Passivhaus Retrofit (EnerPHit): Why High-Performance Windows and Doors Matter

The UK's existing housing stock represents one of the biggest opportunities to reduce energy consumption, lower carbon emissions and create healthier, more comfortable homes. While Passivhaus has become well established for new-build projects, the greatest challenge lies in improving the millions of homes that have already been built.

That's where Passivhaus retrofit, certified through the EnerPHit standard, comes in.

Developed by the Passivhaus Institute specifically for existing buildings, EnerPHit is widely recognised as the gold standard for deep energy retrofit and provides a practical, performance-based approach. Rather than simply upgrading individual elements, it focuses on improving the building as a whole - dramatically reducing energy demand while enhancing comfort, indoor air quality and long-term resilience. As homeowners continue to seek better-performing homes in response to rising energy costs, EnerPHit offers a proven route to creating homes that are healthier to live in, cheaper to heat and built for the future.

As the highlights, improving existing buildings isn't simply about reducing carbon emissions - it's about creating homes that perform better for the people who live in them.

Unlike Passivhaus certification for new-build homes, EnerPHit recognises the practical constraints of renovating existing buildings. Older homes often present structural, planning or architectural challenges that make full Passivhaus certification difficult to achieve, so EnerPHit provides realistic performance targets while maintaining the core principles of Passivhaus.

These include:

• Minimising heat loss

• Improving airtightness

• Eliminating thermal bridges

• Installing high-performance windows and doors

• Providing continuous fresh air through Mechanical Ventilation with Heat Recovery (MVHR)

The result is a home that requires significantly less energy to heat while providing exceptional levels of comfort throughout the year.

Public awareness of Passivhaus retrofit received a major boost with on Channel 4.The programme followed the transformation of a typical 1930s semi-detached house into an EnerPHit Certified home, demonstrating that older properties can achieve remarkable energy performance through a carefully planned, fabric-first renovation.

By combining high levels of insulation, exceptional airtightness, Mechanical Ventilation with Heat Recovery (MVHR) and high-performance triple-glazed windows and doors, the project achieved around a 90% reduction in heating demand while creating a home that is warmer, quieter and healthier to live in.

The programme highlighted an important message: successful retrofit isn't about one product or technology - it's about ensuring every element of the building envelope works together.

Even the highest levels of insulation cannot compensate for poorly performing windows. In many older homes, windows and doors are among the largest sources of heat loss, draughts and condensation. Replacing outdated glazing with modern high-performance systems is therefore one of the most effective ways to improve both energy efficiency and everyday comfort.

However, not all triple glazing performs equally.

The thermal performance of the complete window (Uw value), airtightness, frame construction and insulation, glazing technology and installation quality all contribute to how well a window performs over its lifetime. Alongside reducing heat loss, high-performance windows can also:

• Eliminate cold internal surfaces and uncomfortable draughts

• Reduce condensation and improve indoor comfort

• Improve acoustic insulation for quieter living spaces

• Enhance security

• Increase natural daylight without compromising thermal efficiency

As the continues to emphasise, deep retrofit is about reducing energy demand first. When the building fabric performs exceptionally well, heating systems can be smaller, running costs are reduced and homeowners enjoy healthier, more comfortable living environments.

A successful EnerPHit retrofit depends on every element of the building envelope working together- and windows and doors are among the most important components.

Internorm has been manufacturing triple-glazed windows since 1979 and offers a range of particularly well suited to EnerPHit projects and homes being renovated to Passivhaus standards.

With whole window Uw-values as low as 0.62 W/(m²K), exceptional airtightness and innovative technologies such as , help minimise heat loss while delivering outstanding long-term performance.

In addition to excellent thermal efficiency, homeowners benefit from:

• Triple glazing as standard

• Outstanding acoustic insulation of up to 48 dB

• RC2 security as standard on all window systems

• Low-maintenance and composite options

• Contemporary and traditional design styles to suit every home

At the recent , Vasilis Giannopoulos, Passivhaus Designer and Specifications Manager at Internorm made following comment: "Talk about Passive House differently. Not in technical terms, not with numbers. Talk about the experience that the occupants most likely will feel. Try to make it more personal.”

That perfectly captures the philosophy behind both Passivhaus and Internorm. While lower energy bills are important, the greatest benefit is something homeowners experience every day - a home that is warmer in winter, cooler in summer, quieter, healthier and more comfortable to live in.

Whether you're renovating a Victorian terrace, upgrading a 1970s family home or undertaking a complete refurbishment, the best time to adopt Passivhaus principles is when major improvements are already planned.

The Passivhaus Trust recommends taking a whole-house approach, developing a comprehensive retrofit plan from the outset - even if the work is completed in carefully planned stages. This helps ensure every improvement works together, avoids unnecessary costs and delivers the greatest long-term benefit.

Choosing high-performance windows and doors early in the design process is one of the most effective investments you can make. Together with insulation, airtightness and ventilation, they form a key part of a successful EnerPHit retrofit, helping create homes that are more energy efficient, healthier and built to perform for decades.

Barry McCurran, former chairman of the selected Internorm windows for his certified EnerPHit retrofit: “I had always known of Internorm because of the certified passive house credentials and the long history of use in various projects. When it came to my project and looking at all the various options on the market, it was clear just how much the quality of the product stands out. After this we had our mind made up, really the design and performance were also factors. We just love our front door and the way in which the windows are a real feature of the house.”

If you're considering an EnerPHit project - or simply want to renovate to Passivhaus standards - an approved can help you choose the most suitable Passivhaus window and door systems for your home, ensuring your renovation delivers exceptional comfort, performance and lasting value.