Part 1: Building a Passive House (Passivhaus)

6 April 2016 Paul Newman, Self Build Director
Potton Passivhaus Show House CGI
Amid the hype, everybody seems to want to build a Passive House (known as Passivhaus in most of Europe) at the moment, but it pays to take a step back and examine what a Passive House is — and what building one involves.

The standard Passivhaus design originated in Germany in the 1990s and is based on the idea that a home can be heated using warmed fresh air. However, as air can only hold a small amount of heat, the house must be designed and constructed thoughtfully and carefully.

The amount of heat from the warmed fresh air turns out to be 15kWh/m2/year —about three times more efficient than current UK Building Regulations, and twice the amount required by planned UK regulations for 2016.

How Is Passivhaus Standard Achieved?

To achieve this optimum figure, Passivhaus homes must be ventilated using a mechanical system that preheats 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, windows and open joints between construction elements and are consequently draughty. So many of us are used to having fresh air, provided via draughts resulting from poor construction, without opening windows — or using any 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 uncontrolled heat escape through 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 change. This means Passivhaus homes are cheap to live in — because the heating bills are kept very low. 

What Are the Key Features of a Passivhaus?

  • 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).
The constructor of a Passivhaus needs to put in a lot of effort to make the home highly insulated and airtight. So far in the UK, there are few constructors and manufacturers skilled at doing this. However, as more Passivhaus projects are built, 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 due to the increase in construction quality in passive house builds ought to be offset by the omission of a traditional heating system. However, many homeowners are nervous about leaving conventional heating out as they become concerned when radiators are not there — even if they are not needed.

Potton Elsworth Passivhaus Show House

The Elsworth is open to the public

Come and expereience the Elsworth show house for yourself. Built using the Kingspan TEK Building System, this is the UK's first show home built to Passivhaus standard.

Take me to the Elsworth Show House

Why Should I Build to Passivhaus Standard?

Due to the long payback duration, most people need reasons beyond simple energy efficiency before they decide that building a Passivhaus is a good idea. Well, here are 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 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.
Another huge benefit of Passivhaus buildings — perhaps the least obvious — is that they provide a comfortable internal environment with even internal heat distribution.

What Is the Passivhaus Design?

The stereotypical Passivhaus is a simple box, aimed at containing the most space, as efficiently as possible and maximising windows to make the most of winter sunshine. Optimising the sun can cause the designer some difficulties — for example, a plot may have beautiful views to the north, or may have a lot of trees or hedges that cut out the sunshine to the south.  

As designers become more familiar with the standard, they develop the skills to be more expansive with their design and take advantage of the benefits of different plots. Consequently, the designs they create will 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. Effective 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 used to check that the property will not overheat in service.

The development of automated blinds — which can be set to close if the internal temperature rises beyond a set point — also prevents overheating.  Besides, it is likely that we will soon be able to respond to changes in the temperature of our homes by using our smartphones.

Is There Natural Daylight in a Passivhaus?

Potton Elsworth show home courtyard
A top-lit enclosed “‘courtyard” sits at the heart of the Elsworth show home, filling the deep plan with natural daylight. Flexible in arrangement, the series of interconnected spaces around it are designed to be adaptable to almost any occasion or lifestyle.

You can visualise the ground floor as a layer of nine squares or boxes with the rear-most six almost entirely open plan. We believe that the “nine square” design will provide future customers with plenty of great ways to tailor the design of their houses but still benefit from the care and thoughtfulness of this Passivhaus design.

Heavyweight flooring lines this large south-facing space and 8m of full-height glazing opens out into the garden under the overhang of the roof, blurring the line between interior and exterior. 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 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 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.

Building a Passivhaus Show Home

Potton has 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. As well as being great designers, HTA is a practice we have worked with on previous low-energy housing developments. They 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.

Building a Passivhaus Show Home Guide

Our Building a Passivhaus Show Home manual is a complete step-by-step guide, which takes you through the story of building our Passivhaus show home — from the design process, constructing the superstructure, achieving airtightness right through to the finishing touches.  Download the guide here.
Building a Passivhaus Download

How Is a Passivhaus Built?

Kingspan TEK Passiv standard render
The new showhome will be constructed using the Kingspan TEK® Building System (structural insulated panels) and will have a “thermal bridge-free” construction using additional external insulation.

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 comfortable surface temperature. Most Passivhaus builds use lightweight claddings carried by the structure of the building; insulated render is common.

The house is conceived as a brick-clad box, which reveals spaced timber cladding at points where 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.

Using bricks has added a further level of complication as they need a separate foundation. 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. 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 ground floor of the house consists of a 225mm-thick cast concrete raft sitting on top of a 250mm layer of Kingspan Styrozone insulation. The insulation sits on a levelling layer of sand with hardcore underneath.

Before beginning any work on-site, we commissioned a soil survey to confirm the load-bearing capacity of the soil — thankfully it was excellent. There is a separate and 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 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.

Read Part 2: The Passivhaus Homes Myth-Busting Guide

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