Everyone seems to want to build a Passivhaus at the moment and, amid the hype, it pays to take a step back and examine what it actually means. The standard originated in Germany in the 1990s and is based on the idea that a home can be heated using warmed fresh air. Because air can only hold a small amount of heat, the house must be designed so as to need only a small amount of heating and constructed thoughtfully and carefully. This amount of heat turns out to be 15kWh/m2/year — about three times more efficient than current UK Building Regulations, and still more than twice as good as planned UK regulations for 2016.
To achieve this figure, Passivhaus homes must be ventilated using a mechanical system that pre-heats ventilation air by recovering the heat from stale air as it leaves the building. While this type of system is still relatively uncommon in the UK, it is becoming much more the norm for highly insulated buildings.
Older dwellings frequently have poorly fitting doors and windows and open joints between construction elements and are consequently draughty. Essentially, many of us are used to having fresh air, provided via draughts as the result of poor construction, without having to open windows or to have a mechanical ventilation system. With a Passivhaus the mechanical ventilation system is essential for the winter period, as the buildings are designed to be very airtight to prevent heat escaping in an uncontrolled manner through those gaps and cracks.
The ventilation system runs all the time – even in the summer – at a low level, so the fans don’t make any noise, and the air is kept fresh through constant changes. This all means that heating bills are kept very low and Passivhaus homes are cheap to live in.
The key features of a Passivhaus are:
- Very high levels of insulation
- Extremely high-performance windows with insulated frames
- An airtight building fabric
- ‘Thermal bridge-free’ construction
- A mechanical ventilation system with highly efficient heat recovery
- Accurate design using the Passive House Planning Package (PHPP)
A big effort is needed by the constructor to make the home very highly insulated and airtight and so far there are few constructors and manufacturers in the UK who are skilled at doing it. However, as more of these projects are constructed, the skill levels in the industry will grow to meet the demand and the workforce will gradually become used to the level of care and workmanship required.
Some of the costs of the increase in construction quality ought to be offset by the omission of a traditional heating system. Many homeowners, however, are nervous about doing this — we’re used to seeing radiators and become concerned when they are not there, even if they are not needed.
Reasons to Build to Passivhaus Standard
Because of the long payback duration, most people need reasons beyond simple energy efficiency before they can be convinced that building a Passivhaus is a good idea. Well, here’s a few:
- Low running costs to help prevent future fuel poverty
- Easy to live in with simple control systems
- Thermally comfortable, with no draughts and an even temperature distribution
- Good indoor air quality provided by the mechanical ventilation system
- Excellent acoustic performance provided by a combination of very well insulated walls, airtightness and triple-glazed windows
For me, the biggest benefit of Passivhaus buildings is perhaps the least obvious — the comfortable internal environment with an even internal heat distribution.
The stereotypical Passivhaus is a simple box, aimed at containing the most space as efficiently as possible and maximising windows where they will get winter sunshine. This can cause the designer some difficulties; the plot may have nice views to the north, or may have a lot of trees or hedges to the south that cut out the sunlight, for instance. As designers become more familiar with the standard, they develop the skills to be more expansive with the design and take advantage of the benefits of different plots. The designs they create will also be constructed for a reasonable cost uplift.
With highly energy-efficient homes there is a danger of overheating in the summer if there are large areas of unprotected glazing to the south and west. Good design can overcome this by ensuring that such windows have appropriate shading — including overhangs or shutters, which are a standard feature of southern European houses.
The Passivhaus Planning Package (PHPP) – an essential tool used when designing a Passivhaus – is a monster Excel spreadsheet and checks that the property being designed will not overheat in service. In addition, the development of automated blinds that can be set to close if the internal temperature rises beyond a set point also aids prevention of overheating. It is likely that we will soon be able to respond to changes in the temperature of our homes by using our smartphones.
Potton have previously designed all of our showhomes using in-house designers. This time we took the brave decision to work with an external practice, HTA Design. Apart from being great designers, HTA is a practice we have worked with on previous low-energy housing developments. HTA have a deep and genuine interest in sustainability and energy efficiency.
The Architecture and Sustainable Futures team at HTA worked collaboratively with our technical team to design a house that is ambitious in form and avoids the stereotypical Passivhaus perception of a simple box. Derived from a desire to flood the interior with natural daylight, a series of simple design moves creatively breaks down ‘the Passivhaus box’, employing large openings and a distinctive butterfly roof, while simultaneously challenging and satisfying the requirements of the Passivhaus Planning Package. There are certainly easier ways to design a Passivhaus, but we’re building a showhome to see just how far we can push the performance standard.
A top-lit enclosed ‘courtyard’ sits at the heart of the home, filling the deep plan with natural daylight. Flexible in arrangement, the series of interconnected spaces is designed to be adaptable to almost any occasion or lifestyle. The ground floor can be imagined as a layer of nine squares or boxes; in this house, the rear-most six are almost entirely open plan in arrangement. (We believe that the ‘nine square’ design will provide future customers with almost innumerable great ways that they can tailor the design of their own houses but still benefit from the care and thoughtfulness of this Passivhaus design.)
This large south-facing space is lined with heavyweight flooring and 8m of full-height glazing that opens out to the garden under the overhang of the roof, blurring the line between interior and exterior. The courtyard space at the centre is not only intended to be flexible in use but also the unofficial hub of the home. We are currently developing the interior design with our partners at Dulux Design Service.
The south-facing opening has been maximised to benefit from solar gain, which combined with other Passivhaus principles, helps to reduce the energy consumption of the house to 15kWh/m2/yr for heating. The roof overhang protects this glazing from excessive solar gain and overheating of the interior during the summer, while maximising solar gain during the winter months.
The new showhome will be constructed using the Kingspan TEK® Building System (structural insulated panels) and, with additional external insulation will have a ‘thermal bridge-free’ construction. The windows, which will come from one of our current suppliers Kloeber, are triple glazed and tightly sealed into the envelope to achieve reduced heat losses and a comfortable surface temperature.
The house is conceived as a brick-clad box, which reveals spaced timber cladding at the points at which it is ‘carved’ away. At the entrance, bricks sit above the cladding as it wraps around the corner and seamlessly continues inside to the main living space, guiding visitors into the house.
Most Passivhaus builds use lightweight claddings carried by the structure of the building; insulated render is common. Using bricks has added a further level of complication, due to the need to provide a separate foundation for them (read more here) and we have challenged ourselves further by selecting a longer, narrow-format brick from Wienerberger, with narrow mortar joints, for a contemporary aesthetic.
Progress on Site
Work on site is well underway and has so far progressed smoothly.
The ground floor of the house is constructed of a 225mm-thick cast concrete raft sitting on top of a 250mm layer of Kingspan Styrozone insulation. It’s a difficult concept for some people to believe, but all of the loads applied by the building structure, its occupants and contents get transferred into the concrete and ultimately into the ground by thick layers of ‘load-bearing’ polystyrene. The insulation sits on a levelling layer of sand with hardcore underneath.
Prior to beginning any work on site we commissioned a soil survey to confirm the load-bearing capacity of the soil; thankfully it was very good. There is a separate and completely disconnected foundation to carry the vertical load of the brickwork. A separate 100mm-thick layer of insulation wraps up the sides of the raft and will link into the additional layer of insulation applied outside of the Kingspan TEK® Building System.
These layers of insulation are key parts in providing the well-insulated, thermal bridge-free building envelope. The foundation and ground floor structure is completed, as in almost all timber-based buildings, with a course of thermally efficient aircrete blocks to which the sole plate of the structure will be fixed.
Dr Paul Newman
Self build Director Dr Paul Newman has worked within timber-based construction industries for over 18 years