Glass schedules often show U-values and g-values beside a product name. These numbers answer different questions and should be read with the configuration that produced them.
Separate heat loss from solar gain
U-value describes heat transfer through the assessed element: a lower value means less heat transfer under the stated conditions. The g-value describes the proportion of incident solar energy admitted through the glazing. A low glass U-value does not, by itself, describe solar-control performance.
Ask what has been assessed
Before comparing quotations, request the complete glass build-up and calculation output. Is the U-value for the centre of the glass or the complete framed assembly? Are cavity widths, coatings and gas assumptions consistent between proposals?
For a comparison sheet, place the configuration next to each result and record the assessment method. Leave a separate space for frame and junction performance. This prevents an attractive headline glass value from being mistaken for the performance of the whole façade. Confirm that the proposed build-up is available from the actual supplier.
Reference and further reading
Pilkington: Spectrum — calculated glass properties and supplier checks.
General educational guidance. Use current local requirements, product-specific evidence and competent project assessment for design and construction decisions.