Glass

Special note on laminated glass with embedded coating

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Laminated glass with embedded coating refers to laminated glass in which the coating is in direct contact with the interlayer.

The example below illustrates this configuration, where the low-e coating is located on surface #2 and is in contact with the interlayer.

As clearly documented in THERM 7 / WINDOW 7 NFRC Simulation Manual (page 8-111), the standard calculation method cannot be applied to this type of glass construction.

This is because when a coating is in direct contact with the interlayer, its performance can change compared to when it is exposed to air, and these changes are not fully reflected in the standard calculation models.

For laminated glass with embedded coatings, the most reliable approach to obtaining thermal and optical performance data is to fabricate representative samples and carry out physical measurements.

In practice, calculation methods are sometimes applied to estimate performance data for such glass configurations. However, due to the limitations noted in the NFRC documentation, results obtained in this way may not accurately represent actual performance.

Can the testing fee be reduced by skipping some glass optical property results?

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The following table of results is presented in our glass optical & thermal property test report:

We sometimes receive questions about whether the testing fee can be reduced by excluding some optical property results.

Please refer to this page for information on the testing procedures for glass optical and thermal properties. The optical properties serve as intermediate results in determining thermal properties. The solar heat gain coefficient (SHGC) and shading coefficient (SC) are derived from 5 sets of raw data on spectral transmittance/reflectance. All other results are calculated from this same data set without requiring additional testing.

Therefore, even if some optical properties are omitted, the testing procedures remain unchanged, and the testing fee will not be reduced.

Can glass optical & thermal property test be performed on shattered tempered glass pieces?

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We frequently receive inquiries about whether optical and thermal property testing can be performed on shattered tempered glass pieces.

However, our instruments require a minimum sample size of 1 inch (25 mm) to obtain accurate results. Since shattered tempered glass pieces are typically smaller than this, it is impractical to conduct the tests on such small fragments.

For tempered glasses, the most practical solution is to remove a full and intact piece of glass from the building for testing, as detailed on this page.

Diffuse daylight reflectance of glasses

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Related services Glass optical & thermal properties, Daylight reflectance

As discussed in the glass daylight reflectance page, the diffuse daylight reflectance of a glass is negligible, with the following two relationships:

Diffuse daylight reflectance = 0
Total daylight reflectance = Specular daylight reflectance

In the test report, we will report the glass daylight reflectance only. Below is an example:

There is only 1 result, without the separate total/diffuse/specular reflectance components.

If the result above is expressed in the conventional total/diffuse/specular daylight reflectance format, it would be:

  • Total daylight reflectance = 0.088 (8.8%)
  • Diffuse daylight reflectance = 0.000 (0.0%)
  • Specular daylight reflectance = 0.088 (8.8%)

The diffuse daylight reflectance is negligible, but not exactly 0. It is related to the haze level of the glass. The haze level of typical glasses is less than 0.5%. The diffuse daylight reflectance can be calculated as 0.088 × 0.005 = 0.00044 (0.044%) ≈ 0.000 (0.0%)

In practice, the instrument is not able to accurately measure (or resolve) the diffuse daylight reflectance of a glass. The test method of measuring glass diffuse reflectance is also not defined in the standards.

Therefore, there is only 1 result in the test report, without 3 separate components. The diffuse daylight reflectance of a glass can be estimated theoretically, but cannot the determined by an instrument.

Glass U-value and glass tilt

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Related services Glass optical & thermal properties

We were asked why the U-value of a glass is different when the glass is installed horizontally.

There are 3 heat transfer modes: conduction, convection, and radiation. The convection part is dependent on the glass tilt and it affects the glass U-value.

By default, we evaluate the U-value of a glass with the vertical tilt, which is the most common position of glasses. For a horizontally tilted glass, the U-value is significantly greater than the U-value of the same glass with the vertical tilt.

Besides the dependency on tilt, the U-value is also dependent on the glass height. Other thermal properties (e.g. SHGC) are dependent on the tilt too.

However, it does not mean that the glass U-value shall be evaluated with different tilts. There are primarily two applications:

  • Glass performance rating
  • Fenestration performance rating

For glass performance rating, it is sufficient to evaluate the glass U-value with the vertical tilt only. With this standardized tilt, fair comparisons can be performed conveniently.

For fenestration performance rating, the glass tilt is considered in the evaluation by default.

Are the test methods for glass optical & thermal properties applicable to transparent plastic sheets?

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Related services Glass optical & thermal properties

As an alternative to glasses, several transparent plastic materials are utilized as window glazing panels. Transparent polycarbonate sheets and transparent acrylic sheets are two examples of such transparent plastic materials.

Can the standard NFRC/EN/ISO glass test methods still be employed to determine the optical & thermal properties of such transparent plastic materials?

The NFRC/EN/SIO glass test methods are for glazing materials, which are not limited to glasses. Plastics are a type of glazing material. Transparent plastic sheets can be tested by the NFRC/EN/ISO methods when the following conditions are met:

  1. With specular transmission and reflection only: materials with significant diffuse transmission/reflection are out of the scope (for example, frosted glasses, glasses with ceramic frits, and hazy plastic sheets).

    Note: in the latest NFRC methods (2020 version), diffuse materials are supported, but our lab is not ready to test such diffuse materials.
  2. Homogenous and flat sheet: corrugated plastic sheets and double-wall (multiple-wall) polycarbonate sheets are out of the scope.
  3. Without far-infrared transmission: plastic sheets with significant far-infrared transmission in the 5 µm to 50 µm range are out of the scope.

    Note: in the NFRC methods, it is possible to test materials with far-infrared transmission, but our lab is not ready to test such materials.

In principle, the scope of the NFRC/EN/ISO glass optical & thermal property test methods is based on the optical characteristics, but not on the material type. Transparent plastic sheets with the same optical characteristics as transparent glasses are within the scope.

Glass UV transmittance calculation

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Related services Glass optical & thermal properties

As defined in ISO 9050 or EN 410, the UV transmittance of glass is calculated with the equation below:

In the equation above:

  • τUV: UV transmittance
  • λ: wavelength
  • τλ: spectral transmittance
  • SλΔλ: normalized relative spectral distribution of the UV radiation (part of the standard global solar radiation)

The wavelength range of interest is 300 nm – 380 nm. The term SλΔλ is the weights used in the weighted average of the spectral transmittance. The standard values of SλΔλ are plotted in the figure below.

The peak of the UV radiation distribution is at 375 nm. Glasses with high spectral transmittance near 375 nm are with high UV transmittance.

There are some small differences between the ISO 9050 and EN 410 UV distributions. The UV transmittances calculated according to the two standards could be slightly different.

Import Optics user database into LBNL WINDOW

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Related services Glass optical & thermal properties

We have a post on how to import user IGDB format file into LBNL optics. The next step is to import the user database created into LBNL WINDOW for further calculations. Please refer to the steps below for the operations.

Step 1. Set the user database as the optical data source

In the “File” -> “Preference” -> “Optical Data” tab, browse the Optics user database file and set it as the optical data source, as shown below.

Step 2. Import glass optical data from Optics user database

In the glass library (“Libraries” -> “Glass”), click “Import” (in the “List” view). In the pop-up window, set the format as “IGDB or Optics User Database”, as shown below. The optical data entries in the Optics user database can then be imported in the next pop-up window.

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

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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.