Going for the ‘gold standard’

How ‘zero compromise’ do architects need to be when designing envelopes to the Passivhaus standard and are aesthetic, and other compromises necessary? Sarah Johnson reports on the challenges involved.

The Passivhaus standard is often seen as the ‘gold standard’ for building performance. Passivhaus buildings provide a high level of occupant comfort while using very little energy for heating and cooling. They are built with meticulous attention to detail and rigorous design and construction per principles developed by the Passivhaus Institute in Germany.

The fabric, or envelope, of a Passivhaus, needs to achieve high thermal and airtight performance while providing structural stability and weather protection. For building durability, the building also needs to eliminate the danger of interstitial condensation forming within the structure.1 The key principles of Passivhaus design include:

• continuity of super insulation;

• minimising thermal bridging;

• maintaining airtightness;

• minimising thermal bypass.

A rigorous approach is needed when it comes to designing building envelopes which will provide the necessary performance for a project to achieve Passivhaus accreditation. Passivhaus buildings can be built out of any construction material and therefore a wide variety of building envelope details are required to deal with the challenges of limiting thermal bridging and creating an airtight layout to meet Passivhaus criteria.

Passivhaus criteria do not dictate what construction methods should be used, but the following case studies of certified Passivhaus projects illustrate a range of scales, materials and typologies. In each example, the architects were asked three key questions about their project.

CASE STUDY: The University of Leicester

The University of Leicester’s Centre for Medicine building is currently the largest commercial Passivhaus in the UK. It consists of a concrete frame and curtain walling.

Architect’s View:
Jonathan Chadwick – Associated Architects

Why were the materials and the construction method adopted?

Brick was chosen as the predominant material for the facade as it respected the character of the neighbouring building and the site’s location on the edge of a conservation area. The use of curtain walling modules to panelise the brick facade provides a contemporary architectural language whilst remaining sympathetic to the surrounding context.

Curtain walling lent itself to the rigours of achieving Passivhaus levels of facade performance as it reduced the number of interfaces between different subcontractors’ work packages and provided a high degree of certainty that levels of workmanship would remain consistent. The use of ‘tried and tested’ building components also gave the client confidence that long-term appearance and performance would be maintained without significant additional ongoing costs.

What was the main challenge in meeting Passivhaus?

Refining the details of the curtain walling to minimise thermal bridging and to integrate the controls for the external shading blinds was complex and hadn’t been done to Passivhaus levels of performance in the UK before.

Extensive design development was carried out by the contractor’s design team in conjunction with the facade sub-contractor, Passivhaus specialist, curtain walling manufacturer and facade engineer to ensure the design was robust and repeatable whilst maintaining the required aesthetic.

A large mock-up panel was produced off site at a testing facility to enable the technical resolution of the details to be refined and weather tested to CWCT standards prior to inspection by the client.   

How ‘zero compromise’ was the design and were any aesthetic compromises made?

The Passivhaus standard of performance is very exacting and numerous detail design refinements were reviewed and thermally modelled prior to the curtain walling going into manufacture. This extended detailed design period is necessary with Passivhaus projects to ensure that aesthetics are not compromised in the pursuit of technical performance where new technologies are being pursued.

CASE STUDY: Wilkinson Primary, BILSTON

A second generation Passivhaus Primary School that addresses the future needs of a 21st century learning environment, including various improvements to performance and sustainability. The timber frame is partly clad in Corten steel and polished clay tiles.

Architect’s View:
Project Architect Lee Fordham – Architype

Why were the materials and the construction method adopted?

The construction  is timber frame sourced from domestic FSC timber which is exposed internally. The industrial heritage of the site provided inspiration for the exterior cladding of Corten steel. Corten is a sustainable choice as no maintenance is required, as the surface develops to a stable rust-like appearance. Allowing the steel to rust makes the rust itself form a protective coating that slows the rate of future corrosion.

The smooth dark grey cladding of polished clay tiles provides a striking contrast to the Corten which weathers to a vivid orange patina while complementing the design aesthetic and also linking the school with the site’s industrial past.

What was the main challenge in meeting Passivhaus?

Sharing the site with the occupied portacabins involved careful planning and logistics to ensure the utmost safety and compliance with the programme. Separate access was created, with the close proximity of the temporary accommodation cut off by a fire-rated site hoarding boundary wall.

The site itself is the former Iron Works of John Wilkinson, the namesake of the school and a Black Country industrialist from the industrial revolution. Coincidently, an old forge was identified that restricted the location of the foundations and mine shafts were speculated on initial surveys but never discovered, however an archaeological watching brief was employed for the first stages to ensure site safety.

How ‘zero compromise’ was the design and were any aesthetic compromises made?

Passivhaus certification was a top priority of the design made very clear in the client’s brief. With this commitment from the client, we did not need to compromise the design. Clearly, aspects such as shading and solar-gain influenced the design in terms of form, but of equal importance was the school’s requirement for space and layout.

CASE STUDY: Lansdowne Drive, EAST lONDON

A zinc-clad two-storey Passivhaus home located in a Conservation Area in east London was erected in a few days from pre-fabricated structural cross laminated timber (CLT) panels.

Architect’s View:
Bernard Tulkens – Tectonics Architects

Why were the materials and the construction method adopted?

Early on we chose to use a combination of CLT and wood fibre insulation. This was based on the ease of construction (prefabrication) the sustainability of the natural materials, the good airtightness provided by the base CLT structure and the desire to use breathable construction.

The choice also helped provide a clear layering structure where continuous air tightness and elimination of cold bridges could be achieved. The simple layered logic of the construction system at upper level was also used on the lower ground floor concrete structure, the external EPS insulation and the foundations (Isoquick).

What was the main challenge in meeting Passivhaus?

Direct sun gains from the south were not possible due to the presence of the adjacent Victorian terrace. Additional insulation, efficient MVHR and elimination of the thermal bridges contributed to meeting the standard.

The feedback from the PHPP (Passive House Planning Package) analysis on decisions, and how a response can be found to meet the challenge of Passivhaus, is most important. For instance, as a result of the orientation, the glazing to the west facing windows have a higher g-value (proportion of heat transmitted) as this was more important than their U-value – they are the only windows receiving sufficient quantities of sunlight. Being west-facing windows, external shading was also necessary to control heat gain in summer.

Another challenge is the on-site supervision required. Careful and regular checks, as well as communication to the building team of the Passivhaus principles, are essential.

How ‘zero compromise’ was the design and were any aesthetic compromises made?

We did not begin the project design as a Passivhaus but the principles of the building in terms of form and concept were well suited to being adapted to Passivhaus.

Eliminating thermal bridges made detailing simpler and zero compromise on that aspect is essential.

The PHPP leads to questioning the use and number of elements in a project, and that is also a positive aspect of the Passivhaus analytical process. I was keen to have a skylight in part of the upper level; the PHPP indicated that a skylight at the upper level was not contributing to the energy performance (and Passivhaus-certified skylights were not very developed at the time).

The principles had an impact on the number of windows and guided their form (windows rather than large sliding screens in this case). In the end, they were positive contributions to the design project, and made the building better.

CASE STUDY: Cre8 Barn, Yorkshire

Retrofit with a super-insulated timber frame structure built inside the existing building to preserve the appearance of the cow byre.

Architect’s View:
Bill Butcher – Green Building Store

Why were the materials and the construction method adopted?

Yorkshire Wildlife Trust (YWT) wanted to convert an existing barn at Stirley Community Farm into an education centre. To achieve the required insulation levels and make the airtightness and thermal bridging detailing easier, we decided that a ‘box within a box’ construction offered a practical solution on this project. This was achieved with an inner timber frame construction with a ventilated cavity within the existing masonry barn walls.

What was the main challenge in meeting Passivhaus?

A considerable amount of work was needed to stabilise the walls and underpin the foundations of the barn. To address this problem, the inner timber frame structure also helped support and shore up the outer masonry barn wall, using specially adapted wall connectors with low thermal conductivity.

How ‘zero compromise’ was the design and were any aesthetic compromises made?

The form and orientation of the building were obviously fixed, which impacted on the passive solar gains available in the building. The high ceiling height and reduced level of treatable floor area meant that the project would not have achieved EnerPHit through standard certification using PHPP (Passive House Planning Package) methodology. Fortunately, the Passivhaus Institute had then developed a ‘component’ route to EnerPHit, with set criteria for individual building elements (e.g. walls, roof, floor etc), which the Cre8 Barn could fulfil.

CASE STUDY: Burnham Overy Staithe

A coastal terrace of three Passivhaus units forming part of a mixed development of affordable and open market housing on a planning exception site within the North Norfolk AONB, reflecting the local vernacular of traditional fishermans’ cottages.

Architect’s View:
Chris Parsons – Parsons + Whittley

Why were the materials and the construction method adopted?

In fact, materials here led the choice of construction method. We wanted to respect the character of Burnham Overy Staithe, not least because the whole area is designated as an Area of Outstanding Natural Beauty but also because it was adjacent to the Conservation Area. We studied materials and traditional features to ground the development in its location, and that suggested the use of flint, red brick and clay pantiles, as well as the simple form of the buildings. Given the predominance of masonry type solutions and our previous experience of delivering Passivhaus through masonry construction, the scheme utilised a traditional masonry cavity wall. The increased thermal mass also influenced our decision as did the use of Gypsum plaster (Hardwall) as an airtight membrane.

What was the main challenge in meeting Passivhaus?

The scheme received planning approval in 2009 but had not originally been proposed as a Passivhaus development. Nonetheless, the form of the building and orientation leant itself so the modifications were easily assimilated, with a thickening of wall constructions and a slight height adjustment. Thankfully the planners were very helpful in approving these amendments. This left us short of space for MVHR and other equipment but the use of compact units containing exhaust air heat pumps enabled the whole of the kit to be fitted with the former airing cupboards and still leave enough room for linen storage.

How ‘zero compromise’ was the design and were any aesthetic compromises made?

As always with Passivhaus, the approach starts with ‘zero compromise’ but inevitably there were some on the way. The main compromise was around the heating/MVHR system which is more complicated than we would prefer. The location of the combined units forced us to site the inlet and exhaust for the MVHR through different roof slopes, which is probably not ideal. The overheating strategy couldn’t utilise deeper roof overhangs because of the aesthetic and so we had to resort to electrically operated external blinds to control overheating.

Aesthetic compromises were not admissible given the sensitive nature of the site and the extant planning consent but this did not cause us any real performance issues. Given that the design was based on traditional building forms found in the area, I think it demonstrates that our forebears instinctively understood the principles of Passivhaus design.

The Passivhaus Trust

The Passivhaus Trust is an independent, non-profit organisation that provides leadership in the UK for the adoption of the Passivhaus standard and methodology. Passivhaus is the leading international low energy, design standard. Over 65,000 buildings have been designed, built and tested to this standard worldwide.

The Trust aims to promote the principles of Passivhaus as a highly effective way of reducing energy use and carbon emissions from buildings in the UK, as well as providing high standards of comfort and building health. The 2017 UK Passivhaus Awards take place in July.g

¹How to build a Passivhaus: Rules of thumb.
Chapter 3: Building Fabric