Guest blog: Tips for a Radical Retrofit
Paul Testa is the Founder & Director of HEM Architects, an architecture practice based in Sheffield. After carrying out an extensive retrofit of his own home, he discusses the benefits of retrofitting and the key things to consider before you start.
Why Retrofit?
There are many reasons why people decide to retrofit. You might love your existing home — the location, your neighbours, etc., but perhaps it falls short in terms of space, comfort and performance. Retrofitting your homes means tackling these issues to deliver a home that achieves your space requirements and provides the low energy living and comfort you desire, without the pain of moving.
On the other hand, perhaps you dream of building your own home but have had difficulty finding a plot. Instead of looking for an empty plot; why not look for an existing building, in your desired location, with the opportunity to remodel and retrofit to deliver your dream home with amazing performance?
As an architectural practice, we’re passionate about taking an existing structure and extending its life. It’s not the cheapest option, but it certainly has a lower carbon cost and uses up fewer construction materials than new-build.
What are the challenges?
There are many positives to retrofit including prolonging the life of existing building stock and reducing carbon usage. However, it’s important to acknowledge the challenges it presents.
The big disadvantage is that new-builds are exempt from VAT whereas, retrofit is rated for full VAT.
The other, if you’re already living there, is disruption. The amount of disruption will depend on whether you live in the house or vacate for the duration of the build, and therefore basing your thermal and air-tightness strategy around this.
Things to consider as part of your thermal & air-tightness strategy
- Are you going to have external or internal wall insulation?
- Are you going to put the air-tightness line on the inside or outside of the existing structure?
- Are you going to do the retrofit room by room or the whole house?
It’s generally accepted that retrofit is more technically demanding than new build, and I’ve certainly found this to be the case during my project. Air-tightness is more tricky to achieve; existing buildings always throw up surprises and lots of design options are already discounted by the existing building.
Why EnerPHit?
Unlike with new Passivhaus homes, when you’re considering a retrofit many of the elements like geometry, orientation and structural approach are already decided. You may also have thermal bridges that are difficult to eliminate.
The EnerPHit standard recognises this difficulty and sets the required performance at a lower level to accommodate working with existing buildings. To achieve Enerphit you must achieve a space heating and cooling demand of 25 kWh/m2/year and instead of an air-tightness performance 0.6 air-changes per hour you need to achieve 1.0. We’re talking about the same Passivhaus comfort but utilising slightly more energy. It is still a huge improvement on most existing homes and even new homes currently being built.
For further information about EnerPHit read Paul’s guide: https://www.hemarchitects.co.uk/2019/06/20/what-is-enerphit/
What’s the best insulation strategy?
With any retrofit, you’ll be faced with deciding where to place your insulation. Essentially, inside or outside the existing structure.
Which is best?
They both have advantages and disadvantages. On balance, external insulation (EWI) is more likely to give a better technical solution in most cases, but it’s not always appropriate. Sometimes the building or context decides for you. If you’re in a conservation area or dealing with a beautiful stone cottage, you’re unlikely to want or be able to change the external appearance of the home. Conversely, if you live in a detached, ugly house then EWI and a fresh new look might be exactly what the project needs.
The big risk with most retrofit projects is moisture. If the design and behaviour of moisture in the building fabric aren’t carefully considered, you could end up with unintended consequences; condensation and damp where you don’t want it and where it, perhaps, didn’t exist before.
Will I need to move out?
If you are undertaking a deep retrofit then the answer to this is likely to be yes. My family moved into rented accommodation nearby, and even though it was another cost to factor in, I’m glad we did it. From a build perspective, it makes sense to give the contractor full control over the site to sequence the works and therefore makes the works as cost-effective as possible. When you balance the disruption to both you and the contractor of you living on-site; and the additional costs of prolonging the works to sequence them around your occupation of parts of the house, any perceived benefits of staying soon disappear.
How much will it cost?
Let’s be clear, deep retrofit is not a cheap thing to undertake. We would expect around £1000/m2 to take a building as far as Enerphit. This is plus VAT so is £1200/m2. Compare that to a baseline of £1550/m2 with no VAT for new build and it’s not the most attractive option cost-wise. Whilst it does depend on where you live, it’s unlikely your house will be worth what you’ve spent on it. The payback in bills is not short term either. Cost is not a reason to undertake this kind of work.
My house, post-retrofit and extension, is worth about what we spent on doing the works. That doesn’t include the original purchase price of the house. Thinking about it in those terms doesn’t work. But, we now have a beautifully comfortable home, with excellent air quality and low running costs, that is tailored to the way we live. And we’re in a location we want to live in. As long as we aren’t forced to move, the property value for the next 20 years or more isn’t an issue.
Which contract?
For deep retrofit work, we try and recommend the “cost-plus” or “open book” approach to pay for the work. This is where the client pays for all materials, site time and preliminaries (skips, scaffold, site welfare etc) along with a proportion of contractor’s overheads and a percentage profit. When there is so much unknown with retrofit work, you don’t want to pay extra for the contractor to price in lots of risks but also don’t want to penalise him for doing the good job that will be required to meet the performance target.
If you get a good sense check cost plan produced at the beginning of the project, keeping track of progress is fairly straight forward. You see some items coming in more expensive than cost plan, but conversely, other elements end up cheaper. It’s a very collaborative approach and everyone involved is more invested in the process and the result.
Read Paul’s build diary from the very beginning: https://www.homebuilding.co.uk/choosing-the-right-house-to-eco-retrofit/
Find out more about HEM Architects: https://www.hemarchitects.co.uk/
