Passivhaus Windows Spec: U-value 0.80 or Lower and the Path to Zero Energy

In the same way that a standard UK home is designed for “average” heat retention, a Passivhaus home is designed for total thermal autonomy. The goal is a building that requires almost zero active heating. To achieve this, the windows must transition from being “the weakest link” to being “part of the insulation.”

The Passivhaus Window Specification

For a window to be Passivhaus-certified, it must meet three primary criteria: total U-value, airtightness, and the thermal performance of the frame.

1. The U-Value Target: $\le$ 0.80 W/m²K

While a standard A-rated double-glazed window has a U-value around 1.2 to 1.6, a Passivhaus window must consistently achieve $\le$ 0.80 W/m²K. This is an uncompromising target.

How this is achieved:

• Triple Glazing: Essential. Double glazing simply cannot reach this level of performance consistently across the entire unit.
• Noble Gas Filling: Argon or Krypton is used to fill the gaps between panes, as these gases have lower thermal conductivity than air.
• Warm-Edge Spacers: The aluminium spacers used in cheap windows are replaced with high-performance composites (plastic/carbon) to prevent “cold bridging” at the edges of the glass.

2. Airtightness

A Passivhaus window is not just about heat loss; it’s about airflow. The window must be perfectly sealed to the building’s airtight layer. This is achieved through specialized expansion tapes and gaskets that ensure no air leaks through the frame-to-wall junction.

3. Frame Material and Engineering

The frame is the thermal bridge. In Passivhaus builds, you will typically see:

• Engineered Timber: The most common choice due to its natural thermal properties and low embodied carbon.
• High-Spec Aluminium: Systems like Internorm provide thermally broken aluminium frames that meet the target U-values while offering a modern, slimline aesthetic.

Certified Suppliers in the UK

Achieving these specs is not a matter of “asking for the best” from a local installer. It requires a partnership with specialists who understand the Passivhaus Trust UK standards.

• Internorm: A global leader in bespoke, high-performance windows. Their systems are engineered specifically for the architectural requirements of low-energy builds.
• Green Building Store: A specialist supplier of performance-led systems, providing not only the products but the consultancy needed to ensure the window is installed correctly within the airtightness layer.

Typical Costs for Passivhaus Windows

Passivhaus-certified windows carry a significant price premium over standard products. As a rough guide:

• Engineered timber Passivhaus windows: £1,200–£2,500 per window (unit only), depending on size and specification.
• Aluminium-clad timber Passivhaus windows: £1,500–£3,500 per window. Internorm’s premium ranges sit at the top of this bracket.
• Installation: Specialist installation is essential. Installers familiar with airtightness detailing charge £200–£400 per window above standard rates. The cost reflects the careful sealing, taping, and testing required.
• MVHR system: Budget £3,000–£6,000 for a whole-house MVHR installation. This is not optional in a Passivhaus—it is integral to the building physics.

A complete Passivhaus window package for a typical 3-bedroom UK home (10–15 windows plus MVHR) typically lands between £20,000 and £45,000. This is a substantial investment but should be weighed against near-zero heating bills for the lifetime of the building.

The EnerPHit Retrofit: A Practical Note

Most UK homeowners will never build a new-build Passivhaus, but the EnerPHit standard (the Passivhaus retrofit standard) is increasingly relevant. EnerPHit recognises that retrofits are constrained by existing geometry and allows slightly higher U-values where the building orientation or wall thickness makes the full standard impractical.

The key steps for an EnerPHit window retrofit are:

1. Commission a thermal model: Before specifying any products, model the whole house to understand where windows fit in the overall heat-loss picture.
2. Specify the frame and glass as a system: Do not mix a Passivhaus-certified glazing unit with a standard frame—the thermal bridge at the junction will undermine the performance.
3. Prioritise airtightness detailing: The tapes, gaskets, and expansion foam around the window-to-wall junction are as important as the window itself. Poor detailing can triple the effective U-value of the installed unit.
4. Install MVHR before or alongside the windows: Sealing the building without ventilation will cause condensation and mould problems almost immediately.

UK Climate and Orientation Considerations

The UK’s temperate maritime climate means that Passivhaus windows behave differently here than in continental Europe:

• South-facing windows: In the UK, south-facing triple-glazed windows with low-iron glass frequently deliver a net energy gain during winter, as the solar radiation passing through the glass exceeds the heat lost through the unit. This is the “free heating” that makes Passivhaus viable.
• North-facing windows: These are purely a cost—no solar gain is available. Minimise north-facing glazing areas where possible, and maximise the U-value specification for those you do include.
• East and west: These orientations provide moderate solar gain, mainly in the morning and evening respectively. Overheating risk is low in the UK climate but should not be ignored, particularly with west-facing glazing in bedrooms.

Common Mistakes in Passivhaus Window Specification

1. Mixing and matching systems: Using a Passivhaus glazing unit with a non-certified frame creates a thermal bridge that may double the effective U-value of the installed window.
2. Neglecting the installation detail: Even the best window performs poorly if the airtightness tape is missing or the expansion foam has gaps. Commissioning a blower-door test immediately after installation catches these problems early.
3. Forgetting summer ventilation: Passivhaus homes can overheat in summer because they are so well insulated. Opening windows is the simplest solution, but design for cross-ventilation from the outset.
4. Specifying without MVHR: High-performance windows without mechanical ventilation create condensation, mould, and stale air. This is not a matter of preference—it is a building physics requirement.

What most guides miss: The “MVHR” Synergy

The most common misconception is that Passivhaus windows are “too airtight,” making the house feel stuffy.

In reality, the windows work in tandem with a Mechanical Ventilation with Heat Recovery (MVHR) system. The MVHR system constantly replaces stale indoor air with fresh, filtered outside air, while recovering up to 90% of the heat from the outgoing air.

If you install Passivhaus windows without an MVHR system, you are creating a recipe for condensation and mould. The windows are so efficient that they stop the natural “leaking” of the house, meaning the house no longer breathes. Without MVHR, the high-performance glazing becomes a liability.

For those performing an EnerPHit (Passivhaus Retrofit), combining high-performance glazing with IWantSolar PV systems transforms the property into a power plant. By reducing the heating demand to almost nothing, the solar panels can provide the remaining energy needed for the MVHR and appliances, moving you to absolute energy autonomy.

If your property is currently Unmortgageable because of severe structural decay and outdated glazing, a Passivhaus-led retrofit is the most aggressive way to increase asset value. By transforming a “cold, damp liability” into a “Net Zero asset,” you can leverage the high valuation to secure the financing needed for the completion of the project.

Summary: Passivhaus Setup

• U-Value Target: $\le 0.80$ W/m²K.
• Glazing: Triple-pane, Argon/Krypton filled, warm-edge spacers.
• Sash/Frame: Engineered timber or thermally broken aluminium.
• Ventilation: Must be paired with MVHR for a healthy indoor climate.
• Suppliers: Internorm, Green Building Store.