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Hiley Road Passivhaus retrofit

Architect Patrick Osborne from Eco Design Consultants and Bow Tie Construction have just completed a radical retrofit of a mid-terrace Victorian home in Kensal Green to the Passivhaus standard. Client Peter Land wanted to undertake a low energy retrofit prior to selling the property, utilising the fact of rising London house market prices to help pay for the retrofit.

As an environmental scientist I am acutely aware of the need to reduce carbon emissions, so I feel I have a responsibility to cut the emissions from any property I have an influence over.

Patrick Osborne - Architect

21°’s ULTRA windows and doors were specified for the project and 21°also designed, supplied and commissioned the PAUL Focus 200 MVHR system at the project.

Technical challenges

As a retrofit, there were challenges in terms of thermal bridging, reduced solar gains and fixed form factor. To address these, the design team required very low U values to be achieved in the walls, roof and floor to meet the Passivhaus standard.

 U Values

  • Roof : 0.068 W/m2K
  • Walls: 0.077 W/m2K
  • Ground floor: 0.075 W/m2K

EWI and planning

Setting something of a precedent, the design team managed to overcome initial planning department concerns and specified 250mm of External Wall insulation on the front façade. The planners agreed to it as a ‘permitted development’ so long as the appearance was maintained. Applying EWI to the front of the building proved to be much easier and less expensive than the alternative options and also helped prevent loss of space within the property as well as minimising the risk of moisture build-up and interstitial condensation that can occur with IWI.

We put considerable effort into preparing the front exterior elevation for fitting of the EWI. The bay window, window position, gutters, roof and walls all required extensive preparation to allow the EWI boards to fit snugly in place as designed... uilding good relationships with clients is paramount for us. We are passionate about Passivhaus and will always work with clients to facilitate their needs and deliver their project. In this case Mr.Land advised us early that he could only fund around 2/3 of the building works prior to sale. Together we developed a strategy so that Bow Tie Construction could carry out the works without risk to anyone until the house could be valued and re-mortgaged in its completed state. We don’t think there are many builders who would agree to this and we hope this demonstrates to our clients that we are dedicated to their cause.

Rafael Delimata - Technical Director

Thermal bridging

As well as EWI there was also extensive detailing to reduce thermal bridging, such as the insulation of window reveals internally and the use of IWI on party walls in the loft, including 1 metre in on the party wall returns. The house had to have a new roof, achieving a U value of below 0.1 W/m2K. The design team minimised the risk of interstitial condensation by keeping the majority of the insulation external and outside the airtightness line.

More details of the technical detailing on the project appear in article in PH+ magazine ‘Sensitive passive retrofit transforms Victorian North London home’, Issue 17, September 2017, page 64-71.


Passivhaus certified triple glazed windows

21°’s Passivhaus certified outward opening ULTRA windows with insulated frames (Uw 0.68 kWh/m2) were specified for the project, alongside an ULTRA entrance door (insulated with VIPs panels) and fully (triple) glazed French doors.

The decision to select the ULTRA windows for the Hiley Road retrofit project was a combination of thermal performance, aesthetics and price. Although the project is not in a conservation area, the replacement windows were required to be approved as part of the planning application, and the ULTRA frames were received well by the planners. The outward opening function of the ULTRAs was also a consideration, as we were replacing like with like, and is a preference for some clients. The low U values of the frames, and good g values assisted with reducing the heat losses for the house, which was critical for this project and other retrofits, where the orientation and opening sizes are relatively fixed. We often recommend GBS ULTRA windows for retrofit or new build projects, due to their thermal properties and materials.

Patrick Osborne - Architect

There is no shortage of window suppliers in the UK however we mostly work with 21° due to reliability of delivery timings which are essential to meet strict deadlines and stay on budget. The design team respond quickly and are very knowledgeable about their products. We’ve also found their after-sales team very helpful and respond quickly when it comes to needing spare parts.

Rafael Delimata - Technical Director

MVHR mechanical ventilation with heat recovery system

21° designed, supplied and commissioned the PAUL Focus 200 unit for the project, which was installed by Bow Tie Construction.

We always recommend 21°’s MVHR design and supply of systems, as the design, supply and commissioning in a joined up process is important for making a system work as intended. Having the MVHR designed at an early stage was important to ensure the system could be coordinated with the structural design and other alterations we made to the house. Installing a robust system is imperative to ensure the house is well ventilated and energy efficient.

Patrick Osborne - Architect

21° are MVHR market leaders in the UK and we feel we can rely on them for a design that meets our client’s needs. We have worked with Alan Clarke on design of systems several times and he has seen our installations and been very satisfied with them. We were particularly pleased with this one, the design was very well thought through, easy to fit and very robust. This project was challenging as it was a retrofit of a Victorian house. Suitable locations for duct runs between beams had to be found. We have found the MVHR system to be quiet and effective. The duct grilles have blended seamlessly into the room design. Filters are easy to change.

Hagop Matossian from Bow Tie Construction

Project details
Life changing homes