Aluminium was long seen as a difficult material for low-energy buildings because it conducts heat so well. Modern thermally broken systems have changed that. Several aluminium window systems now reach values suitable for passive buildings and carry Passive House certification. For developers, architects and contractors, the question is no longer whether aluminium can work, but which system, glazing and installation details deliver the performance the energy calculation relies on.
Uw, Uf and Ug: what the values mean
Thermal transmittance, or U-value, shows how much heat passes through one square metre of a building element for each degree of temperature difference, in W/(m²K). The lower the value, the better the insulation. For windows, three values are used:
- Ug is the U-value of the glazing unit at the centre of the glass.
- Uf is the U-value of the frame and sash profiles.
- Uw is the U-value of the whole window, combining glass, frame and the linear heat loss at the glass edge, where the spacer bar joins the glass to the frame.
Uw depends on the window size and proportions as well as on the components. A large fixed window with a high share of glass usually has a better Uw than a small opening window with the same system and glazing, because the frame makes up a smaller share of the area. Published values are calculated for a reference window size, so the Uw for your actual window schedule should be checked for the real sizes.
For curtain walling the equivalent value is Ucw, the U-value of the whole facade including mullions, transoms, glass and panels.
Why glazing and installation matter as much as the profile
A high-performance frame will not deliver passive-level results with average glazing. Triple glazing with low-emissivity coatings, gas filling and a warm-edge spacer is normally needed to reach the lowest Uw values. The spacer affects the edge heat loss, which is significant in windows with narrow glass panes.
Installation is the other half. The window must sit in the insulation layer of the wall, with the frame fixed and sealed so that the insulation continues around it. A window mounted too far out or too far in, or with gaps around the frame, creates a thermal bridge that can cancel much of the profile's advantage and cause condensation. Airtight inner seals and weather-resistant outer seals complete the connection.
- Position the window in the insulation plane, or use brackets that allow it.
- Plan airtight and vapour-controlling membranes inside, weatherproof membranes outside.
- Check the installation thermal bridge in the energy calculation, not just the window Uw.
- Coordinate sill, head and jamb details with the wall build-up early.
Which aluminium systems reach which values
These are the systems in our range most often used on passive and low-energy projects. The values are best-case figures for the system with suitable glazing.
- Aluprof MB-104 PASSIVE: windows and doors with Uw from 0.53 W/(m²K), certified by the Passive House Institute in Darmstadt.
- Aluprof MB-86N: 86 mm profiles with Uw from 0.62 W/(m²K) and burglar resistance up to RC4.
- Aluprof MB-79N: Uw from 0.64 W/(m²K) with security up to RC3, a good balance of performance and profile depth.
- Procural PE96HI: 96 mm system for energy-efficient and passive buildings, Uw 0.66 W/(m²K) with triple glazing.
- Schüco AWS 90.SI+: 90 mm basic depth with Passive House certification.
- Schüco AWS 120 CC.SI: Passive House certified composite window with integrated Schüco CCB solar control.
For large glazed areas, curtain walling can also reach passive level: Aluprof MB-SR50N HI+ achieves Uf from 0.59 W/(m²K) and Procural PF152HI is certified for Passivhaus with Ucw 0.62 W/(m²K). For large openings to terraces, Aluprof MB-82HS lift and slide doors reach Uw from 0.60 W/(m²K).
Beyond the U-value
Energy performance is not only about heat loss in winter. Large glazed areas in offices and apartment blocks can overheat in summer, so solar control glass, external shading or integrated systems such as the CCB in AWS 120 CC.SI should be considered together with U-values. Airtightness classes and water tightness affect comfort and durability, and acoustic requirements may change the glass build-up and therefore the Uw.
National requirements for new buildings differ between countries and are updated regularly. Always check the requirement that applies to your project and the method your energy calculation uses, rather than relying on a single number from another market.
When comparing systems, ask for Uw calculated for your real window sizes and glazing, not only the best-case figure from the brochure.
Choosing the right system for your project
- 1Start with the energy target and the calculation method used for the building.
- 2Decide on glazing: triple glazing, solar control, acoustic and safety requirements.
- 3Choose the system depth and profile width that meet the target at your window sizes.
- 4Add security (RC class), opening types and hardware.
- 5Detail the installation in the wall with the architect and the contractor.
You can see all Aluprof, Procural and Schüco systems on our aluminium window systems page, and passive-level facades on the curtain walling systems page.
Send us your window schedule
Finestra Solution manufactures and supplies aluminium windows from Liepāja, Latvia, for commercial, public and multi-residential projects. Send your window schedule, drawings and energy targets through our contact page and we will reply within 1–2 working days with a system proposal and a quote. The first project consultation is free.
