A U-value measures heat transfer: the lower the number, the better the thermal performance. Ug normally describes the centre-pane glass value, while the declared rooflight or product value should account for the complete unit and its orientation.
Ug is not the complete rooflight
The glass-only Ug value does not include the frame, edge spacer, junctions or installation. Comparing two rooflights only by their centre-pane value can therefore be misleading.
Ask which value is being quoted, what orientation was used and whether it represents the complete rooflight dimensions being considered.
Orientation matters
Heat flow and gas movement within a sealed unit change when glazing is installed horizontally or on a slope. A value calculated for vertical glazing should not automatically be used for a rooflight.
Frame design, glass size, cavities, low-emissivity coatings and thermal breaks all influence the result.
Project checklist
Building Regulations and calculation methods change. The project energy assessor or approved calculation should confirm the value required for compliance.
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