20 Years On, Passivhaus ‘Out of the Blue’ Still Excels in Comfort & Efficiency

Established in 2004 by Tomas O’Leary, MD of mosart in Wicklow, Ireland’s first Passivhaus, “Out of the Blue”, continues to deliver exceptional energy efficiency, thermal comfort, and resilience over two decades later, proving the enduring value of Passive House design.

Imagine living in a 20-year-old home that still outperforms most brand-new houses in comfort and energy efficiency. That’s the reality for architect Tomás O’Leary, whose family home “Out of the Blue” in County Wicklow was Ireland’s very first Passivhaus. Built in 2004, this sustainable building was ahead of its time and today it continues to set the standard for low-energy living. Nicknamed Out of the Blue, the 4,000 sq ft (≈350 m²) house maintains a high level of comfort year-round while using roughly 90% less energy than a conventional house. In other words, it delivers cosy, healthy living with just a fraction of the energy use and cost of a typical home.

For professionals in architecture, development, and sustainability, Out of the Blue offers a rare long-term case study in Passive House performance. Two decades on, the home’s measured data and lived experience provide compelling evidence that Passive House design yields enduring benefits. Below, we delve into the project background and design highlights, examine its performance today (with real data), and highlight key lessons from 20+ years of ultra-efficient, comfortable living. All in service of inspiring the next generation of Passive House and sustainable building projects.

Project Background: A Passivhaus Ireland Pioneer

Tomás O’Leary embarked on building Out of the Blue in the early 2000s, inspired by the groundbreaking Passivhaus standard, which was then virtually unheard of in Ireland. Completed in 2004, his Wicklow home became the first certified Passivhaus in Ireland, a trailblazing project that proved this ultra-efficient approach could work in the Irish context. At about 350 m² in size, the house was a substantial family home rather than an experimental cabin, making its performance all the more impressive. Tomás effectively turned his personal residence into a living lab for Passive House Ireland ambitions.

From the outset, Out of the Blue demonstrated exceptional efficiency. It was reported to cost only a few hundred euro per year to run at a comfortable temperature, an almost inconceivably low operating cost for a house of its size. In fact, by 2008 the annual heating bill was a mere €250, thanks to the home’s minimal heat demand. Such performance was achieved even as the home provided modern levels of comfort year-round. Unsurprisingly, this first-of-its-kind project attracted huge public interest and became a touchstone for low-energy design. Out of the Blue was regularly featured in tours, studies, and press articles as a real-world proof of concept for Passive House design in Ireland. It showed policymakers, builders, and fellow architects that ultra-efficient, low-energy buildings were not only possible but also desirable, comfortable, and affordable in the long run.

Design Highlights: Why ‘Out of the Blue’ Excels

What made this house so efficient from day one? Out of the Blue was designed and built with a “fabric first” approach, incorporating cutting-edge Passive House principles that were revolutionary in Ireland at the time. Key design highlights include:

• Super-Insulated Envelope: The house is wrapped in an exceptionally thick layer of insulation (e.g. ~315 mm of EPS in the walls) to minimise heat loss. This high-performance thermal envelope keeps the interior warm in winter and cool in summer, using the building itself like a cosy thermal blanket.
• Airtight Construction: Extraordinary attention was given to airtightness, achieving about 0.6 air changes per hour at 50 Pa, meeting the rigorous Passive House standard. By sealing every crack and penetration, the design prevents cold drafts and heat leakage. Even decades later, the home remains far tighter than conventional new builds, contributing to its efficiency and comfort.
• Optimised Orientation & Shading: Out of the Blue is oriented due south to capture passive solar gains in winter. Large south-facing windows bring in free heat from the sun, while carefully designed shading elements (informed by lessons from Frank Lloyd Wright’s Usonian houses) prevent overheating by blocking high summer sun. The result is a bright, warm home in winter and a cool, glare-free interior in summer, all achieved naturally.
• High-Performance Windows: The house features high-spec triple-glazed windows and doors that insulate almost as well as walls. These windows virtually eliminate cold surfaces and condensation, meaning no winter chill or mould growth even after years of service. They also keep out unwanted noise and help maintain an even temperature throughout the space.
• Mechanical Ventilation with Heat Recovery (MVHR): A heat recovery ventilation system continuously supplies fresh filtered air to the house while recovering around 90% of the heat from exhaust air. This MVHR refreshes indoor air roughly 12 times per day, ensuring excellent indoor air quality and even humidity. The occupants enjoy a healthy, draft-free environment without an energy penalty, as the system retains warmth in winter (and expels excess heat in summer).
• Efficient Heating System: Thanks to the Passivhaus fabric, the home needs only a tiny heating load. A small wood pellet boiler/stove was installed to provide space heating and hot water as needed. In practice, it runs infrequently and uses very little fuel, as reflected by the token annual heating cost. (Originally, solar thermal panels covering ~7 m² were also integrated to harness renewable energy for hot water.) There are a few small radiators for backup, but most of the time, passive heat gains and insulation do the heavy lifting.
• Thoughtful Detailing: Every aspect of construction aimed to minimise thermal bridges and inefficiencies. For example, the design and workmanship addressed junctions and connections to ensure there are no “cold spots” where heat could leak. This meticulous detailing, along with the use of local materials and suppliers, set a new quality benchmark in Irish homebuilding.

Collectively, these design features enabled Out of the Blue to use only ~10% of the energy of a typical house while maintaining modern comfort. The house essentially redefined what “energy efficient” meant in Irish residential design, without compromising on aesthetics or amenity. It looked and functioned like a normal family home, just one that quietly slashed energy waste by an order of magnitude.

Performance After 20 Years: Comfort and Resilience in 2025

Figure: Recent performance data from “Out of the Blue” demonstrates its stable indoor climate. Top: Outdoor (pink) vs. indoor (blue) humidity over one week in August 2025 – indoor humidity stays around a comfortable 50–60% even as outdoor humidity swings much higher. Bottom: Outdoor (green) vs. indoor (red) temperature for the same period – while outside temperatures ranged from chilly ~12°C nights up to 23°C days, the indoor temperature remained rock-steady around 21–22°C without active cooling.

Fast forward to today, and Out of the Blue is still delivering outstanding performance. The house’s internal environment remains remarkably stable and comfortable, as ongoing monitoring shows. For instance, during a typical summer week, indoor temperatures stayed around 21°C consistently while outdoor temperatures fluctuated by over 10°C each day (from cool nights around 12°C to warm days above 22°C). Indoor relative humidity stayed in the 50–60% range, right in the sweet spot for comfort, even as outside humidity spiked toward 80% or more. This level of indoor comfort is achieved without any air-conditioning – it’s purely a result of the Passivhaus design regulating the climate naturally. Even on the hottest days or coolest nights, the indoor conditions barely budge, meaning the family enjoys a continuously comfortable atmosphere year-round.

Crucially, this performance has endured with minimal energy input. The home’s heating energy demand remains so low that even in winter, the heating system runs only occasionally. In fact, the house demonstrated its thermal resilience in dramatic fashion during a severe winter storm in late 2024. Storm Darragh brought a multi-day power outage to the region, cutting off the O’Leary home’s heat source for 72 hours, yet the indoor temperature dropped by only 0.5°C in that time. In other words, after three days with no heating, the house was still essentially at the same comfortable temperature. Tomás O’Leary noted that while neighbours in conventional houses were layering up as their homes went cold, his family remained warm and comfortable, crediting the superior insulation and airtight construction for preserving the heat. “Storm Darragh reminded us why Passive House homes are about so much more than just energy savings,” O’Leary reflected. “They ensure resilience, comfort, and reliability even in extreme weather. It’s future-ready living.” This real-world event underscored that a well-built Passive House is not only energy-efficient in day-to-day use, but can also buffer its occupants from crises like power cuts or fuel shortages. Resilience is built into the design.

Over 20+ years, Out of the Blue’s core systems and envelope have also proven incredibly durable. The house still scores in the top tier for airtightness and insulation in Ireland, with no evidence of envelope deterioration. There have been no issues with mould or condensation, thanks to the continuously balanced ventilation and high-performance windows – the indoor air has remained fresh and healthy throughout. The only significant upgrade made was an addition to further reduce its carbon footprint: in 2023, to celebrate the home’s 20th “birthday,” O’Leary installed a 6.5 kW solar PV array on the property. This photovoltaic system now generates roughly two-thirds of the home’s annual electricity needs, covering most of the remaining energy consumption that the Passivhaus design doesn’t already eliminate. In combination with the house’s drastically reduced demand, the solar panels have pushed Out of the Blue even closer to net-zero energy status. Essentially, the home is now largely self-sufficient and future-proofed for the next decades of operation.

The measurable outcomes speak volumes: ultra-low energy usage, rock-solid thermal comfort, and proven resilience over decades. Out of the Blue hasn’t just aged gracefully; it arguably performs better today than when it was built, thanks to small improvements and the verification that all those Passive House features do exactly what they were meant to do. It stands as living evidence that investing in quality Passivhaus design yields payback not just in the first few years, but for the long haul of a building’s life.

Key Lessons from Two Decades of Passive House Living

Out of the Blue’s 20-year journey offers several powerful takeaways for architects, developers, and sustainability professionals looking to create the next generation of low-energy buildings. Here are some of the key lessons and insights from this long-term success story:

• Passive House Delivers for Decades: The house emphatically proves that the Passive House standard isn’t just a theoretical or short-term win; it continues delivering value decades later. Exceptional energy efficiency, comfortable interiors, and low running costs aren’t just initial selling points; they are enduring features. Even as building codes and technologies evolve, a well-built Passive House remains ahead of the curve in performance. This longevity dispels any notion that high-performance design might “fade” over time.
• Ultra-Low Energy = Real Savings and Security: Reducing heating demand by ~90% isn’t an abstract number; it translates into very tangible benefits for occupants year after year. Tomás’ family have enjoyed extremely low energy bills and a degree of insulation from energy price volatility. Moreover, the home’s ability to stay warm through a winter outage shows how energy efficiency equals resilience. In an era of rising fuel costs and climate uncertainty, Passive House provides a form of insurance: a Passive House home stays habitable and comfortable even if the power goes out or the heating system fails. That level of reliability is priceless for homeowners and adds long-term value to the property.
• Comfort and Health Are Paramount: Technical metrics aside, one of the most important outcomes is how comfortable and healthy the indoor environment has been, continuously. The consistent temperatures (around 20–21°C) and balanced humidity, with ample fresh air, mean no cold spots, no drafts, and no stuffiness or mould, simply put, an excellent quality of life indoors. The O’Leary family’s experience underscores that Passivhaus design is not about sacrificing comfort for efficiency but rather enhancing comfort through efficiency. This is a powerful lesson: sustainability can go hand-in-hand with superior living conditions, keeping occupants happier and healthier.
• Quality Construction Has Lasting Payoffs: The project highlights the importance of meticulous design and construction quality. Investing in high-grade materials (insulation, windows, etc.) and careful execution (airtight detailing, thermal bridge avoidance) resulted in a building that needed virtually no “energy retrofit” even after 20 years; it was built right from the start. Aside from routine maintenance, the house’s envelope and systems didn’t require expensive upgrades to maintain performance. This suggests a strong ROI for quality: money spent upfront on a Passivhaus can save a fortune in operational costs and future retrofit costs. It’s an approach that aligns with sustainable building best practices, reducing lifecycle environmental impact.
• Inspiration and Industry Impact: Finally, Out of the Blue’s story shows how one pioneering project can catalyse broader change. This home helped spark a paradigm shift in Ireland’s construction industry, inspiring a new wave of architects and builders to embrace Passivhaus principles. Its success proved that genuinely “low energy buildings” are achievable, encouraging colleagues to upskill through Passive House courses and pursue similarly ambitious projects. In fact, Ireland’s Passive House community grew significantly in the wake of this project’s influence, with many professionals (and even manufacturers) pivoting toward Passivhaus design and high-performance building. The key lesson here is that demonstrating real-world success is a powerful way to drive adoption of sustainable innovations. Out of the Blue became more than just one house; it became a lighthouse project guiding an entire movement toward better building standards.

In summary, the enduring performance of Out of the Blue is an uplifting validation of Passivhaus as a holistic, future-proof approach to building. The house achieves what few conventional buildings do: it provides exceptional comfort, trivially low energy use, and resilience, all at once, and keeps on doing it year after year. These lessons are invaluable as we aim to design buildings for a low-carbon, climate-resilient future.

Embrace Passive House. Your Next Step with Mosart

Out of the Blue’s 20-year success story is a testament to what forward-thinking design and craftsmanship can achieve. The good news is that these principles are not limited to one home; they can be replicated and adapted to new projects, whether you’re planning a residential development or a commercial building. If you’re feeling inspired to explore Passive House for your own project or to deepen your expertise in sustainable building, Mosart is here to help you take that next step.

Mosart’s team of certified Passivhaus architects and consultants has decades of experience delivering high-performance, low-energy buildings in Ireland and abroad. We offer expert Passive House consulting services to guide developers and design teams through the process from feasibility and Passive House Planning Package (PHPP) modelling to detailing, certification, and on-site execution. Our goal is to ensure your project realises the same enduring benefits you’ve read about: superior energy efficiency, comfort, and quality.

For those looking to build their personal expertise, Mosart also provides internationally accredited Passivhaus training programs. As Ireland’s leading Passive House educators (with courses for designers, consultants, and tradespeople), we can equip you with the knowledge and skills to implement Passivhaus principles successfully. Whether it’s our Certified Passive House Designer course or hands-on workshops for building professionals, our training is designed to empower you to build the next generation of low-energy homes.

Sources: Mosart Project Data, Passive House Plus; Passive House Association of Ireland; Coldwells Build Journal; ShaneKelly Passive House blog.