Some example calculations were conducted to illustrate the possible differences between EN, ISO and NFRC glazing system test methods. The three standards are widely used, but specific national standards may exist for respective countries (e.g. JIS R3106:1998 and JIS R3107:1998 for Japan).

Sample glazing systems

Two sample glazing systems were selected:

Double glazing unit with high spectral selectivity: AGC Stopray Clearvision 60T 6 mm Stopray Clearvision 60T with coating on #2 (IGDB #4383) + 12 mm air space + 6 mm generic clear glass (IGDB #103)

Single glazing with hard low-e coating: AGC Sunergy Clear 6 mm Sunergy Clear with coating on #2 (IGDB #4210)



The two representative examples were selected because relatively large discrepancies can be expected based on our experiences.

Calculation methods

Glass material optical data available in IGDB were used in the calculations (accessed through Optics).

WINDOW was used for NFRC calculations.

Proprietary calculation tools developed by OTM was used for EN and ISO calculations.

Results

The tables below list calculation results obtained according to the three methods:

For the double glazing unit

Mehod EN ISO NFRC Visible light transmittance (VLT) 0.605 0.605 0.605 Visible light reflectance, front (VLR f ) 0.151 0.151 0.151 Visible light reflectance, back (VLR b ) 0.198 0.198 0.198 Solar transmittance (ST) 0.349 0.344 0.317 Solar reflectance, front (SR f ) 0.374 0.384 0.424 Solar reflectance, back (SR b ) 0.290 0.296 0.323 Solar heat gain coefficient (G) 0.393 0.390 0.365 Shading coefficient (SC) 0.452 0.448 0.419 Thermal transmittance, winter (U w ) 1.57 1.57 1.63 Thermal transmittance, summer (U s ) N/A N/A 1.57

For the single glazing system

Method EN ISO NFRC Visible light transmittance (VLT) 0.677 0.677 0.677 Visible light reflectance, front (VLR f ) 0.086 0.086 0.086 Visible light reflectance, back (VLR b ) 0.102 0.102 0.102 Solar transmittance (ST) 0.538 0.535 0.517 Solar reflectance, front (SR f ) 0.092 0.092 0.094 Solar reflectance, back (SR b ) 0.103 0.103 0.105 Solar heat gain coefficient (G) 0.600 0.603 0.599 Shading coefficient (SC) 0.690 0.694 0.688 Thermal transmittance, winter (U w ) 4.10 4.11 4.12 Thermal transmittance, summer (U s ) N/A N/A 3.36

All quantities are unit-less, except the thermal transmittance (U), with the SI unit of W/(m2K).

Observations and interpretations

The visible light quantities (VLT and VLR) calculated by the three methods are the same All of them are based on CIE 015:2004 (or equivalent)

There are relatively large discrepancies among the solar related quantities (ST, SR, G and SC) calculated by the three methods, particularly for the double glazing unit with high spectral selectivity The reference solar spectrum in EN 410 is based on the AM1 global spectrum (direct + diffuse) defined in CIE 085:1989 ISO 9050 uses AM1.5 global spectrum (direct + diffuse) defined in ISO 9845-1:1992 NFRC 300 uses AM1.5 direct normal spectrum defined in ISO 9845-1:1992

There are some discrepancies among the thermal transmittance quantities (U) calculated by the three methods, particularly for the single glazing system with hard low-e coating on surface #2 (summer condition U-value is significantly lower) The internal and external surface heat transfer coefficients defined in the three methods are different There is no separate summer condition thermal transmittance in the EN and ISO methods



Which test method shall I use?

All methods are widely accepted and fair comparison can be made only if the results are from the same method

It is not appropriate to compare results obtained with different methods

It is at the end user’s discretion on method selection. Your end user could be internal staffs, project owners, certification bodies or government agencies.

If the end user has no preference, NFRC method is recommended due to the availability of a suite of software tools. The summer condition thermal transmittance is also more relevant for summer condition applications

Please feel free to contact us for any enquiries.