What affects the thermal performance of roof glazing?

In any construction project, having an early idea of likely material specifications is helpful to avoid last minute shocks and surprises. It’s particularly relevant to rooflights and roof windows, though, because designing a healthy building means not taking quoted U-values at face value. The composition of roof glazing units – including overall size, relative areas of glazing and frame, and the thermal performance of the materials used – varies, so U-values quoted in a specification should be based on an actual product or a detailed calculation model. Unfortunately, there exists an element of misunderstanding and potential confusion when specifying U-values for rooflights. A performance specification may not be clear about whether a whole-unit or centre pane value is required, and some manufacturers are not clear about which they are quoting. What is a centre pane U-value? Centre pane U-values address the thermal performance of the glass only. They appear lower because the cold bridging effect of the spacer and edge seal are not accounted for. Where two different types of glass are being considered for the same frame, centre pane values are a useful comparison tool. Conservation projects make good use of centre pane values, because traditional frame designs offer no meaningful thermal performance but the glass needs to be shown to help achieve regulatory compliance. In most other circumstances U-values should be for the whole unit, including glazing and frame, but too many manufacturers still rely on quoting centre pane U-values.

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