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Thermal transmittance
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Thermal loss through an insulating glass unit is expressed in thermal transmittance factor (k-factor) or U-value, which is normally expressed in SI metric system as W/m²K, is the loss of heat through one square metre in constant conditions divided by the difference in temperature per one Kelvini or degree Celsius between the internal and external environment separated by a glass pane or some other construction element.

In other words U-value shows how much heat energy (in watts) is transmitted via an insulating glass unit with the size of 1m² or any other construction element in one hour, when the difference of external and internal temperature is 1° C.

There are three different causes of heat loss via an insulating glass unit:
• heat loss from the room on the surface of an internal glass pane
• heat loss through the glass
• heat loss from the surface of the external glass pane

Heat loss from the room through the surface of an internal glass pane occurs when the temperature of the surface of the glass pane is lower than the internal air temperature. A single glass provides little protection against heat transfer since glass in general is a good thermal conductor. One of the possibilities to reduce heat transfer is to add another glass pane so that the two glass panes are separated with an air cavity. An air cavity reduces heat exchange between the glass panes of an insulating glass unit thanks to the fact that compared to glass, heat transfer of air is relatively low. In addition the second glass pane prevents transmittance of long wave radiation through the insulating glass unit.

Glass panes

The most commonly used glass to improve thermal insulation and to achieve low energy consumption is selective glass (Pilkington Optitherm SN). Externally a selective glass pane is similar to a normal float glass pane, one side of which is covered with a special transparent coating with low emissivity (Low-E). The coating is transparent, with slight, almost invisible colour shade and it has a minimum impact on light transfer and reflection properties. Using a selective glass pane in an insulating glass unit reduces heat loss via windows up to 30%. Low-E coating of the glass surface lets short wave radiation (light) inside the building but prevents loss of long wave heat radiation generated by the heating systems through windows. A selective glass pane is installed as an internal glass pane in the insulating glass unit so that the coated surface remains inside. In this way the internal surfaces warm up which in its turn reduce temperature fluctuation and generation of condensation and unpleasant cold is not emitting from the surface of the internal glass in to air. In some combinations of an insulating glass unit the selective glass pane is installed as an external glass, in which case the U-value does not change but the transfer of solar energy reduces up to 4%. Selective glass panes with hard surface coating can be used in an insulating glass unit with glass panes blocking UV-radiation. Glass panes with hard surface coating can be toughened and laminated, which cannot be done with glass panes with soft surface coating. Soft surface coating is applied on a glass pane after laminating and toughening.


To improve the U-value the insulating glass units with selective glass panes may also contain gases heavier than air such as argon or krypton. Today mostly argon is used, in which case price plays an important role.

Spacer, air cavity

Presence of an aluminium spacer generally increases the U-value of an insulating glass unit. A spacer installed in an insulating glass unit functions as a thermal “short circuiter” in the edges of the insulating glass unit, which sometimes causes generation of condensation in the edges of the insulating glass unit and in cold climates also frost and ice. To reduce or eliminate this we suggest to use triple insulating glass unit, where required.

In addition to glass and gas type, also increasing of the width of the spacer may increase thermal insulation up to 10%. But the width of the spacer cannot be increased unlimited due to convection, which occurs in the air cavity after a slight improvement in thermal insulation is achieved. The approximate limit is 16–18 mm. When a third glass pane is added to the insulating glass unit, a second air cavity is created and the k-factor is even lower. In Estonian climate it is not recommended to use an insulating glass unit with a k-factor less than 1.2 W/m²K, since there is a risk that condensation is generated on the external surface of the insulating glass unit. This risk is especially big during humid periods from autumn till spring.



Insulating glass units Spacer width, mm
6 9 12 15 18 20
Float, air, float 3.3 3 2.9 2.8 2.7 2.8
Float, argon, float 3 2.8 2.7 2.6 2.6 2.6
Float, air, Pilkington Optiherm SN 2.5 2 1.6 1.4 1.4 1.4
Float, argon, Pilkington Optiherm SN 2 1.5 1.3 1.1 1.1 1.1

Total thickness of the insulating glass unit, mm

14 17 20 23 26 28


Insulating glass units Spacer width, mm
6+6  9+9  12+12 6+9  6+12 9+12
Float, air, float, air, float 2.3 2 1.9 2.2 2.1 2
Float, air, float, argon, float 2.2 1.9 1.8 2.1 2 1.9
Float, air, float, air, Pilkington Optiherm SN 1.9 1.5 1.3 1.6 1.4 1.3
Float, air, float, argon, Pilkington Optiherm SN 1.6 1.2 1 1.3 1.1 1.1
Total thickness of the insulating glass unit, mm 24 30 36 27 30 33


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