NFRC/EN/ISO test method differences

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What are the differences between the NFRC, EN, and ISO glass optical & thermal test methods?

There are 3 widely adopted glass optical & thermal property test methods:

  • NFRC method: NFRC 100/200/300/301
  • EN method: EN 410/673
  • ISO method: ISO 9050/10292

This article aims to compare the 3 test methods in detail:

  1. 5 examples: are they different?
  2. The theory: what are different?
  3. The practice: which method to use?

More information:

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5 examples: are they different?

In order to illustrate the result differences between the 3 test methods, listed below are the result comparisons of 5 typical glass types. Please expand one of the blocks below for the comparison.

If you are interested in the technical background, please read the theory part.

If you are interested in other glass types, please leave your comments at the bottom of the page or contact us.

Example 1: Clear & uncoated laminated glass

Glass configuration: 6 mm clear glass + 1.52 mm PVB + 6 mm clear glass

Spectral transmittance and reflectance curves
NFRCENISO
Visible light transmittance0.8650.8650.865
Visible light reflectance, front0.0770.0770.077
Visible light reflectance, back0.0770.0770.077
Solar energy transmittance0.6700.6700.675
Solar energy reflectance, front0.0630.0640.064
Solar energy reflectance, back0.0640.0650.065
Solar heat gain coefficient (SHGC)0.7550.7330.743
Shading coefficient (SC)0.8680.8420.854
Summer condition U-value [W/(m2K)]4.92N/AN/A
Winter condition U-value [W/(m2K)]5.425.315.35
Example 2: Tinted & uncoated glass

Glass configuration: 8 mm tinted glass

Spectral transmittance and reflectance curves
NFRCENISO
Visible light transmittance0.3680.3680.368
Visible light reflectance, front0.0480.0480.048
Visible light reflectance, back0.0530.0530.053
Solar energy transmittance0.3640.3600.362
Solar energy reflectance, front0.0470.0480.047
Solar energy reflectance, back0.0510.0520.052
Solar heat gain coefficient (SHGC)0.5490.4990.514
Shading coefficient (SC)0.6310.5740.591
Summer condition U-value [W/(m2K)]5.21N/AN/A
Winter condition U-value [W/(m2K)]5.775.635.68
Example 3: Tinted & laminated glass with hard low-e coating

Glass configuration: 6 mm tinted glass + 1.52 mm PVB + 6 mm glass with hard low-e on #4

Spectral transmittance and reflectance curves
NFRCENISO
Visible light transmittance0.3270.3270.327
Visible light reflectance, front0.0530.0530.053
Visible light reflectance, back0.0870.0870.087
Solar energy transmittance0.2360.2420.243
Solar energy reflectance, front0.0510.0510.051
Solar energy reflectance, back0.0950.0910.092
Solar heat gain coefficient (SHGC)0.4010.3590.370
Shading coefficient (SC)0.4610.4120.425
Summer condition U-value [W/(m2K)]3.13N/AN/A
Winter condition U-value [W/(m2K)]3.833.833.84
Example 4: Clear DGU with soft low-e coating

Glass configuration: 8 mm clear glass with soft low-e on #2 + 12 mm air space + 5 mm low iron glass + 1.52 mm PVB + 5 mm low iron glass

Spectral transmittance and reflectance curves
NFRCENISO
Visible light transmittance0.7010.7010.701
Visible light reflectance, front0.1300.1300.130
Visible light reflectance, back0.1360.1360.136
Solar energy transmittance0.2770.3110.306
Solar energy reflectance, front0.5030.4540.466
Solar energy reflectance, back0.4520.4040.418
Solar heat gain coefficient (SHGC)0.3060.3410.335
Shading coefficient (SC)0.3510.3920.385
Summer condition U-value [W/(m2K)]1.52N/AN/A
Winter condition U-value [W/(m2K)]1.581.521.53
Example 5: Tinted DGU with soft low-e coating

Glass configuration: 6 mm tinted glass with soft low-e #2 + 12 mm air space + 6 mm clear glass

Spectral transmittance and reflectance curves
NFRCENISO
Visible light transmittance0.2670.2670.267
Visible light reflectance, front0.0980.0980.098
Visible light reflectance, back0.2430.2430.243
Solar energy transmittance0.1260.1400.138
Solar energy reflectance, front0.1540.1450.147
Solar energy reflectance, back0.4260.4010.407
Solar heat gain coefficient (SHGC)0.1950.1910.193
Shading coefficient (SC)0.2240.2200.222
Summer condition U-value [W/(m2K)]1.57N/AN/A
Winter condition U-value [W/(m2K)]1.631.571.57

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Observations from the comparisons

Based on the result comparisons listed above, we have the following observations:

  • The visible light transmittance/reflectance results are the same.
  • The solar energy transmittance/reflectance results are different:
    • The differences are small for the uncoated glasses, but are relatively larger for the low-e coated glasses
    • For the low-e coated glasses, the EN and ISO results are close, but the NFRC results are noticeably different from them.
    • For the low-e coated glasses, in general, the NFRC solar energy transmittance results are lower and the NFRC solar energy reflectance results are higher.
  • The solar heat gain coefficient (SHGC) and shading coefficient (SC)  results are different:
    • In general, the EN and ISO results are close, but the NFRC results are noticeably different from them.
    • For the single glazing glasses, the NFRC results are higher.
    • For the clear DGU with soft low-e coating, the NFRC results are lower.
    • For the tinted DGU with soft low-e coating, the result differences between the 3 methods are small.
  • There is no summer condition U-value in the EN and ISO methods.
  • The winter condition U-value results are close between the 3 methods.

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Different result names

It is also necessary to mention the following naming differences in the EN and ISO test methods:

  • Solar energy transmittance/reflectance: they are named as direct solar transmittance/reflectance in the EN and ISO methods.
  • Solar heat gain coefficient (SHGC): it is named as total solar energy transmittance (TSET) in the EN and ISO methods.
  • Winter condition U-value: it is simply named as U-value in the EN and ISO methods.

In this article, the default names used for the NFRC method at OTM, i.e. solar energy transmittance/reflectance, SHGC, and winter condition U-value, are used.

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The theory: What are different?

As described in our glass testing procedures article, there are the testing part (Steps 2 & 3) and the calculation part (steps 4 & 5) in glass optical & thermal property testing.

Summarized below are the differences between the NFRC, EN and ISO test methods:

  • Identical measurement principles
  • Testing part: the NFRC method is more detailed
  • Calculation part: different solvers and environmental conditions
  • Results: large differences in SHGC and SC results

At OTM, the testing procedures are the same and the result differences are caused by the different calculation models for the 3 test methods.

If you are interested in more details, please expand one of the blocks below.

Identical measurement principles

The 3 test methods are based on the same measurement principles and testing procedures. The same set of instruments can be used for all 3 test methods.

The measurement principles for glass optical and thermal property testing can be briefly described as:

  • All optical & thermal properties are not directly measured.
  • The spectral transmittance/reflectance results are directly measured by instruments.
  • The optical properties are calculated from the spectral transmittance/reflectance results with weighted averaging.
  • The thermal properties are calculated from the optical properties, according to the calculation models and environmental conditions defined in the test methods.
Testing part: the NFRC method is more detailed

For the testing part, the NFRC method is more detailed, with the testing procedures defined in NFRC 300 (solar optical properties) and NFRC 301 (emissivity). In contrast, the testing procedures described in the EN and ISO methods are very brief.

For example, NFRC 300 is a 16-page document on glass spectral transmittance and reflectance measurement in the solar spectrum range (300 nm – 2500 nm) and it covers all details from instrument requirements to operational guidelines. In contrast, ISO 9050 only provides brief guidelines on the testing part, with 1-page content only.

The spectral transmittance/reflectance results collected according to the NFRC method can be directly used for EN or ISO calculation, but not vice versa.

For this reason, at OTM, our laboratory testing work instructions are based on NFRC 300/301 and the spectral transmittance/reflectance results collected by us can be used for NFRC, EN or ISO calculation.

Calculation part: different solvers and environmental conditions

The differences in the calculation part of the 3 test methods are substantial:

  • Identical in visible light transmittance/reflectance calculations
  • Different solar spectra in solar energy transmittance/reflectance calculations
  • Different solvers and environmental conditions in thermal property calculations

Identical in visible light transmittance/reflectance calculations

The calculation models for visible light transmittance/reflectance are the same in the 3 test methods.

This is the reason that, in the examples presented above, the visible light transmittance/reflectance results by the 3 methods are always the same.

Different solar spectra in solar energy transmittance/reflectance calculations

The solar energy transmittance/reflectance results are calculated from the spectral transmittance/reflectance results in the 300 nm -2500 nm range, with weighted average and a solar energy distribution spectrum as the weights. The solar spectra defined in the EN and ISO methods are close, but the NFRC spectrum is significantly different from the other two.

This is the reason that, in the examples presented above, the solar energy transmittance/reflectance results by the 3 methods are different (refer to the observations).

Different solvers and environmental conditions in thermal property calculations

For thermal property calculations, the NFRC calculation model is solved numerically, whereas the EN and ISO calculation models are solved analytically. The environmental conditions, such as solar spectrum, indoor/outdoor temperature and wind speed, are different too. The different environmental conditions result in different thermal resistances and solar energy transmittance/reflectance.

This is the reason that, in the examples presented above, the SHGC, SC and U-value results by the 3 methods are different (refer to the observations).

Additionally, as the EN and ISO methods use analytical solver, we are able to code our own online glass U-value, SHGC & shading coefficient calculator, based on the two methods.

Results: large difference in SHGC and SC results

As discussed in the observations section and the calculation part section above, the SHGC and SC results by the 3 methods are different. The NFRC SHGC and SC results could be higher or lower than the other two methods.

This could be further explained with the concepts of primary and secondary solar heat gain components, as described in our glass testing service page.

The NFRC method tends to result in a smaller primary solar heat gain component, particularly for glasses with strong spectral selectivity (e.g. low-e coated glasses). On the other hand, the NFRC method tends to result in a larger secondary solar heat gain component, particularly for glasses with high solar absorptance (e.g. tinted glasses) or glasses with poor insulation (e.g. single glazing glasses).

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The practice: Which method to use?

It is important to select a right test method for fair material evaluation.

OTM: we are neutral and technically ready

At OTM, we are technically ready to test glasses according to the NFRC, EN or ISO test method.

We are neutral to the customer’s selection of the test method. We provide the relevant information in this article for customer’s decision-making, and it is the customer’s responsibility to select the right test method.

By default, we use the NFRC method if a customer does not specify a test method.

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Customers: check your project requirements

In Australia and USA, it is clear that the NFRC method shall be used. However, it is not so straightforward for projects in the other parts of the world.

In such cases, a customer needs to check the project requirements or negotiate with the other relevant project members to mutually agree on a test method.

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If you are interested in more in-depth information, please continue reading this article. If your concerned questions are not explained, please feel free to leave a comment at the bottom of this page. This article will be reviewed and updated regularly.

More information

How to identify the test method used in a glass specification sheet?

Most glass manufacturers prepare their glass specifications based on the NFRC method. A small number of glass manufacturers prepare the specifications based on the EN method. Very few glass manufacturers prepare the specifications based on the ISO method.

To identify the test method used in a glass specification sheet, please read the fine prints on the specification sheet:

  • If texts such as “WINDOW software” are mentioned, the specifications are based on the NFRC method.
  • If texts such as “EN 410” are mentioned, the specifications are based on the EN method.

In certain cases, the glass specification sheet is based on the NFRC method, with an additional U-value result based on EN 673.

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Can the test method be changed without re-testing the glass?

As explained in the theory part, at OTM, our laboratory testing work instructions are based on NFRC 300/301 and the spectral transmittance/reflectance results collected can be used for NFRC, EN or ISO calculations.

We can calculate the results according to another test method, without re-testing the glass.

In certain cases, we can issue multiple test reports based on different test methods. For example, on the customer’s request, we can issue two separate reports for one glass sample, one based on the NFRC method and the other based on the EN method.

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Last update: 28/09/2020

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