U-values are central to how we specify and compare windows across every energy-efficiency framework in use today. They appear in planning documents, energy models and product datasheets at every stage of a project. Yet the number itself is only part of the story: different standards use U-values in very different ways, and real-world performance depends on more than a single figure can convey.
Key Takeaways
- The Future Homes Standard requires a whole-window U-value (Uw) of 1.2 W/m²K or lower in new builds.
- Passive House projects typically require a whole-window Uw of ≤0.80 W/m²K; once installed, the effective threshold including the frame-to-wall thermal bridge is ≤0.85 W/m²K.
- Meeting 1.2 W/m²K is compliant, but it is not enough to eliminate cold spots, condensation risk, or meaningful heating demand in well-insulated homes.
- Always specify and compare whole-window U-values (Uw), not centre-pane values (Ug). The difference can be substantial.
- Installation quality can make or break the performance a window achieves on paper.
What Is a U-Value?
A U-value measures the rate at which heat passes through a building element: the lower the number, the better the insulation. For more on how glazing specification affects thermal performance in practice, Norrsken’s guide to triple glazed windows is a useful starting point.
Future Homes Standard vs Passive House: The Numbers Side by Side
The Future Homes Standard (FHS) uses a notional building comparison rather than an absolute energy target. For new builds, the notional dwelling specification sets a whole-window U-value of 1.2 W/m²K – a significant improvement on the 1.6 W/m²K that was acceptable under earlier Part L rules, with a backstop limiting value of 1.4 W/m²K. Full implementation is expected by December 2026, and while triple glazed windows aren’t mandatory, the required U-values make it the default route for new builds.
For Passive House (PHI-certified new builds), the primary requirement is an annual heating demand of ≤15 kWh/m²/year, with internal surface temperatures sufficient to prevent condensation and cold discomfort. In the UK’s cool-temperate climate, this typically means a whole-window Uw of ≤0.80 W/m²K – the PHI’s component certification threshold. Once installed, the linear thermal bridge at the frame-to-wall junction raises the effective threshold to ≤0.85 W/m²K. Because PHPP models the building as a whole, a window marginally above 0.80 W/m²K may still be compatible with certification if the wider fabric compensates. The 0.80 figure is a reliable benchmark, not an absolute gate for every opening.
Projects targeting net zero may choose to go beyond the FHS minimum depending on design strategy – the two standards are not competing, but they do reflect different levels of ambition.
Is 1.2 W/m²K Enough for a Net-Zero Home?
In a well-insulated, airtight home with a heat pump, windows become proportionally more significant as a source of heat loss. Meeting the minimum backstop reduces energy waste, but it won’t completely eliminate cold spots near the glazing or the downdraught sensation that homeowners notice on cold winter mornings.
It also does not guarantee low heating demand in PHPP modelling or SAP calculations – both of which depend on actual Uw values across the whole envelope. For regulatory compliance under the FHS, 1.2 W/m²K is sufficient, but projects with net-zero ambitions often target Uw in the 0.7–0.85 W/m²K range.
Whole-Window U-Value vs Centre-Pane U-Value: Know the Difference
One persistent source of confusion is the difference between the glazing U-value (Ug) and the whole-window U-value (Uw).
The Ug (centre-pane value) measures the glass unit alone, taken from the middle of the pane where performance is best. The Uw (whole-window value) accounts for the entire window assembly – frame, glazing, spacer bars, and seals combined.
Because frames and spacers conduct more heat than the glass itself, the Uw is always higher than the Ug. A glass unit achieving 0.50 W/m²K at its centre pane may produce a whole-window Uw of 0.70–0.80 W/m²K once the full assembly is factored in. Warm-edge spacer bars help close this gap by reducing heat loss at the glazing perimeter.
When comparing products, always confirm which figure is being quoted: it is the Uw that matters for compliance and meaningful comparison.
Internal Surface Temperatures: Comfort Beyond the Number
Two windows with relatively similar U-values can perform quite differently in terms of how a room feels.
A window with a lower U-value maintains a warmer internal surface in cold weather and, when surfaces fall significantly below room temperature, occupants experience radiant heat loss even when the air temperature is nominally correct. This is the origin of cold spots and the downdrafts that make sitting beside a window uncomfortable in winter.
For Passive House-level projects, meeting comfort criteria – surface temperature ≥17°C (at –5°C externally and 21°C internally) – not just the target U-value, is central to the specification brief.
Overheating and the g-Value trade-off
In well-insulated, airtight buildings, summer overheating is a genuine risk.
Part O of the Building Regulations requires designers of new residential buildings to limit solar gain and provide means of removing excess heat. The solar transmittance of glazing (the g-value) becomes particularly important here, and balancing U-value, g-value, orientation, and shading is a design exercise requiring detailed energy modelling rather than a straightforward product decision.
Norrsken’s guide to what Part O means for your project covers the key considerations in full.
Does Installation Matter as Much as U-Value?
Even high-performance windows rely on correct installation to achieve their rated performance.
Thermal bridging at the frame-to-wall junction, inadequate airtightness detailing, and glazing positioned incorrectly within the wall build-up all reduce what a product is capable of delivering.
In Passive House projects, the Psi-value of the installation detail is itself a PHPP input – the installed performance of the window, not just its product rating, affects the energy balance of the building.
Do You Need Passive House-Certified Windows?
Not necessarily. A window that meets ≤0.80 W/m²K without PHI certification can still perform to Passive House standards – what matters is the verified performance data and whether it is sufficient for the project’s energy model. Norrsken’s article on passive house certified windows explores the distinction between certified components and certified design intent in more depth.
U-values remain a critical part of specification – but the most successful projects consider them alongside thermal comfort, installation quality, and overall building performance. Getting that balance right, from early design stage through to completion, is what makes the difference between a building that meets a target on paper and one that delivers a return on specification that’s felt every winter for the lifetime of the building.
Norrsken supplies triple-glazed aluminium-clad timber windows and doors with Uw values as low as 0.64 W/m²K. Our technical team supports specification from early design through to installation — see what our customers say.
