A U-value describes heat transfer, but the number on a quotation does not always describe the complete window or door. Understanding what has been measured makes specifications easier to compare.
What a U-value tells you
A U-value describes the rate at which heat passes through a building element. It is expressed in watts per square metre kelvin, written as W/m²K. A lower number indicates less heat transfer and therefore better thermal performance.
The figure is useful, but only when it refers to the same part of the product. A glass-only value cannot be compared directly with a whole-window value.
Ug, Uf and Uw are not interchangeable
- Ug relates to the centre-pane glazing.
- Uf relates to the frame.
- Uw relates to the complete window, including glass, frame and the effect of the spacer at the glass edge.
For doors, you may also see Ud. When comparing products, ask for the value that represents the complete proposed unit. A very low Ug figure can look impressive in a quotation while giving an incomplete picture of overall performance.
Use the complete unit at the proposed size. A generic brochure figure is a useful indication, but it is not a substitute for the calculation or declaration that applies to the actual configuration.
Why the size and layout change the result
Glass and frame do not perform identically. A large fixed pane has a higher proportion of glass and less frame than a small opening window. Add mullions, transoms and opening sashes, and the proportion changes again.
This is why two windows made from the same system can have different whole-window U-values. Glass thickness, coatings, gas fill, spacer bars and frame reinforcement can also affect the result.
The current Virtus brochure lists indicative system capabilities, including Uw values from 1.0 W/m²K for its slim casement platform and from 0.66 W/m²K for selected COR 80 arrangements. These figures should be checked against the chosen size, opening type and glass package.
The opening around the frame still matters
A high-performing unit can be undermined by a poorly detailed perimeter. The position of the frame within the wall, insulation at the reveal, airtightness, membranes and the continuity of the damp-proofing all influence the installed result.
Thermal bridging and surface temperatures around the reveal can contribute to local condensation even when the glass and frame are performing as specified. The interface between glazing and wall should therefore be included in the construction details.
How to compare quotations
- Confirm whether the figure is Ug, Uf, Uw or Ud.
- Ask which size and configuration the value represents.
- Check that the glass specification is the same across quotes.
- Review spacer bars, coatings and gas fill.
- Confirm what is included for perimeter sealing and insulation.
- Make sure the proposed result suits the project requirements.
U-values are an important part of product selection, but they sit alongside solar control, daylight, ventilation, acoustics and comfort. The best specification balances those needs rather than chasing one number in isolation.