A guide to earth-shelter homes
How to build a house into, or even under, the ground
Earth-sheltered homes are built into, or even under the ground to help regulate their temperature. Cooler in summer and warmer in winter, they are highly energy efficient.
The concept of houses using the ground as part of their construction came to the attention of the eco-building scene thanks to the Earthship. Originally developed in the 1980s in Mexico, Earthship homes were built with reclaimed materials such as bottles and tyres. They faced towards the sun and were backed by a big earth mound to even out the building’s heat gains and losses.
In the UK the idea was taken up by pioneers such as the Hockerton Housing Project in Nottinghamshire, which was completed in 1998. With their thick insulation and modern ventilation systems, these homes brought the Earthship home concept up to date.
The idea of using earth-sheltering to create energy-efficient homes has been somewhat left behind by advances in construction. However, there are still reasons why a house might be dug into, or sit under, the ground. Sometimes the site leaves no other option – either because it is too steep to build on it in any other way, or because the planners want the property to be inconspicuous.
Some self-builds have turned this necessity into a virtue, with many benefitting from sloping sites that look out over fantastic views. Successful semi-underground homes have also been built to Passivhaus standard, so there should be no need to compromise on energy performance.
If you are pondering on whether to buy a sloping plot and build an earth-sheltered home, the first question should be: is this the best way to go? For instance, if you want to use natural, breathable construction materials, retaining the ground away from the house may make the build more straightforward. Such materials need to be separated carefully from the damp earth – they are much better left open to the air.
On some sloping earth-shelter home sites it’s possible to level more of the ground. Instead of supporting the slope against the back of the house, the ground could be retained further back using terraced planting, gabions or a courtyard wall and creating a sheltered patio.
It’s worth investigating your options before making a final decision – preferably at the pre-planning stage. An initial half-day or day of preliminary advice from a structural engineer before you make an offer on the site could prove to be a great investment.
In any case, a thorough site survey before work begins is essential. A delay because the site collapses, or turns out to have an unexpected pipe, cable or watercourse, could do a lot of damage to your budget.
Get expert help
If digging into the ground turns out to be your best way forward, you will need the advice of a structural engineer as well as an architect. The structure of the earth sheltered home must be strong, well-insulated and waterproof. You need to have absolute confidence in the strength of the retaining structure, but an insulation or waterproofing failure underground will be very bad news, and may be almost impossible to remedy.
The design of Earthship homes will depend entirely on the nature of each site. A shallow slope is likely to require less substantial ground retention than steeper or more unstable sites. Only an expert structural engineer will be able to advise on this.
The structural work involved in building into or beneath the ground is more expensive than a standard construction, and may involve lots of concrete and steel.
Keeping Earthship homes dry
Reliable waterproofing is the first line of defence against water ingress. But it is also often recommended to include drainage, such as a French drain, to help remove groundwater that could otherwise remain trapped behind the house. This type of drain is perforated and set in a rubble-filled trench, which allows water to trickle away, but also allows air in to help remove moisture by evaporation.
As there may be limited through-ventilation due to the absence of windows on all sides, a well-designed, whole-house solution such as a mechanical ventilation with heat recovery (MVHR) system is a must.
There should be good provision for purge ventilation upwards too. This flushes several changes of outside air rapidly though the house to remove excess heat that might have built up during the day, or to clear any cooking smells.
5 factors to consider
Advice from Beth Williams, chartered civil engineer and certified Passivhaus designer with Build Collective, on building Earthship homes.
- Get a geotechnical investigation: When undertaking a below-ground building project, always obtain a geotechnical investigation. Budget for around £2,500 to £4,500, depending on the complexity of the site. Spending money on this at an early stage could save a lot more in the long term. But if you are worried about cost, start with a desktop study by a geotechnical specialist.
- How close are your neighbours? Consider any structure that will be within 6m of your new-build, subject to how deep the construction is, and the lie of the surrounding land. The Party Wall Act could apply to your neighbour’s neighbour, the people behind – or even across the road – as well as potentially relating to drainage and services in the area.
- Have you read the small print? This mantra applies to all building products, but it is particularly relevant when it comes to waterproofing systems – 80 per cent of basement failures come down to waterproofing issues. A product’s approval is specific to the use and installation conditions. Deviation from these will invalidate any warranties. If you are unsure whether a product is appropriate, the British Board of Agrément (BBA) certificate (bbacerts.co.uk) will tell you its intended use situations.
- Who’s in charge? Below-ground construction requires specialist design and oversight. This type of work is high risk. A temporary works engineer is needed – they are responsible for planning the process by which the construction is done, and for site safety measures. Check who is appointed to this essential role.
- Avoiding the highway/railway: If your excavation might impact on a road or railway, you will need local authority highways or Network Rail approval – and these aren’t cheap. If you can, avoid a construction where you need to retain any infrastructure like this, as costs will run into the thousands.