This is one of those cases where the label performance should not be taken at face value: U-value of glass units – if the glass is meant to be installed horizontal or with a tilt angle. The analysis shown here illustrates the performance of two different IGUs, depending on the tilt angle.
Thermal performance of insulated glass units (IGUs) is driven by the combination of 1) the number of gas pockets (or in layman’s terms, the number of glass panes), the thickness and the fill of the glass pockets, and the low-emissivity coatings (low-e) on the glass panes).
The gas pockets provide insulation because the gas (whether air, Argon, or Krypton) conducts less heat than a solid. The more still the gas remains, the higher its insulation property, the lower the heat losses.
However, the gas does move. In doing so, it is subject to gravity, meaning that its ability to provide insulation is impacted by the tilt angle of the IGU.
Unfortunately, manufacturers provide glass U-values assuming the glass is vertical, even for IGUs intended to be installed horizontally or at an angle – as in the case of skylights. In other words, the label value may not represent the actual performance of the glass.
The performance gap is not negligible – the heat losses through the glass can be up to 30-35% higher than the label value.
Of course, this does impact the risk for condensation, as well as the whole building energy performance.
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