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EON Retrofit Test House: Vacuum Insulation Components

Solid-wall, ‘hard to heat, hard to treat’.


This case study is part of the CALEBRE project, 'Consumer Appealing Low Energy technologies for Building REtrofitting', which focused on solid wall 'hard to heat, hard to treat' properties.

The project considered a range of current, medium and longer-term technologies for domestic energy efficiency refurbishment, with the perspective of the householder at the heart of all thinking and decision making.


Window energy performance is a crucial factor in the thermal performance of a building envelope. In Europe, 44% of buildings still have single glazing, and only around 15% use energy efficient glazing.

Vacuum glazing can be used to improve the thermal performance of a building. It consists of two or more sheets of glass hermetically sealed around the edges to enclose a vacuum between the sheets.

Tiny pillars are used to separate the glass panes and to stop them touching due to differences in atmospheric pressure. Low emittance (low-e) coatings are used on one or more of the internal surfaces to further reduce heat transfer through the glazing.

In this case study, a number of vacuum glazed units were fabricated and laboratory tested to assess thermal performance and durability.

A third pane was then added to create a hybrid unit, with a vacuum cavity and an air filled cavity. The hybrid glazing units were installed in one of the bedrooms in the EOn Retrofit Test House.

Once the hybrid glazing units were installed in the EOn Retrofit Test House, a range of testing to determine acoustic performance and thermal transmission were undertaken. The windows were then removed again, and re-tested within a laboratory to identify if there had been any change in thermal performance during their installation in the property.



The vacuum glazing which manufactured using an optimised method was found to have a centre pane U-value as low as 0.85W/m²K (for a 400mm x 400mm unit with two low-e coated panes).

For the hybrid unit, with a 12mm air cavity, the U-value was reduced to 0.67W/m²K.

The U-value was measured again once the unit had been installed and then removed from the test house, and was found to have increased to 2.11W/m²K. This change was thought to be attributed to the effects of solar radiation causing a drop in vacuum pressure. This could be resolved by introducing suitable getters to prevent out-gassing.

Simulation results indicate that with optimised design, a centre pane U-value of 0.26W/m²K could be achieved for triple vacuum glazing units.


This case study work identified that further research is needed into vacuum glazing, particularly before commercialisation can occur. The practical development of the modelled triple vacuum glazed unit is also required.

Alternative sealing materials need to be identified to reduce the cost of vacuum glazing.

Further testing of in-situ performance is also required.

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