the test results of glasses 1 & 2, the window film only optical data can be calculated. The window film only optical data can then be added to all glass substrates tested in step 3 to get the combined glass with window film optical & thermal properties.

This method is recommended for product development applications, with a large number of glass substrate types.

On-site measurement of wall system U-value

Related services Thermal conductivity, On-site testing & monitoring

We are now able to test wall system U-value on-site. The test method is based on ISO 9869-1, with some improvements for Singapore’s environmental conditions.

The measurement instrument setup is illustrated below:

The following 3 quantities are measured:

  • Indoor side wall surface temperature (by a temperature sensor)
  • Outdoor side wall surface temperature (by a temperature sensor)
  • Heat flux through the wall (by a heat flux sensor)

The instruments need to be deployed on-site for a few days. The thermal resistance (R-value) of the wall system is calculated from the averaged results. The thermal transmittance (U-value) of the wall system is calculated from the R-value and the surface film resistances defined in the BCA ETTV code.

For better measurement accuracy, a surface electric heater of the size 0.5 m x 0.5 m is attached to the indoor side of the wall system to increase the indoor/outdoor temperature difference across the wall system.

Sample thickness in ASTM C518 thermal conductivity testing

Related services Thermal conductivity

In thermal conductivity testing according to ASTM C518, a test sample is clamped between two plates and compressed to certain thickness.

Some customers are concerned if thermal conductivity results are affected by the compression. This article aims to provide some explanations to this concern.

3 thicknesses

Conceptually, there are 3 thicknesses for a sample:

  • Uncompressed thickness: the thickness of a sample in the free state without compression;
  • Installation thickness: the thickness of a sample in the intended installation. In an installation, the sample may or may not be compressed;
  • Testing thickness: the thickness of a sample during thermal conductivity testing. During testing, a sample is always compressed for good thermal contact.

Among the 3 thicknesses:

  • The installation thickness could be the same as the uncompressed thickness (if the sample is not compressed in an installation), or smaller than the uncompressed thickness (if the sample is compressed in an installation).
  • The testing thickness is always smaller than the uncompressed thickness, as a sample is always compressed during testing.
  • The testing thickness should be as close to the installation thickness as possible, for fair product performance rating.

Testing thickness and thermal conductivity

When a sample is compressed to a smaller thickness, its density increases and the increased density affects the thermal conductivity measurement result.

A sensitivity study was performed by OTM in 2020. In the study, when the sample was compressed by 10%, the result variation was less than 1.7%. The thermal conductivity measurement result is not so sensitive to the testing thickness variation. If the compression is small (e.g. less than 5%), the result variation is negligible for general engineering applications.

Determination of testing thickness

The heat flow meter used by OTM supports two thickness control modes:

  • Automatic thickness: a sample is compressed by the instrument with a constant pressure of approximately 2.5 kPa and the sample thickness under compression is automatically measured as the testing thickness.
  • Manual thickness: a thickness is input manually and the sample is compressed to the manually input thickness as the testing thickness (provided that the sample can be compressed to this thickness with less than 2.5 kPa of pressure).

In practice, there are two scenarios:

  • Rigid or firm materials
  • Soft materials

Testing thickness of rigid or firm materials

For rigid materials (e.g. polystyrene foam or polyurethane foam) or firm materials (e.g. high-density rockwool), they cannot be compressed significantly in nomral installations (e.g. more than 5% of compression).

The testing thickness of a rigid or firm material is determined with the automatic thickness mode mentioned above.

Testing thickness of soft materials

For soft materials (e.g. low-density rockwool or glasswool), they can be compressed significantly in normal installations (e.g. more than 10% of compresssion).

The testing thickness of a soft material is determined with the manual thickness mode mentioned above.

The customer needs to declare the installation thickness of a soft material sample. The declared installation thickness will be used as the testing thicknes.

If an installation thickness is not declared, we will assume that the installation thickness is the same as the sample nominal thickness. If the installation thickness is the same as the uncompressed thickness, the sample may be compressed by up to 5% for good thermal contact.

Solar spectra in ASTM E903 solar reflectance calculations

Related services Solar reflectance index (SRI)

Various solar spectra (i.e. solar spectral irradiance distributions) could be used in solar reflectance (of course, solar transmittance and absorptance too) calculations according to ASTM E903, as listed below:

  • ASTM E891:
    • Default solar spectrum used by the building industry and by OTM in SRI testing.
    • It is the default solar spectrum due to historical reasons and for compatibility with existing practices, data and instruments.
    • ASTM E891 is withdrawn and the same data set is available in ISO 9845-1.
  • ASTM E892:
    • Default solar spectrum for solar reflectance testing of non-roofing materials, according to LEED (note: solar reflectance only, not SRI)
    • Similarly, ASTM E892 is withdrawn and the data set is availble in ISO 9845-1.
    • ASTM E891 and E892 are different. ASTM E891 is in terms of direct normal solar radiation (no diffuse radiation); ASTM E892 is in terms of hemispherical solar radiation (with diffuse radiation).
  • ASTM G173:
    • Default solar spectrum included in ASTM E903, including both direct normal solar spectrum (equivalent to ASTM E891) and hemispherical solar spectrum (equivalent to ASTM E892)
    • It is not often used in the buidling industry, due to the historical reasons stated under ASTM E891.
  • ASTM E490:
    • Used in extraterrestrial applications, with AM0.
    • All other spectra mentioned above are with AM1.5 (refer to this article on air mass).