Why the R-value printed on a glazing specification does not necessarily tell you how a skylight will perform.

When considering the thermal performance of a skylight, it can be tempting to look for a single R-value and use that across a project.

Unfortunately, skylights are not quite that simple.

The construction R-value of a skylight is influenced by the complete unit, not just the glass. Under H1, the calculation includes both the glazing and frame, with the glazing performance also assessed at the angle at which it will be installed.

This means several factors can change the final performance.

Glazing

Double and triple glazing can be specified with a range of Low-E coatings, spacer systems and gas fills. Each combination has different thermal characteristics.

Frame

The glass is only part of the skylight. Heat also travels through the frame, making the thermal performance of the frame and its thermal breaks an important part of the overall result.

Pitch

Glass does not necessarily perform the same horizontally as it does vertically. As the angle changes, convection within the insulated glass unit changes too. For this reason, the glazing performance used for a roof window needs to account for its installed angle.

Size

The proportion of frame to glass also changes with skylight size. Two roof windows manufactured from exactly the same frame and glazing can therefore have different overall construction R-values.

This is why a generic R-value should not simply be applied to every skylight on a project.

At Adlux, we can provide project-specific thermal information for Harmony Roof Windows based on the size, glazing and installation conditions being considered.

The result is better information at specification stage and a clearer understanding of what the roof glazing is contributing to the overall thermal envelope.