The Marvel of Passive Houses
Maximal performance, comfort and air quality
Passivhaus buildings, or passive houses, often referred to as a gold standard in sustainable architecture, are a remarkable example of energy efficiency and environmental consciousness. The key principle is that a Passivhaus building’s heat losses are reduced so much that the need for ‘active’ heating is minimised. This is achieved by optimising the effect of ‘passive’ heat sources such as the sun, the building’s occupants, and heat recovered from extracted air in the ventilation system. Similarly, the building’s design helps keep out excess heat in the summer. Put simply, Passivhaus buildings are designed to maintain a comfortable temperature and a constantly fresh indoor environment throughout the year with minimal energy costs.
Understanding Passive Houses
Passivhaus is a building standard pioneered in Germany. It employs meticulous design, superior levels of insulation and airtightness, very high-performing windows and doors, and a mechanical ventilation system with heat recovery (MVHR). A Passivhaus building maintains its internal environment without the need for traditional heating and cooling systems.
You can find out more in this first of a series of posts about our own certified Passivhaus self-build home.
The main principles of a passive house: optimising heat from the sun and retaining heat from within, with mechanical ventilation with heat recovery. Picture from The Earthbound report Jeremy Williams
Exceptional thermal comfort
The success of passive houses lies mainly in their ability to capture and retain heat efficiently in colder weather and to keep excess heat out when it’s warmer. Their optimisation of solar energy, internal heat sources, and heat recovery mechanisms, make them not just environmentally friendly but also very cheap to run.
Indoor and outdoor temperatures during the July 2022 heatwave at Woodlands Malvern B&B. The indoor temperature exceeded 25℃ (just!) for only 60 minutes over the four days of the heatwave, whist neighbours’ non-passive houses reportedly went up to at 33-34℃ indoors
Depending on choice of design and equipment, passive house buildings are simple to live in. Windows can be opened anytime for additional ventilation, and doing so on hot evenings and nights (‘Mediterranean purging’) helps expel unwanted heat accumulated in the day. Many mechanical ventilation heat recovery systems automatically bypass their heat recovery function in warmer weather, and can be adjusted at the push of a button to a lower fan speed if you are away or to a higher one if you have lots of people round.
Overheating in summer can be easily managed using external shading, and what minimal active heating is required in winter can be managed automatically via a conventional thermostat controlling, for example, a heat pump and a small area of underfloor heating pipes.
And you can also spend as long as you want saying goodbye to guests on a Passivhaus doorstep in winter without worrying about the house temperature dropping or people elsewhere in the house feeling cold. The airtightness of passive houses means that there are no draughts so the effect an open door can’t be felt elsewhere — there’s nowhere for the outside cold air to flow to.
If there are problems with passive houses they are that (i) you won’t be able to tell how hot or cold it is outside without going there (or having an outdoor thermometer), and (ii) you’ll no longer need your thick duvet or an electric blanket in winter or bedroom fans for sultry summer nights.
Exceptional indoor air quality
The mechanical ventilation heat recovery (MVHR) system in a passive house ensures that there is always a noiseless supply of 100% fresh, filtered air throughout with stale air is removed. It’s like having a window open in every room permanently without draughts or significant heat loss (Passivhaus Institut-certified MVHR units recover at least 90% of the heat in the outgoing air and use this to warm the incoming air).
The MVHR system also prevents condensation on internal surfaces, keeping relative humidity levels optimal most of the time (see graph and diagram opposite).
Relative humidity (%) at Woodlands Malvern B&B: 8 January 2022 – 7 January 2023
In our view, the constantly high-quality indoor air quality and thermal comfort is the most important feature of a passive house - minute heating bills are a bonus!
Build as designed, and simplest not to change things later
Passivhaus design needs specialist skills. And building needs meticulous attention to detail and some additional processes. For example, if you breach the airtightness layer during construction and don’t properly repair it or don’t install wall insulation or the windows properly, the building is unlikely to perform as designed.
Based on our experience, you won’t necessarily need a builder who has constructed a passive house before, but you will need a builder, and their whole team, who are fully committed to the high standards and different processes required to build to ensure achievement of the Passivhaus standard.
Passive house triple-glazed windows and doors need special installation. The picture shows a window frame fixed with brackets and not screwed through the frame to avoid ‘cold bridging’ the insulation. They are then sealed with airtightness tape (and, in our case, the window frame was first surrounded with Vitaseal expanding foam tape to provide further airtightness and insulation). Not shown here is the additional insulation subsequently placed around the whole frame inside and out to further reduce heat loss through the window frame. There is much more information about this in another post.
This raises another point: Passivhaus buildings are not the easiest to structurally alter afterwards (say, to build an extension) if you want them to continue to perform properly. To do this you’ll almost certainly need the skills of an experienced Passivhaus designer as well as a builder who will ensure the continuity of the original insulation and airtightness layer in the new part of the building. (The greater meticulousness and additional processes required in passive house building are exemplified in the picture above. This shows a passive house window installation which, in a conventional build, would be screwed into the wall through the frame and then, usually, ‘sealed’ on its outer edges with mastic. It would also probably be located on the outer edge of the wall rather than in the thermally more efficient mid-point of the wall insulation. This traditional method is likely to lead to substantial heat loss through the frame itself (counterintuitively, there’s less heat loss through the glass than the frame) compared with a passive house window installation.
Some passive house facts
Noting that the carbon footprint attributable to heating UK homes is equivalent to that of all its petrol and diesel cars:
Passivhaus is the world’s leading building energy efficiency standard
The Passivhaus standard far exceeds the current UK Building Regulations and is becoming increasingly more popular because it avoids over-engineering and (if built as designed) guarantees performance as designed
Passive houses can save up to 90% on heating and cooling costs compared to average UK homes (75% compared to those built to current Building Regulation standards) because their ‘active’ heating and cooling needs are so low
Passive houses have very high levels of comfort: there is a near-constant temperature everywhere with no ‘thermal layering’; there are no cold spots or draughts; there is a constant supply of 100% filtered, fresh air; there is no condensation (other than maybe a brief, light misting of the bathroom mirror when showering); and there is a pervading relaxing quietness and peacefulness
The minimal (or no) need for wall-hung radiators in a passive house, and the inner pane of the triple-glazed windows being the same temperature as the room, means that there is more wall and floor space to use
Financial benefits
Building a passive house can have some higher up-front costs, principally due to (i) higher design costs, (ii) higher levels of insulation, (iii) using triple glazing rather than double, and (iv) the need for a mechanical ventilation heat recovery system (a necessity for any building with high levels of air tightness).
However, these are offset by (i) lower heating system installation costs (you won’t need full central heating, just one that’s equivalent to the output of a hairdryer), (ii) minuscule heating and cooling costs, (iii) a more durable building because the MVHR will prevent condensation damage on surfaces and within the building’s fabric.
In addition, you’ll benefit from superior comfort because a Passivhaus will have a consistent temperature throughout with no draughts, and a constantly fresh, peaceful indoor environment.
Passivhaus UK-based Insights
As of August 2023, there were more than 1,680 certified Passivhaus buildings in the UK; there are more that have been built without undergoing certification.
The Passive House Trust has observed that ‘Passivhaus buildings are optimised for Net Zero and meet the predicted capacity of our future decarbonised grid’. and according to the Royal Institute of British Architects, there is increasing interest in building to the Passivhaus standard in the UK (as is reported elsewhere), with the Scottish government now developing legislation to require all future housing to be built to a Scottish equivalent of the Passivhaus standard.
But just building all new houses (or, indeed, all new buildings) to the Passivhaus standard will not meet current UK targets to decarbonise homes by 2050 as part of achieving ‘net zero’. This is because more than 80% of the homes likely to be used in 2050 are already built and these fall way short of the energy efficiency standards required for this.
It’s therefore important that we retrofit as much of our existing housing stock as possible learning from what’s achievable with the Passivhaus standard because this will reduce energy requirements for heating and cooling. At the very least, this means upgrading insulation and draught-proofing. You can read more about retrofitting in another post.
Find out about Passivhaus for yourself
If you’d like to experience the Passivhaus difference then stay at Woodlands Malvern B&B! Set in the Malvern Hills Area of Outstanding Natural Beauty (now called National Landscapes), you can enjoy stunning scenery and tranquillity, and we’d also be pleased to talk with you about our experiences of building and living in a passive house.