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CIBSE Case Study: Residential Retrofit to Passivhaus Standards
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CIBSE Case Study: Residential Retrofit to Passivhaus Standards

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Article from the January 2014 edition of the CIBSE Journal written by Andy Pearson.

When the founder of Green Tomato Energy bought a leaky, dilapidated Victorian terrace house in London, it was the perfect test project for his company's new low energy refurbishment service. The result is one of the UK's first retrofit to Passivhaus standards. Andy Pearson reports.

Domestic energy-efficient retrofit projects do not come more ambitious than the Victorian midterrace house in Lena Gardens. Its refurbishment had to tackle the issue of its heat-pervious solid brick walls and leaky sash windows. The task was made all the more demanding because the West London property is located in a conservation area, which prevented alteration of the building's exterior. Despite these challenges the scheme's designers -Refurbishment Project winners in the 2013 Building Performance Awards, Green Tomato Energy - managed successfully to transform this unpromising dwelling into the UK's first private domestic Passivhaus-certified retrofit.

Passivhaus buildings are super-insulated with controlled ventilation to provide high levels of occupant comfort while using very little energy for heating and cooling. The key to their design and construction is meticulous attention to detail, according to principles developed by the Passivhaus Institut in Germany.

Green Tomato Energy's feat is even more impressive because the project was undertaken between January and October 2010, which meant it was completed before the launch of EnerPHit, the Passivhausrefurbishment standard, which takes into account the bigger challenges associated with retrofit. Equally remarkable, Lena Gardens was the first Passivhaus scheme the practice had designed. It was undertaken at the same time as Princedale Road, another Passivhaus retrofit in West London.

The four-storey house belongs to Tom Pakenham, co-founder of Green Tomato Energy. "Because we were doing everything for the first time, we probably wouldn't have attempted a Passivhaus solution if it had been a client's project, because the risks would have been too great," says Akta Raja, Pakenham's co-founding partner at Green Tomato Energy.

In 2009, the firm was looking at setting up a business to deliver low energy improvements to homes. As Pakenham had just started renovating his home, the team seized the opportunity to test their plans.

"It was intended to be a research project, not just from a technical perspective but also from the client's perspective - because it enabled us to work out how to deliver a strong client experience," Raja explains.

The Passivhaus design was developed with input from building physics engineer Edward Borgstein, who helped come up with the most effective low energy interventions. The team was fortunate in that Lena Gardens was in need of significant renovation, including replacement of the windows, which enabled the low energy element of the works to be cost-effectively incorporated into the scheme. Passivhaus Planning Package (PHPP), the software design tool produced by the Passivhaus Institut, was used to model performance of the proposed interventions.

"The house was in need of a full refurbishment even before we decided to try for Passivhaus standard," says Raja.

On site, the building's roof was demolished, the chimney breasts removed, and plaster was stripped from the walls to enable a thorough investigation of the building fabric early in the project. As a result, remedial works were carried out to the walls and additional structural beams installed to provide additional support.  Nasty surprises came from things like the walls not being straight," says Raja.

The walls were then insulated internally, using phenolic foam in a two-layer system. A layer of Oriented Strand Board (OSB), was incorporated into the walls, and the joints taped to form a continuous, airtight layer. The building's second and third floors were replaced because their timbers were rotten and similar OSB layers were installed under the floors and over the ceilings.

The party walls were also insulated to eliminate thermal bridges and to protect against low temperatures in the neighbouring property. Internal walls were then rebuilt inside the building's insulated shell. The team faced some challenges over predicted condensation risks because of the high levels of insulation, which resulted in the scheme being modelled extensively and the risks minimised through design.

Similarly, there were concerns that the building might overheat in summer, because it faces south and solar shading was not permitted within the conservation area. As an alternative, temperatures have been managed using a combination of night purging and cross-ventilation, which post-occupancy evaluation has shown to be effective.

The roof was rebuilt as a mansard loft extension, incorporating the same two-layer insulation system as in the walls.

Replacing the windows was more of a challenge. At the time of the scheme's construction, there were no manufacturers producing high-performance, air-tight tripleglazed windows that looked identical to the building's original sliding sash units. As a consequence, a second project was initiated with the UK Technology Strategy Board and Ryder Strategies to develop and manufacture a suitable replacement.

The result was a new triple-glazed window, indistinguishable in appearance from the original, but with a fixed upper triple-glazed unit and a tilt-and-turn opening lower section. The tilt-and-turn solution of the casement windows enables the building's overall airtightness levels to achieve Passivhaus standards.

The timber floor on the building's ground floor was lowered to allow insulation to be added. The timber joists of the first, second and third-floors - which would have bridged the wall insulation - were also rehung inside the thermal envelope both to reduce thermal bridges and to eliminate the risk of condensation and rot occurring where the timber would have been enclosed in the wall's insulation layer. Instead, they are supported from steel beams, slotted into insulated pockets set in the party walls at the front, middle and rear of each floor.

In the kitchen, the original concrete floor slab was removed, and replaced with 150mm polyurethane foam insulation topped off by a new slab. The floor in the basement was also replaced. Here the team took the opportunity to install a labyrinthine ground-to-air heat exchanger during the works. The labyrinth preheats and pre-cools the fresh air being drawn into the building through the mechanical ventilation heat recovery (MVHR) system.

The tempered fresh air is drawn into a Passivhaus-certified Genvex Combi unit, housed in the basement, which combines an MVHR system with an air source heat pump. The unit supplies fresh air throughout the house via a network of spiral wound ducts hidden within floor voids and stud walls. Air exhausted from the kitchen and bathrooms is returned to the unit, where a heat exchanger transfers its heat to the incoming fresh air, before being expelled.

Because Passivhaus space heat needs are so low, the Genvex unit also heats the Lena Gardens house using the supply air as the heat transfer medium. Heat is supplied from the air-source heat pump contained in the unit to the MVHR supply duct.

The air source heat pump also functions as an air-to-water heat pump to top up the domestic hot water temperature if the building's three rooftop solar thermal array panels fail to provide sufficient heat to the 400-litre tank.

"You reduce the heat load and then use the MVHR system with air heating to warm the house," explains Raja.

The roof also houses the scheme's 1.1kWp array of photovoltaic panels, which supply 801kWh of electricity annually (based on a twoyear average). This provides roughly 16% of the total annual electricity needs.

Raja says the project team were "privileged" with the Lena Gardens Passivhaus refurbishment because they had a client that let the team experiment; they had a building team that was "interested" in Passivhaus and because the retrofit was an in-houseproject the design engineer was on sitealmost every day.

10 Lessons Learned from Lena Gardens

1. Teamwork is essential

"What is absolutely key on a retrofit project is that the contracting and design teams have to work closely together to overcome site issues and to share knowledge," says Marine Sanchez, building physics engineer and Passivhaus designer at Green Tomato Energy. "The projects that work best are when everybody acknowledges that there are things that they don't know, and that we're only going to achieve the best outcome if we all work together.  Designs must be buildable and contractors must buy-in to the concept, since they will be delivering the project.

2. Allow sufficient design time

The Passivhaus concept requires a wholehouse approach. This means actions taken in one part of the building have consequences elsewhere, which could affect performance."Use as much time as possible to work through the details. This will reduce time and cost during the on-site phase, where choices might be more limited," says Sanchez.

3. Investigate potential problem areas in advance

Examine key areas to expose the existing form of construction and prevent surprises once work starts. "A lot of schemes we work on are in conservation areas in London. As you have to insulate internally, you need to know how the building has been constructed - otherwise you risk developing a design that will increase the risk to the fabric or causing an escalation in cost," says Sanchez.

4. Be prepared to adapt the design

Provision must always be made for unforeseen problems with the building fabric. For example, don't expect walls to be straight, or build quality to be high, which can add to costs.

5. Develop the most energy-efficient design possible, based on the client's requirements,the building, its location and the retrofit budget

Renovation is the best opportunity to make significant, cost-effective improvements to the building's energy performance. "We're not saying every building retrofit should be Passivhaus, because existing buildings are hard to work with and costs can escalate," says Raja. "On most projects, where we've not gone as far as Passivhaus, we've managed to achieve at least 50% improvement on what was there before".If no renovations are planned, there are many measures that can be applied in the meantimeto reduce energy consumption and cost.

6. Provide the client with sufficient information to enable them to make informed decisions

"The beauty of using a Passivhaus approach is that PHPP is an Excel spreadsheet, which enables you to show clients how different measures will impact on the scheme's performance," says Raja. If reliable costinformation is factored into the discussion, the works can be considered in terms of a potential payback period, which allows clients to decide where best to spend their money. Client engagement with the proposals is important and Green Tomato Energy recommends adopting an energy target early in the project. "We're not forcing people to go down a particular route -  we're just giving them the figures they need to make a decision," adds Sanchez. It is also a good idea to make the homeowner aware of the scale of disruption caused by a whole-house retrofit.

7. Be aware that some historic elements will not be Passivhaus-compliant

"To achieve an airtight building with an air change rate of less than 0.6 per hour, you need a good seal on windows, which is difficult to achieve with a sash-style windows," says Sanchez. Because Lena Gardens is in a conservation area, Green Tomato Energy worked with contractor Philip Proffit and Princedale EcoHaus to develop a new form of window that looks like a sash, but which has a fixed upper glazing unit with a tilt-and-turn lower section that could take triple-glazing.

8. Be prepared to give extra support on site

If a scheme has been poorly designed or installed, the retrofit measures are unlikely to perform as planned, which means that they could fall short of the occupier's expectations for both comfort and energy savings. Passivhaus retrofits require greater attention to detail than is typical in the UK renovation sector. "One of the lessons we learned from Lena, and other retrofits, was that we needed to provide support on site. All our design expertise will be wasted if the design is not properly applied on site, or if the design doesn't take account of site issues," says Raja.

9. Allow time to show people how to use a low energy house

One of the most crucial factors is the handover. The original design team should ensure all systems are functioning as designed, and they are there to explain their operation to the client both to ensure optimum performance and to prevent problems occurring in the future. Handover documentation alone is insufficient.

10. Treat each project as a learning experience

Measure the performance of everything so you know how well it is working, or not, so that you can learn from it and prevent every project turning into a trial project. Keep an open mind,and continue to learn from every project.

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