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EnerPHit Retrofit Project Relies on EWIS

EnerPHit retrofitted terraced house, Balham

Prewett Bizley architects received a 'Highly Commended' in the AJ Retrofit Awards 2012 for the refurbishment of a terraced house in Balham.

Project architect Bob Prewett approached this pilot project with the objective of meeting the extremely demanding Passivhaus standard for refurbishment, EnerPHit. With a price tag of over £90,000, does this set a realistic model for replication? Bob Prewett reflects:

"The cost of this pilot would obviously come down significantly if it were repeated in volume. R&D, as well as management and coordination costs, were very high, as this was an experimental project that did a lot of things that would be seen as unusual. There was also a premium to be paid for mobilising several specialist teams for relatively small bits of work.

Because we set out with the objective of meeting the EnerPHit standard, the house is highly engineered. Is it worth the effort and expense to work to that level? My heart says it's a worthwhile standard and it is just about achievable in many cases. If energy prices continue to rise as they are, then the long term investment will, I think, be shown to be attractive. But I accept for the time being that for many homeowners it may be seen as an aspiration that they can't quite reach just yet. I think over time as the industry improves delivery and economics of retrofitting, I hope that will change.

Being pragmatic and thinking about the scale of the retrofit challenge, there are obvious easy economic wins that could be upscaled widely and quickly.

This particular house is part of a classic English terrace: there are hundreds of thousands of terraces like this throughout the UK, so taking just some of the effective measures from the pilot and rolling them out would bring significant results.

In the Balham house, for example, we made extensive use of external wall insulation (EWI). This is a very effective and practical way to bring solid brick walls up to modern levels of thermal efficiency. The Passivhaus Institute recognised this and the early drafts of the EnerPHit standard only allowed buildings to be certified if they used EWI.

It would make sense to treat, at once, a whole row of terraced houses together. The end result would be a unified look and the energy savings would be huge. This process is not problem-free and I have seen some very poor attempts (under CESP) where the detailing, particularly at the eaves and around the windows, has not been properly thought through.

These problems can easily be rectified with good initial design and effective project management and the new ECO funding, with its emphasis on improving solid wall insulation, may well provide a stimulus for housing associations to increase the volume of this type of work.

Our own project in Balham also highlighted some of the issues relating to UK building design. The often shallow eaves of the existing roofs can be a challenge for EWI solutions. We designed a standard product to deal with this cost effectively and making the installer's job easier.

By the way, we were unable to use external wall insulation on the front elevation for aesthetic reasons. Using internal insulation was an effective alternative, but it did involve a fairly significant reduction in the already limited footprint of the rooms.

Recognising these difficulties, the EnerPHit standard that was finally published earlier this year does now allow properties to use internal as well as external wall insulation."