How *Not* to Build a Passivhaus: 2 Costly Lessons [and How to Avoid Them]

The Passivhaus Standard takes a 'fabric first' approach to energy efficiency. This means your home’s materials and components [the building fabric] do the hard work to reduce the amount of heating and cooling required to keep your home comfortable. It achieves this by using 5 complementary building principles:  

• Continuous amounts of high-performance insulation: 
  Keeps you comfortable all year round and reduces energy demand.
   
• Airtight construction: 
  A continuous, high-performance airtight layer is formed around your home. Then any gaps around doors, windows, electrical outlets, pipes and lights are taped and sealed. This stops heat escaping, cuts down on energy use and keeps your home comfortable with no draughts. Two airtightness tests are performed both during and after construction to make sure your home hits the strict Passivhaus airtightness target. 
   
• Triple-glazed windows: 
  These provide optimum comfort and efficiency in summer and winter. Triple glazing doesn't feel cold to touch and also helps reduce outdoor noise, keeping your home quiet.
   
• Mechanical ventilation with heat recovery [MVHR]: 
  The lungs of your home, quietly delivering fresh, filtered air 24/7 while recovering heat from outgoing air. Continuous air circulation means your bathroom doesn't steam up, cooking smells vanish and laundry dries faster. If you've got allergies or asthma, the MVHR's filters remove dust, pollen and pollutants, keeping your home healthy.
   
• Eliminate thermal bridges: 
  These are weak spots in a building envelope. They often occur around corners, junctions and where materials meet (ie- a steel beam passing through insultation). The problem is, these areas become cold spots, which can lead to condensation, damp, mould or structural decay. The Passivhaus Standard requires that any junctions, corners, connections and penetrations are carefully planned in the design phase to avoid thermal bridging. This is cheaper and easier to address early, rather than on the building site.

Certified Passivhaus designers and tradespeople will ensure these 5 building principles are integrated into your home. However, these alone don't reliably deliver Passivhaus performance. Three approach principles are also essential.

• Strict Performance and Comfort Criteria: 
  Your home must meet strict targets for space‑heating/peak heating load, overall energy demand, summer temperatures, airtightness, ventilation and acoustic performance.
   
• Passivhaus Planning Package Modelling: 
  The Passivhaus Planning Package [PHPP] is powerful, physics-based design software. A certified Passivhaus designer uses it to calculate, model and optimise your home's year‑round energy use and comfort. It’s been proven to be reliable in giving accurate predictions of actual energy use in a finished home. 
  
  PHPP is used early in the design process, before key decisions are fixed, such as how many storeys your home has, its siting, orientation, building form and window layout. These factors have a direct impact on your home’s energy efficiency. PHPP instantly tests and compares design options, clarifies the impact of each decision and flags risks such as poor performance, occupant discomfort (ie- too hot, too cold) and high bills. It helps you get the best value for money by highlighting when design ideas work against energy efficiency and by identifying opportunities for optimisation.
   
• Quality Assurance/Passivhaus Certification: 
  Passivhaus Certification is an independent, third-party quality-assurance process that protects your interests. A qualified Passivhaus Certifier reviews the technical drawings and the PHPP energy model, checks detailed construction evidence at key stages of the build and verifies test results [eg - airtightness]. When all criteria are met, your home is certified and it receives the official Passivhaus plaque. Certification gives you confidence that you’re living in a genuinely low-energy, high-comfort home. The quality assurance process also ensures the design and build teams are trained in the Passivhaus Standard.

It's the combination of all these principles [building AND approach] that delivers the Passivhaus Standard. Skip one and overall performance can be compromised.

Two housing projects in Wales serve as a cautionary tale.

• 13-home social housing scheme in Wales
• Aim: lower residents' energy bills and operational carbon
• The design and build contract vaguely referred to "Passivhaus principles" / "design inspired by Passivhaus" but didn't require third-party Passivhaus certification
• Mention of "meeting Passivhaus Standards" without any definition of what was meant by this. Such terms are unenforceable in a contract.
• No PHPP models were prepared during the design phase
• No Certified Passivhaus Designer appointed

A retrospective PHPP model created after the homes were built exposed issues that would’ve been cheap and easy to fix during design, but instead were carried into construction. 

The building envelope, including the insulated foundations, wall and roof build-ups and window specifications, was good enough to achieve Passivhaus Certification had other factors been appropriate. However, in relation to the key principles outlined above:

• The airtightness level specified was required to meet the Passivhaus standard. However, none of the homes achieved the airtightness target. There was no evidence that an interim airtightness test had been carried out during construction, so leaks went undetected.
• Most junctions, corners, connections and penetrations were detailed to avoid thermal bridging, but the external door thresholds were missed.
• The later PHPP model showed that solar gain and shading hadn’t been properly considered. Several window sizes and positions could've been adjusted to improve the balance between heat gains and losses.
• The retrospective PHPP model also showed that the choice of heating system for the homes wasn't suitable.
• Building orientations were also suboptimal, with an excess of north-facing sliding patio doors leading to high heat losses. 

• After moving in, residents immediately experienced comfort issues
• Several homes were very cold in winter and residents resorted to using portable electric radiators
• Other homes experienced overheating in summer
• Energy bills were much higher than expected, with many residents unable to pay and officially in fuel poverty.
• Residents energy bills had to be subsidised on an ongoing basis
• The absence of clear quality-assurance requirements during construction led to inconsistent airtightness results and no reliable audit trail to demonstrate build quality
• Huge rectification costs, far exceeding the uplift needed [4-8%] to build certified Passivhaus homes

Although the project clearly intended to follow some Passivhaus principles, they were applied inconsistently. If you want Passivhaus results, use the whole recipe. Choosing to build a certified Passivhaus from the outset typically adds 4-8% to your project costs (often less), but it's an investment that saves money, hassle and comfort issues later. Case Study A's partial adoption of Passivhaus principles resulted in hefty rectification costs to address performance shortcomings. 

Which wouldn’t have been needed if the client had required Passivhaus-certified homes from the start:

While Case Study B wasn't targeting the Passivhaus Standard or claiming to follow Passivhaus principles, the Passivhaus Trust included it in its paper to highlight the risks of a technology-driven approach without a strong 'fabric-first' methodology.

• 16-home social housing scheme in Wales
• Aim: lower residents' energy bills and operational carbon
• Big focus on using "innovative, low carbon technologies" such as solar PV panels with battery storage, domestic hot water from individual Air Source Heat Pumps [ASHPs], preheated by Transpired Solar Collectors [TSCs] and Mechanical Ventilation with Heat Recovery Systems [MVHRs]
• No PHPP modelling

• High-tech, low-carbon systems added 18% to the project's costs versus the 4–8% needed to build certified Passivhaus homes
• Promises of generating 80% of required energy on site and cutting bills by up to 50% [vs residents' previous homes] didn't materialise
• Overreliance on manufacturers offering innovative but unproven tech solutions
• Limited technical support for the products installed
• Design issues started to show up before the homes were finished, leading to last-minute changes to the heating system
• The homes included high levels of insulation and MVHR systems. However, a certified Passivhaus designer wasn’t involved, nor was the use of the Passivhaus Planning Package [PHPP]
   

• Residents in brand-new "low-energy" homes were effectively pushed into fuel poverty
• Homes did not perform as intended
• Avoidable overspend on tech with higher maintenance needs and shorter lifespans
• Delays to occupants moving in
• Substantial rectification cost

Starting with a 'fabric first' approach and adopting Passivhaus principles from the outset would've provided a proven route to creating energy-efficient and sustainable homes, ensuring better outcomes for residents. Case Study B demonstrates why prioritising Passivhaus Planning Package (PHPP) modelling and Passivhaus levels of Quality Assurance throughout construction are key tools. Without these measures, unintended consequences can occur.

Building a Passivhaus may cost a little more than a standard build, but bolt-on tech typically costs more again, needs more maintenance, has shorter lifespans and still doesn’t guarantee lower bills, lower carbon, or better comfort.

Which wouldn’t have been needed if the client had required Passivhaus-certified homes from the start:

As awareness of Passivhaus has grown, some of the fundamental building principles (for example, continuous insulation, airtight construction and MVHR systems) have also spread throughout the industry. That must be seen as a positive. 

However, this has led to projects that have declared they're following “Passivhaus principles” because they incorporate some or all those characteristics, while neglecting to follow the principles of accurate PHPP modelling, quality assurance and meeting performance and comfort criteria, which are central to all certified Passivhaus homes and deliver so many of the health, comfort and energy benefits for occupants. 

Passivhaus isn't a shopping list, it's a recipe. When every ingredient works together, the result is a home that feels brilliant: cosy in winter, comfortable in summer, fresh air always, bills you barely notice.

At Coldwells Build, we model our Passivhaus homes using the Passivhaus Planning Package. We design and build with Passivhaus certified architects and tradespeople. We prove the results with independent Passivhaus certification.