Glasses from existing buildings

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How to test optical & thermal properties of glasses from existing buildings?

It is often required to test the optical & thermal properties of glasses taken out from existing buildings, for a building retrofit project or performance verification purpose.

This article aims to answer the following 2 frequently asked questions:

  1. Can glass optical & thermal properties be tested on-site?
  2. How to test over-sized glasses removed from existing buildings in the lab?

More information:

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Question 1. Can glass optical & thermal properties be tested on-site?

The short answer is technically yes, but practically no.

Technically yes

There exist a number of handheld or portable instruments for on-site measurement of glass optical & thermal properties:

  • Color and visible light reflectance: most handheld colorimeters can give readings
  • U-value: a number of portable devices exist
  • Emissivity: handheld emissometers can be used on-site
  • SHGC and shading coefficient: portable devices are available for on-site measurement

Practically no

However, there is no consensus in the industry that the results obtained with the on-site instruments are as genuine as the results obtained in lab.

The standard lab test methods have been used for decades. Both the first party (owners, architects and governments) and second party (material suppliers and builders) rely on them for quality control and decision making.

There is currently no confidence on the on-site test instruments. In the worst case, if the on-site results are in favor of the second party, for example, they can become loopholes in quality control, as many will use the on-site methods to get some advantages.

In practice, impartiality is vital to third-party testing. On-site testing of glasses is impractical before consensus standards are agreed. For commercial reasons, OTM also decides not to do on-site measurements at this moment.

More importantly, many glasses from existing buildings can be directly tested in the lab, without deviation from the existing standards. Please continue reading the answers to question 2 below.

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Question 2. How to test over-sized glasses removed from existing buildings in the lab?

Most glasses removed from existing buildings are large in size. Since it is not practical to test them on-site, it is then necessary to test over-sized glasses in the lab.

Sample size requirements

First of all, we need to explain our sample size requirements for glass lab testing.

Listed below are our sample size requirements in the normal circumstances (typically for new glasses):

  • Minimum sample size: 40 mm x 40 mm
  • Preferred sample size: 100 mm x 100 mm or slightly smaller
  • Maximum sample size: 300 mm x 300 mm

The sizes listed above are mainly determined by the instrument UV/VIS/NIR spectrophotometer.

The minimum sample size is based on the measurement port size (1 inch or 25 mm) of the instrument. Samples smaller than 25 mm cannot fully cover the measurement port. We specify the minimum sample size as 40 mm x 40 mm with some allowance.

Transmission port of UV/VIS/NIR spectrophotometer
Reflection port of UV/VIS/NIR spectrophotometer

The preferred sample size is based on the measurement compartment sizes of the instrument. For samples smaller than the preferred size (100 mm x 100 mm), the compartment covers can be fully closed and the measurements can be proceeded directly.

Transmission compartment of UV/VIS/NIR spectrophotometer
Reflection compartment of UV/VIS/NIR spectrophotometer

The maximum sample size is based on the industry’s common practice in preparing inspection samples. Most glass manufacturers prepare 300 mm x 300 mm for owners or architects’ inspection. For this size the compartment covers cannot be fully closed. At OTM, some accessories are used to resolve this issue, without deviation from the test standards.

For test samples between 40 mm x 40 mm and 300 mm x 300 mm in size, we apply a flat rate at OTM.

Oversized samples

For samples larger than 300 mm x 300 mm, they still can be tested, but they are considered oversized samples. Our accessories allow us to measure large glasses. The largest glass we’ve tested so far is 1000 mm x 1900 mm.

For the oversized samples, the testing procedures are identical to those for non-oversized samples, as described in our previous articles.

For over-sized samples, the main challenge is to safely lift and alignment them. The processing time required is also significantly longer. We therefore apply certain surcharge for over-sized samples and the surcharge is dependent on the size. As a general guideline, the largest over-sized sample we can safely handle is 1000 mm x 1000 mm.

With the understanding of the size restrictions, we are now ready to explain how to test over-sized glasses removed from existing buildings. The basic strategy is to reduce the sample sizes as much as possible.

A glass can be without heat treatment or with heat treatment. There are two types of heat treated glasses: heat strengthened glasses and tempered glasses. Therefore, there are 3 scenarios:

Scenario A: glasses without heat treatment

For glasses without heat treatment, it is straightforward to directly cut them to any size between 40 mm x 40 mm and 300 mm x 300 mm, with smaller than 100 mm x 100 mm preferred.

Scenario B: heat strengthened glasses

Heat strengthened glasses cannot be cut to rectangular shapes with straight edges, but can be broken to smaller sizes, with a hammer or equivalent tools. A few small samples, in irregular shapes and smaller than 300 mm x 300 mm, can be picked for lab testing.

Shown below is an example.

Two small glass pieces selected for lab testing, from a broken heat strengthened glass

If the glass is a laminated glass, there might be some crack lines in the broken piece. The crack lines will be avoided during testing and they does not affect the measurement results. Shown below is an example.

The crack lines in the sample above will be avoided during testing

Scenario C: tempered glasses

Tempered glasses cannot be cut nor broken. The small pieces from a broken tempered glasses are too small for lab testing, as shown in the photo below.

A shattered tempered glass and the broken pieces are too small for lab testing

For tempered glasses, below are our suggestions:

  • Please identify the smallest glass size in the building. If it is possible to get a small piece of sample smaller than 1000 mm x 1000 mm, it can be directly tested;
  • If there are no samples smaller than 1000 mm x 1000 mm, please negotiate between the first party and the second party if it is acceptable to test a replacement sample fabricated by the glass manufacturer with the same configuration.
  • Technically, it is not safe for us to test samples larger than 1000 mm x 1000 mm. For samples slightly larger than 1000 mm x 1000 mm, we can justify if they can be tested on case by case basis. As mentioned above, the largest sample we’ve tested so far is around 1000 mm x 1900 mm.

Shown below is an over-sized tempered glass tested by us:

An oversized tempered glass tested by us

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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Can OTM perform glass optical & thermal property testing on-site?


It needs to be determined by the market mechanism. We will provide such on-site services, when:

  • Both the first party and second party agree OTM to use non-standard on-site test methods;
  • There are a sufficient number of such jobs

If the number of jobs is insufficient, it is not practical for us to run such tests in a sustainable way. Based on our observation, the demand for on-site glass optical & thermal property testing is low, when factors including cost and accuracy are taken into account.

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Do the accessories for large glasses affect measurement accuracy?


The accessories used do not deviate from the test standards. We’ve also performed some verification tests and confirmed that the accessories do not affect the measurement accuracy.

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Are the measurement results dependent on the sample size?


The area to be measured in each measurement is fixed, regardless of the sample size. For the UV/VIS/NIR spectrophotometer available at OTM, the area sampled in each measurement is smaller than 1 inch (25 mm) in diameter.

For non-uniform samples, our procedures require us to sample a few randomly selected points, until the mean results are consistent. In that case, large samples have certain advantage, as the measurement points selected are more scattered, but the improvement is negligible for most glasses.

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What are the processing time and lead time for over-sized glasses?

The typical processing time for testing an over-sized glass is significantly longer, due to the extra efforts required to lift and align the larger and heavier test samples. We estimate that it takes around 6 – 9 hours to test one glass pane.

The typical lead time is 5 working days. We may schedule the activities on different days, due to operational reasons.

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