Bob Prewett, a director of Prewett Bizley Architects, shares his approach to low energy retrofits and explains why he sets energy targets on a project by project basis.
Interview with Bob Prewett
Prewett Bizley Architects specialises in low energy architecture and Bob has a particular interest in retrofit. He explains that housing is responsible for around 27% of the UK's carbon emissions, so buildings of all ages and styles have a part to play in terms of energy efficiency.
There is a Distinction Between Retrofit and Renovation
Bob's partner, Marion Baeli, defines renovation as being smartening things up to make them look good and dealing with the top 3mm of construction. Retrofit on the other hand is about getting under the skin of things and sorting them out at a fundamental, architectural and building physics level.
What to Consider when Commencing a Retrofit Project:
- Get advice at an early stage
- Create a whole house plan which identifies where the energy savings might be, along with the risks associated with tackling them, and how these might be actioned
- Form a clear strategy of how these will be dealt with, either all at once or as stages over an extended period
- Be aware that there is no ‘one size fits all' when it comes to retrofit.
“It's not for the faint hearted, either in spirit or in finance”
“Understanding What the Quality of the Building is and Then Designing and Planning Very Carefully to Not Upset That Balance”
Bob is concerned by evidence of the careless nature of works being carried out to some buildings which, with moisture being allowed in and then trapped, can actually be causing damage. He looks at the example set by Germany on the other hand which takes a more considered approach, doing gradual, quality improvements over time, instead of trying to do multiple houses quickly and cheaply.
“Germany still have a confidence about the future and progress and a belief that just doing things well now is an investment for tomorrow. “
“We're Interested in Taking the Building to Where it's Going to Sit Happily”
Rather than starting off with a target value in mind, Bob believes that it is important to understand the building as a whole and take it to where it's going to sit comfortably, and not push it too hard. Some buildings will get to a U-value of better than 0.15 or 15kWh per square metre relatively easily, but for others, such as historic solid wall construction, they are probably not going to want to go there. Bob explains that they are interested in taking the building to where it sits happily, even if those values are a little higher. He would expect most buildings that don't have insulation or good quality windows to easily achieve a 60% energy reduction, and perhaps even going up to 90% in some cases.
Significant energy savings will be achieved by using a holistic approach to deal with insulation, airtightness, cold bridges, windows, walls, roofs and floors. It is important to understand the building and how these elements interact and work together.
“Some buildings will be predisposed to very high energy savings, some will be predisposed to doing a bit less.”
Be Strategic About What You Purchase if You're Actively Looking to Retrofit
Bob has purchased a 1960s small terrace house which should leave them with enough money left over to do a good retrofit. He deliberately decided against listed properties and those with planning restrictions as he felt it would be a higher risk approach with energy savings being more difficult to achieve.
They intend to take advantage while the roof on this property is being done to pass brushes between the cavity walls and give them a good clear out before adding cavity wall insulation.
Carefully Consider Your Insulation Options
External Insulation – This can seem like the most simple option. There is potentially very little disruption and you are likely to be able to continue living there while the work is being carried out. You're cloaking the building, keeping it warm and dry and therefore less likely to suffer moisture problems. The biggest drawback is that it completely transforms the building architecturally, which may not be to yours or the planners liking. In terraced houses you can end up with cold bridges left over through party walls.
Internal Wall Insulation – This has the benefit that it does not have an impact externally so there aren't the same planning issues as above. There will be far greater disruption inside however and possibly not be in a livable state. The process can be quite slow and be impacted by various pipes, electrics and other services. There are also cold bridge issues when getting to things like window junctions.
Cavity Wall Insulation – If you have cavity walls then filling them can be a good option. Bob however advises caution and not always accepting the claims of the installer that it is suitable for the wall, without following up with further questioning. There may be some situations, such as bad pointing or a coastal property with porous bricks, where it might not be an appropriate choice.
The 80% House was a Defining Project
This was one of Prewett Bizley Architects' first deep retrofit projects. The client wanted excellence and was prepared to pay to achieve it. During the closing stages of the project, in an extremely hostile winter and before the heating system had been commissioned, Bob would visit the house which maintained a temperature of around 18 degrees. Even though the systems weren't all running, he was struck by the quality of the environment. Even leaving aside the benefits of energy efficiency, it was the feeling of the house that made a huge and defining impact on him and was the reason why they continue doing these kinds of projects.
“If you do it really well you just get this amazing quality of environment as a backdrop to our life.”
Find Out More
International Passive House Days
As Bob Prewett explains, it's really about the feel of the house and the quality of the environment, which is why the International Passive House Days are the ideal opportunity to experience it for yourself. Each year Passive House residents fling open their doors and invite people in to have a look around and ask questions. This year they will be taking place around the world on 13-15 November 2015. Keep an eye on the International Passive House Association website to find out which buildings near you will be open to view.