How to build sustainable home extensions
Photograph of the reclaimed Suffolk White brick facade
With high energy prices and a greater emphasis from the media on the climate crisis we’re finding our clients are looking more and more for how they can build their home extensions more sustainably. Here we take a look at how to build sustainable home extensions with a case study of a recently completed project. Downton Avenue located in London is an extension and renovation of a large Edwardian home.
Traditionally such an extension would be built with a concrete slab to cover the area o the ground floor extension and concrete foundations. The walls would usually be constructed from blockwork (concrete blocks) and facing brick. The usual approach to insulation would be to use rigid foam insulation. Steelwork is also often used to create larger openings, open plan spaces and big spans.
A step by step guide to reducing carbon emissions and lowering heating bills.
1. Efficient design of spaces
Firstly we consider the size of the extension - the bigger the extension the larger the carbon footprint is likely to be. Can we make clever use of space to make the most out of less? Often we find that an extension if designed correctly can unlock unused or under utilised existing space in the house it’s attached to.
2. Reduce the embodied carbon in materials
Concrete and brick, although very durable and long-lasting, are high in embodied carbon (229kg CO2 eq/m3 and 565kg CO2 eq/m3 respectively). Even materials like foam insulation can have a large carbon footprint as the quantity can be fairly large (123.3kg CO2 eq/m3).
So the next thing we consider when designing is how can we reduce these more intensive items. Here, we’ve broken it down into the key building elements:
Low Embodied Carbon Wall Construction
In this project we’re using timber panels (known as SIPs) for the wall construction with recycled wood fibre insulation between – both of these materials (and all timbers) store carbon and are therefore either low or carbon positive, provided that the timber is sustainably sourced.
Low Embodied Carbon Floor Construction
We haven’t yet figured out a way to avoid using concrete in foundations (although we’re working on it and have a project outside of the UK using stone foundations). However, we have found ways to reduce the amount of concrete used. At Downton Avenue we have used a timber floor which sits above a ground cavity with some blinding beneath. This has saved about 2.7m3 of concrete in total or 618kg CO2 eq.
Low Embodied Carbon Facade
A building’s façade is often the highest source of its embodied carbon, at Downton we’ve wrapped the timber frame in textured reclaimed brick. Using reclaimed bricks can reduce the carbon significantly (to circa. 4.9kg CO2 eq/m3) because the carbon in the bricks has already been spent – when a material is re-used we’re also saving it from landfill. It takes more time and effort to source reclaimed bricks and they can be more expensive – but we find that it’s often a price our clients feel is worth paying. Finding the right brick can make all the difference and knowing the inherent history of a material can also have a positive effect on how it feels.
3. Improving Energy Performance
A building’s energy performance uses about ⅓ of its total whole life carbon emissions, so is a vital part of the puzzle to get right. However, when dealing with existing homes the biggest wins can often be found in the existing building rather than the new extension.
At Downton Avenue, as well as ensuring that the new extension was very highly performing: with better insulation, higher performing glazing, we are also preventing energy waste and improving efficiency with an underfloor heating system and a more air tight frame. We’ve added a green roof into the design which attracts insects and helps to mediate rainwater runoff.
To address the existing home, we installed insulation below the floorboards in the ground floor (which was particularly leaky), and would also usually insulate the existing roof - but in this case we create a large roof extension too which did the job for us.
4. Greater than the sum of the parts - environmental benefits plus
It’s not just the environment that benefits – there are other plus points too: the timber frame construction is faster than blockwork and involves fewer wet trades on site; the façade has a visual richness that would be hard to achieve with new bricks; and there’s a simple joy in knowing that we’ve managed to make more with less.
Learn more about the Downton Avenue project here
