A timber window U-value describes how readily heat passes through it. A lower value means less heat transfer. When you compare replacement windows, ask for the whole-window figure: a number for the glass alone leaves out the frame.
Keep that distinction beside you when reading a quotation. It helps you compare the proposed windows without mistaking a glazing specification for the performance of the finished product.
What does a window U-value measure?
U-values are expressed in W/m²K. They describe heat transfer per square metre for each degree of temperature difference between the two sides. The Australian Glass and Window Association's technical explanation distinguishes the whole-window value from figures for individual components. The physics applies to timber windows too; its Australian building requirements are not UK rules.
A useful first question is: “Does this number describe the complete window I am being quoted for?” Ask the supplier to identify the product and the evidence behind the figure.
Whole-window and glass figures are different
Uw refers to the whole window. Ug refers to the glazing, while Uf refers to the frame. The edge where glass meets frame also contributes to overall heat transfer. BRE's U-value calculation conventions explain that a complete window's value includes the frame and depends on its size and configuration.
If one quote gives a glass figure and another gives a whole-window figure, ask for a comparable measure before choosing. Keep the opening style, size and glazing specification alongside it. A single number copied from a product range does not tell you which configuration it describes.
Is a Window Energy Rating the same thing?
A Window Energy Rating considers more than heat loss. BFRC explains that its ratings include heat loss, solar gain and air leakage. A U-value and an energy-rating band therefore answer different questions.
If a quotation includes an energy-rating label, ask which product it covers. If it includes only a U-value, ask for the supporting calculation or test information. Neither approach should rely on a label borrowed from a different window.
A specification checklist for your quotation
- Give each opening a room name and reference, such as “bedroom, front left”.
- Record whether it is a fixed light, casement or sliding sash.
- Ask for the proposed glazing and the whole-window U-value, with the basis of the figure stated.
- Keep any energy-rating evidence with that specification.
- Discuss ventilation, fitting and any heritage constraints before approving the order.
You can copy those headings into a simple list and use the same list for each quotation. Leave a space for unanswered questions. This makes omissions easier to resolve before comparing totals.
Choosing glazing for a timber window
Schnauber's timber window collection includes double and triple glazing, with slim double and vacuum glazing options in the Heritage Slimline range. Ask which specification suits your opening and, for a period home, which details need to be retained.
Our double versus triple glazing guide covers the wider choice. Use the U-value to compare heat transfer, then discuss how the window will open, look and fit. This guide does not set a regulatory target for your project; confirm the applicable requirements with your installer or building control adviser.
Can a U-value tell me what I will save?
It cannot give you a household saving on its own. Ask what assumptions sit behind any savings estimate, including your existing windows, heating and use of the rooms. Keep a product comparison separate from a prediction about your bills.
Discuss the specification at a free survey
For homes in Cambridge, Bedford or Chelmsford, send your postcode and the openings you want to discuss. You can also check all areas we cover. Schnauber offers a free home survey and no-obligation quotation. Ask us to explain the glazing and performance figures in your proposed specification.