Double glazing testing procedures

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Double Glazing Unit (DGU) Optical & Thermal Property Testing Procedures

Double glazing units (DGUs) are glazing systems consisting of two single glazing panes and one gas space (e.g. air or argon).

Instead of testing the DGU assembly directly, the two single glazing panes are separated and tested one by one, according to the identical procedures of testing standalone single glazing glasses. The gas space is not physically tested, and only considered in the calculation.

This article aims to explain the step-by-step procedures for DGU optical & thermal property testing.

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Step 1. Separation of DGU into standalone outer pane and inner pane

A DGU cannot be directly tested in its assembly form. Upon receiving a DGU sample, the sample is separated into two standalone single glazing samples, one outer pane and one inner pane, as the example shown below.

DGU assembly before separation
Standalone outer pane glass
Standalone inner pane glass

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Step 2. Individual single glazing testing

The two glasses are tested individually, in the same way as standalone single glazing glasses, with the following procedures:

  1. Cleaning and labelling
  2. Spectral reflectance measurement with FTIR
  3. Spectral transmittance/reflectance measurement with UV/VIS/NIR
  4. IGDB file generation with IGDB@OTM
  5. Result calculation with LBNL Optics/WINDOW or Glazing@OTM

Refer to the article on single glazing testing procedures for more details.

In step 4, two IGDB files are generated, one for each glass.

In step 5, the optical properties of the two standalone glasses are calculated. The DGU system optical & thermal properties are calculated in the next step.

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Step 3. DGU result calculation  

The optical & thermal properties of the DGU are calculated by combining the outer pane, the gas gap and the inner pane virtually in the LBNL Optics/WINDOW or Glazing@OTM software.

Shown below are the screenshots of the software tools in DGU result calculation.

DGU result calculation with LBNL WINDOW software
DGU result calculation with Glazing@OTM software

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Step 4. Report writing

The last step is to write the test report according to the software calculation results.

There are 3 result tables, one for the outer pane, one for the inner pane and one for the DGU. The first two result table are intermediate results for reference only. The last result table lists the optical & thermal properties of the DGU.

Shown below are 3 example result tables based on the NFRC methods.

Example result table of an outer pane glass
Example result table of an inner pane glass
Example result table of a DGU

In summary, a DGU is not directly tested in its assembly form. It is separated into standalone outer pane and inner pane glasses. The two glasses are tested as two independent single glazing glasses. 

The outer pane glass, the gas space and the inner pane glass are assembled virtually in the calculation software to get the optical & thermal properties of the DGU.

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 end of this page. This article will be reviewed and updated regularly.

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What are the processing time and lead time?

The typical processing time for testing a DGU is around 1.5 times of that for testing a single glazing, i.e. 6 – 9 hours. 

The typical lead time is 3 working days. We may schedule the activities on different days, due to operational reasons.

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Can the test samples be submitted in the standalone form?  


It is possible for us to test glass samples in the standalone form. In that case, we don’t need to separate the inner and outer panes.

It is also possible for us to re-use the optical data of a tested glass sample. For example, if there are 2 DGU samples with the same type of inner pane glass, it is possible to test the inner pane once only and combine the inner pane results with the 2 sets of outer pane results.

However, for test samples with soft low-e coating, the coating may degrade when the test sample is submitted in the standalone form. For this reason, we still encourage DGU samples to be submitted in the DGU assembly form.

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Can a DGU with argon space be tested?


In fact, the gas type and gas space thickness are not physically tested, but declared by the client. The gas type can be air or argon.

Normally, the client needs to provide the configuration of the DGU to be tested. The lab retrieves the gas gap type and thickness information from the configuration information.

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Is it possible to test a DGU assembly directly, without  separating it?

According to the NFRC, ISO or EN standards, a DGU has to be tested pane by pane and cannot be tested directly in the assembly form.

There are mainly two reasons: 1) pane by pane information is required for the thermal calculation; 2) the measurement instruments are only accurate for thin samples.

However, technically, the following optical properties can be obtained by directly testing a DGU assembly:

  • Visible light transmittance & reflectance
  • Solar energy transmittance & reflectance

The accuracy is satisfying for typical DGU samples.

It is not possible to test the other optical & thermal properties without separating the outer pane and inner pane.

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Last update: 05/04/2020

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