Thermal transmittance of glass: the Ug value

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Insulated glass units, IGU for short, have been around for a long time.

These elements are of primary importance for the energy balance of highly performing buildings and passive houses: that is why we dedicate this article to the thermal transmittance of insulated glass.

The energy balance of a glass unit is determined on one side by its heat losses, and on the other one by its solar heat gains. Glazings are very dynamic elements in the thermal envelope, and need to be designed with great attention, including the shading of them.

The thermal transmittance and resistance of opaque building components is calculated according to ISO 6946 (respectively, U-value and R-value). For IGUs, the thermal transmittance is calculated according to EN 673.

GLASS IN THE SUN: WHERE’S THE SURPRISE?

In the first place, we’d like to bust a common myth that we run into every day.

In summer, a very performing glass DOES NOT cause the building to overheat. To cause overheating is the design mistake that leaves the glazing exposed.

In summer, ANY unshaded glass causes overheating.

Anybody can test this: you just need to park your car in the shade first, and then in the sun. The car is provided with 1-pane glass, with an approximate Ug of 5,7 W/m2K (1.0 BTU/h*ft2*°F). Such a glass allows the car to “get rid” of the heat about ten times faster than with a triple-pane glass.

Does a car in the sun not get hot?

car in the sun

For buildings as for cars, to avoid overheating, you need to shade the glazings. An available alternative for buildings is to work on the “g” value of the glass: we’re going to cover this in a specific article.

Once it is shaded, a highly performing glass unit protect the building from overheating, because it allows for less heat to enter the envelope via transmission than a lesser performing glass.

INSULATED GLASS UNITS: THE STATE OF THE ART

Glass units are currently available in most countries around the world as double pane or triple pane glass. In Germany, double pane glass is no longer available, because triple pane has taken over the market. At the moment, quadruple pane is not economically feasible even in a cold climate such as central Europe.

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Vacuum glazing instead is still in a development phase, and not available on the market (it is also facing a technical limit constituted of an extremely high glass edge thermal bridge).

Overall, the insulative effect of a glass unit is given by two factors:

  • low-e coating of the glass panes;
  • gas fill of the cavities.

The thickness of the glass panes has little or no role in the thermal performance of the unit, because of the very high conductivity value of the glass.

We’re going to cover other parameters of the overall energy performance of glass in dedicated articles, including the “g” value, light transmission and the glass edge thermal bridge.

ALTERNATIVES TO TRIPLE PANE GLAZING

In countries such as the US, some companies offer alternative technologies for performing IGUs to triple pane glass. This is called “heat mirror” technology, where the central glass pane is replaced by one or two plastic sheets.

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This technology allows for lighter glass units, and a higher number of low-e coatings (with a lower “g” value of the glass). However, this technology requires at least one stainless steel spacer per sheet, for the structural stability of the unit.

THE VALUE OF DECIMAL FIGURES

For highly performing buildings and passive houses, glazings have a primary role.

When you deal with the thermal transmittance of the glass (as well as with its “g” value) a certain degree of precision is required:

the Ug value of the glass has to be expressed with at least two decimal figures.

Many data sheets only report one decimal figure, for example: Ug = 0,6 W/m2K (0.1 BTU/h*ft2*°F). Such a value covers a range of different IGUs, ranging from Ug = 0,55 W/m2K (0.097 BTU/h*ft2*°F), all the way down to 0,64 W/m2K (0.113 BTU/h*ft2*°F). This variability in the performance of the glass (7-8%) can compromise the comfort level inside the building and its overall energy performance.

DOUBLE PANE VS TRIPLE PANE: COMFORT AND LIFE CYCLE COST

When choosing between double pane and triple pane glass, the main requisites are:

  • thermal comfort in the room;
  • energy and economic balance of the building.

The first one is accomplished if the temperature on the internal surface of the glass (and of the window) is similar to the average radiant temperature of the rest of the room even on colder days. This requisite is therefore set by local climate conditions.

Radian symmetry

When choosing the type of glass, the first parameter to consider is the local climate.

The second requirement - energy and cost balance - depends on other factors such as the cost of the IGUs (double vs triple pane), the type of heating system and the energy market.

For example, in Rome, the first requirement may be met with a double pane glass, as the external temperatures never drop very low. However, if you consider the life cycle of a window (at least 30 years), the most economic choice may be the triple pane.

In the life cycle of a window (at least 30 years), a triple pane glass can be the most economic choice even in a mild climate.

LOW-E COATINGS

Low-e coatings are made by layer of metal oxides, that are spread over internal surfaces of the glass (surface 2 and 5), to enhance the thermal performance of the glass unit.

IGU faces

PHI glass coating

On one hand, the low-e coatings lower heat losses through the glass (decreasing the Ug value). On the other, they may also lower the solar heat gains provided by the same glass (the “g” value), as well as the amount of light that is able to pass through the glass (light transmission).

This is why more advanced glass units are provided with “selective” coating, that can reflect infrared waves, and let visible light through. In a coming article, we’re going to cover how to properly balance the Ug and g values of a glass unit, according to the use of the building and the local climate.

GAS FILL OF THE CAVITIES

In Europe, the cavities of a glass unit are now by default filled with a gas (Argon, Krypton or Xenon), to improve the thermal transmittance of the glazing. In North America, the market of IGUs is still dominated by silicone-sealed glass units (as opposed to polysulfide), so that the durability of the gas inside the cavities is a lot shorter. Durability of IGUs is going to be covered by an article in the near future.

IGU sealants in EU and US

Considering the same low-e coatings, the transmittance of the unit (Ug value) varies significantly depending on the gas fill.

Emu - Ug gas and cavity

The optimal thickness of the cavities depends on the type of gas. The total width of the cavities, however, is bound to a structural limit that cannot be crossed. The glass units are subject to a “pump” effect due to changes in temperature: in order to guarantee the durability of the gas inside the cavities, this effect has to be contained by limiting the total width of the cavities.

solar pumping

This “pump” effect can damage the primary seal of the glass unit, potentially decreasing the amount of gas inside it, and exposing the glass to damages caused by moisture (internal fogging).

TILTED GLASS UNITS

In “handling” the Ug value of a glass unit, the first wrong step could be to consider this value as constant, regardless of the tilt angle of the glass.

With the same glass assembly, the Ug value varies depending on the inclination from the vertical position.

PlatinumRoofingSkylight

Considering a standard glass unit 4le/18Ar/4/18Ar/4le, its Ug value is:

  • 0,53 W/m2K (0.09 BTU/h*ft2*°F) at 90° (vertical);
  • 0,74 W/m2K (0.13 BTU/h*ft2*°F) at 45°;
  • 0,87 W/m2K (0.15 BTU/h*ft2*°F) at 0° (horizontal).

Of course, this is the case of skylights, where manufacturers provide a Ug value for the unit in vertical position. The design thermal transmittance depends on the slope of the actual roof.

THERMAL TRANSMITTANCE AT DIFFERENT TEMPERATURE

Another aspect of IGUs that is often overlooked is the variation of the Ug value depending on different conditions of internal and external temperature.

The thermal transmittance of opaque assemblies varies depending on moisture content and temperature. The transmittance of a glass unit depends on the temperature of operation. A glass unit is a very dynamic element. The convective movement of the gas contained in the unit depends directly from the temperature of the assembly.

The variability depends of course on the type of gas (air, argon, krypton or xenon), and on the width of the cavities.

We calculated the variability of the thermal transmittance of the same glass unit of the previous example (4le/18Ar/4/18Ar/4le), which has two 18mm cavities filled with argon gas.

Within an usual range of external temperature (from -5°C to +40°C, 23°F to 104°F), the Ug value remains in a 5% range compared to the baseline. For more extreme temperatures (lower than -10°C or higher than 45°C, respectively 14°F and 113°F), the Ug value gets worse in a way it cannot be overlooked.

Emu - Ug value

NOTE: the values of external temperature Te refer to the average external temperature, in the case of a shaded IGU.

CONCLUSIONS

As the glass units cover a role of primary importance in the thermal envelope of highly performing buildings and passive houses, their transmittance value Ug needs to be analysed in detail.

Designers need to choose the Ug value together with other energy parameters, such as the “g” value and light transmission. Caring about the Ug value alone, overlooking the other parameters, is just poor design.

Unlike opaque assemblies, glass units are very dynamic elements: their performance may vary significantly due to glass inclination, as well as to external weather conditions.

In the economy of the design, he right choice of the glass is never pre-determined: it has to meet comfort requirements (which depend on local climate), and the life cycle cost of the window.

NOTE

The variations of Ug values depending on glass inclination, and on different temperature conditions, according to EN 673, have been calculated with Dartwin software.