Work out the Uw of the whole window
Combine glass, frame and spacer. See the Uw and how much each part weighs. Free, and what you enter stays in your browser.
Your values
The starting values are only an example to show the sum. Use those from the data sheets of your glass and profile.
Result
1.26 W/(m²·K)
- Glass area
- 1.12 m²
- Frame area
- 0.32 m²
- Glass perimeter
- 4.24 m
How much each part weighs in the Uw
- Glass68%
- Frame23%
- Glass edge (spacer)9%
This is a window with a single opening, with the same frame width on all four sides. Windows with several sashes, mullions or a shutter box have more frame and a different result. The certified Uw comes from the maker.
How it works
How it works
The Uw adds up three heat losses: through the glass, through the frame and through the edge of the glass, where the spacer is.
Each loss is multiplied by its area (or, at the edge, by its length), added up, and divided by the area of the window.
The formula
- Uw = (Ag × Ug + Af × Uf + Lg × ψ) ÷ (Ag + Af)
- Ag is the glass area, Af the frame area, Lg the glass perimeter and ψ the linear coefficient of the spacer. It is the structure of the EN ISO 10077-1 method.
- The calculator treats the glass as one piece and the frame with the same width on all four sides.
Limits
- It does not replace the manufacturer's certified value or the full calculation of the standard.
- The Ug, Uf and ψ values must come from data sheets: we give no typical values.
- It does not count the installation (the thermal bridges between window and wall).
Sources
- 21degrees: Understanding windows, U values, psi values. Explains the structure of the Uw calculation, with the ψ of the glass edge and the role of each term.
- The EN ISO 10077-1 standard defines the method; we did not read its full text. For a design calculation, check the standard and the maker's data sheets.
Frequently asked questions
It is the heat loss along the edge of the glass, per metre, in W/(m·K). It depends on the spacer. Ask the maker for the value.
When the glass is very good, the frame and the glass edge weigh more in the total. So the window's Uw can end up above the glass's Ug.
Yes, as long as you use the values from the data sheets of both. Also compare the certified Uw each maker declares.
Yes, for windows in housing in new buildings or with renovated windows, with exceptions. The maximum depends on the winter climate zone (Portaria n.º 138-I/2021). See the "Climate zone" block on your town's page.
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