Max Fordham takes on Britain’s biggest Passivhaus

The redevelopment of the Agar Grove estate, in Camden, is not only expected to be the biggest residential Passivhaus development in the UK. It is also highly challenging for the designers involved.

As well as having an 18-storey Passivhaus residential tower on a tight inner-city site, the development is subject to environmental planning requirements that are not always compatible with Passivhaus principles. For example, heat networks might appear to be a prerequisite for large housing schemes in London, which means incorporating a network of heating pipes. The heat loss from the pipework introduces an increase in annual ‘primary energy demand’,^I which is limited in the Passivhaus standard, so the project team had to work hard to come up with an ultra low-loss network design.

So, if Passivhaus can introduce tensions with London planning policy, why attempt to apply Passivhaus to 345 apartments? The drive comes from the client, Camden Council, which owns and manages the estate. It believes a Passivhaus design can reduce fuel bills and cut carbon, while minimising the council’s costs of maintaining the heating system.

Michelle Christensen, senior development manager at the London Borough of Camden, explains: ‘The simplicity of the concept, low maintenance requirements and low fuel costs for residents are the driving forces behind this approach, which has been integral to the proposals from the outset.’

Buildings constructed to the Passivhaus standard are designed to use the minimal amount of energy, and the focus is on minimising heat gains. Buildings must have an annual heating and cooling demand of not more than 15 kWh/m², or be designed with a peak heat load of 10 W/m². To minimise heat loss from the building fabric, the structure must not leak more air than 0.6 times the house volume per hour at 50 Pa. With such low heating demand the heat could be delivered on the ventilation air rate required for hygiene, although heating via warm air is not necessarily proscribed.

The standard is as much about comfort as it is about energy. It sets numerous standards for summer and winter comfort. Some of these are described in Table 1.

Arguably more important than the energy and comfort targets is the process of design, check and improve that is central to the Passivhaus process. It is more rigorous than conventional procurement, and this applies to design, to the manufacture of certified components, to construction and, finally, to the commissioning process.

Passivhaus has been proved, over 25 years, to produce a significantly smaller performance gap, and does not have to cost more than conventional building¹ – so why is it not more commonplace? It is hoped that Agar Grove will prove the relevance of Passivhaus in the UK, and its viability in volume housebuilding.

The Agar Grove estate

Agar Grove was built in the inner-London borough of Camden in the 1960s, and is bounded by a busy road and two mainline railways. Its refurbishment will result in 493 new and refurbished homes, including 240 for private sale. All 345 new-build units are designed to Passivhaus standards and Code for Sustainable Homes level 4.

The project also includes a deep refurbishment of the 148-unit Lulworth tower, which will meet BREEAM Domestic Refurbishment Excellent (148 units), but not the Passivhaus retrofit standard, EnerPHit. Phase 1 includes the construction of 95 units.

Current residents will be rehoused on the site to maintain social cohesion. For the scheme to be financially viable, the housing density has been doubled to allow the inclusion of apartments for sale on the open market (see Table 2).

Design challenges

Architects Hawkins Brown and Mae were responsible for masterplanning the development, and Hawkins Brown associate Emma Lynn says designing to Passivhaus on an urban site posed some challenges.

‘Building Passivhaus in the city is quite unlike building on a greenfield plot – there are myriad requirements on the building form which must be resolved,’ she says.

The masterplan is based upon the traditional concept of streets and squares, adds Lynn, with an emphasis on ‘creating liveable spaces between buildings and allowing people to move across, through and within the site.’

As a result, it was not always possible to position the buildings north to south, which is the ideal orientation for beneficial controlled solar gains. To ‘stitch in’ the massing to the neighbouring streets, the taller buildings were placed to the south of the site, which caused some shading. This had to be taken into account when considering the lower heat gains in some flats during the winter. (See panel, ‘The north-south divide’).

Another challenge was the balcony design. They are often recessed to reduce visual impact on the façade and complement the aesthetics of the existing housing and maisonettes.

‘This introduces complexity in the structure and associated thermal bridging,’ says Lynn. ‘The Passivhaus consultant and the design team resolved these tensions by using the Passivhaus Planning Package (PHPP) model rigorously and iteratively to test design choices – such as window size, U values, and M&E efficiencies – to select the most cost- and technically efficient specification. 

Figure 2 shows the effect of such design choices on the heating demand in two blocks over the course of RIBA work stages 3 and 4.^III The contractor design team have since developed this much further.

Passivhaus and other design codes

Agar Grove meets the strict requirements of Passivhaus, as well as of the Code for Sustainable Homes and other metrics of sustainability and design quality. Part of Max Fordham’s role was to resolve any conflicts arising from these overlapping standards.

Planning policy in London sets challenging carbon targets, which are assessed via the ‘SAP methodology’ calculation. This is known to be a poor measure of performance in use². SAP does not model the expected significant reduction in performance gap that Passivhaus achieves, which makes it difficult to justify the perceived increased cost or complexity. Even so, a less stringent CO₂ target than the policy was accepted by planners in recognition of the quality of the scheme.

Through iterative design, the window-to-wall ratio was set around a conservative 35%, to limit summer solar gains without resorting to solar control glass, and to limit winter heat losses. The groundfloor units are generally duplex to avoid street-level bedrooms, where occupants may not wish to open the windows at night. The bedrooms are of lightweight construction, to allow fast purging of the day’s heat in the early evening.

Working with district heating

As the primary energy demand is limited by the Passivhaus standard, the designers focused on reducing the length of pipework wherever possible. The boilers are not in a single central location, as preferred by the Greater London Authority, but block by block. As well as easing the phased development, this approach reduces the required system temperature and pressure regime, and allows a reduced scale of plant and controls. It also allows more of the maintenance to be done by Camden’s in-house facilities management (FM) team.

Rather than a traditional network of one riser with long lateral branches at each level, the pipes are arranged in many vertical risers. This simple change reduces pipework length considerably. By using hydraulic interface units (HIUs), the LTHW flow and return temperatures are reduced – both are critical to reducing losses. The system was designed at 70/40°C, and the contractor team is looking to push this even lower.

Secondary benefits of this approach are lower capital and running costs, reduced risk of overheating, and reduced risk of legionella-favourable conditions in the mains water supply (as the water main takes a different route into the building ot the heating mains so there is less risk of heat gain into the mains water).

Mechanical ventilation

Modern homes are ‘built tight’, but are not always ‘ventilated right’, so they end up being ‘stuffy and stinky’³. This is often because the ventilation is not being designed, installed and commissioned properly. The scrutiny of Passivhaus addresses these issues. On Agar Grove, private-sale and duplex apartments are served by their own MVHR units, which are mounted directly onto the external wall, for efficiency.

Social tenure flats on a repeating floor plate can be more suited to communal MVHR, with a single unit at the head of the riser serving many flats. At a marginal cost increase, or cost parity, communal MVHR offers enhanced thermal efficiency, air quality and reduced plant space within the flat. It also means simple but essential maintenance can be carried out by facilities workers, rather than by untrained residents. Impacts of communal MVHR to be considered include increased riser space, and consideration of how individual control of ventilation rates is provided. (See Figure 1).

Summertime heat purging at Agar Grove is primarily via opening the windows, and the MVHR in bypass mode provides a small contribution towards this. If it is noisy outside, opening the windows wide may cause a short-term acoustic nuisance, but once the heat is purged, the window can be returned to crack opening and it is once more comfortable. A cracked-open window provides some acoustic attenuation and background ventilation, too.

Passivhaus in UK housing

For affordable housing providers, the lower heating bills, comfort and health benefits are very attractive. Although cost trends are showing that larger Passivhaus projects – greater than 40 units – need not cost any more to construct⁴, a marginal increase in the cost could be borne by these housebuilders.

When it comes to private sale, the average buyer is unlikely to be aware of the benefits, so there may be little premium value for building a new Passivhaus. However, private developers are starting to appreciate the smaller plant allocation, improved build quality and reduced landlord overheads.

Agar Grove has been a steep learning experience for everyone. But at this early stage we feel we have demonstrated that Passivhaus can be applied successfully at a very large scale. The challenge of  procurement is the management of cost and risk while achieving continuity of design. This is more tricky when there is a commitment to the rigorous and, initially, unfamiliar Passivhaus standard.

A clear brief is required, as is strong support from the client, and buy-in from the design and contractor teams. Early advice (at RIBA stage A/B) from an experienced Passivhaus designer is crucial to achieving a cost effective and successful result for all concerned.

Christensen sums up why Camden is committed to Passivhaus. ‘We are determined to tackle fuel poverty and reduce CO₂ without the need for complex energy systems with high lifetime costs. This approach provides thermal comfort and air quality in a way that alternatives do not match.’

‘Although this can increase the initial capital costs, Camden Council – as both developer and landlord – believes that it will see the benefits of this approach, in higher build quality and reduced maintenance costs over the lifetime of the buildings.’

Notes
I Primary energy demand for all heat and electricity, kWh/m²  floor area per yr. This is a comparable to carbon dioxide emission rate of a building.
II Includes heat loss from frame, spacer & installation.
III Developed design and first part of technical design stages.

References
1 Designs for the same building task In €/m² in relation to high energy efficient additional costs, Evaluation Of Competitions, Ebök 2004.
2 End of Term Report, Zero Carbon Hub, 2014.
3 Howieson, CIBSE Journal October 2015.
4 Passivhaus Capital Cost Research Project January 2015, PHT.