What’s stopping us from reducing the amount of carbon in basements?

We all know that in terms of embodied carbon the construction of new basements is pretty terrible, particularly when you’re trying to re-support an existing building. But maybe we shouldn’t resign ourselves to the fact they’re bad and move on, but instead work to try and reduce their impact as much as possible.

Basement construction has two main features that contribute to the carbon footprint; firstly, basements are very difficult to construct in anything other than concrete and secondly, carbon dioxide (and other gases) are stored in the soil and released into the atmosphere during excavation. If we’re going to try and reduce the carbon associated with a basement then we have to focus on these two aspects of the design.

Constructing a basement in anything other than concrete is both difficult and expensive, so let’s assume we’re going to use concrete even though we know the embodied carbon will be high. The easiest way to reduce carbon is to reduce the amount of structure and there are a few ways this could be achieved.

By involving the structural engineer earlier in the design, a layout could be developed that reduces the need for large transfer structures

Often with these types of projects, particularly homes, layouts for the basement have been produced before the structural engineer is involved. As a result, the place that would work best for a structural column is right in the middle of the dining room table. By involving the structural engineer earlier in the design, a layout could be developed that reduces the need for large transfer structures and, with a few strategically placed columns, reduces the amount of material required overall.

Additionally, details such as lightwells should be carefully considered or allow for some structure to cross them. Whilst these can hugely improve a basement space, they can also massively increase the structure required if they lead to an unpropped retaining wall.

When we consider the structure at ground floor level, this is often also constructed in concrete because it’s required to prop the retaining walls and because it’s easier. But constructing the ground floor structure in steel, where there’s the potential to reuse steel, or even timber, can still provide adequate propping to the retaining walls.

The second part of the problem is reducing the amount of excavation

Once all of the options for reducing the amount of concrete have been investigated then cement replacement products can also be considered to reduce the embodied carbon of the concrete. When specified carefully there can be significant reductions in terms of carbon but it’s important to ensure that the properties of the concrete are not compromised and that cement replacement products are not being transported from the other side of the world.

The second part of the problem is reducing the amount of excavation. Reducing the overall depth of the basement, even by a small amount, can significantly reduce the amount of excavation required. Similarly considering different methods of construction for the retaining walls and not extending right to the limits of the site can reduce both the amount of carbon and the amount of excavation.

Involving the contractor early on in the design process can also lead to savings in terms of carbon. There are often small changes to the design that can have a huge impact on the way the contractor needs to work and could easily be eliminated if they are considered early enough. Changing the design to allow for staggered construction or leaving existing structure in place can reduce the amount of temporary works required which can again reduce the overall amount of material required.

Reducing carbon needs to be a consideration on every project, not just the ones we know will be good

There are also a number of projects that have found creative ways to re-use excavated materials on site, for example by using clay-based soils to form air dried bricks which could be used for internal walls. Whilst this doesn’t have a massive impact on the carbon, it does reduce the amount of waste material being sent to landfill.

Of course, in terms of the environment it would be better not to build at all and maybe this option needs more consideration. But if you’re already going ahead with a new basement then it’s important to remember that you can still make large savings in terms of carbon on a project that you wouldn’t consider to be particularly sustainable. Reducing carbon needs to be a consideration on every project, not just the ones we know will be good.