Marianne Suhr: Laying limecrete slab foundations

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The project: Picketts Cottage, a Grade II listed, 16th-century house in need of sensitive modernisation
This month's challenge: To lay a limecrete floor to the new kitchen extension and add underfloor heating.

When I first met Alan Tierney, I knew he was a ‘lime’ fanatic, but I didn’t quite realise what a complete anti-cement obsessive he was. He was so opposed to using the ‘c’ word in his new kitchen extension, that he pushed the boundaries of hydraulic lime to ensure that even the foundations were cementfree. Instead, he decided to construct a slab of limecrete (lime concrete), to support his single storey timber-framed extension.

‘It’s great working on your own house,’ Alan told me, ‘because you can experiment in a way that you probably wouldn’t if you were working for a client.’ The limecrete slab was jointly evolved between Alan, his structural engineer and his architect. The primary concern was to avoid putting a rigid, impermeable concrete foundation next to his 400-year-old, timberframed house. He knew this could cause problems, and didn’t want to take any chances.

Clever thinking
After the ground had been prepared, Alan spread a loose insulation material of LECA (lightweight expanded clay aggregate) directly on the earth. This would allow any moisture to rise and fall within the layer – effectively providing a breathable, damp-proof membrane but, unlike a plastic sheet, it won’t push damp to the edges and up the wall of the old house.

The LECA layer also provided excellent insulation for the base of the new floor. Over the top of the LECA Alan spread a breathable separation membrane. He then constructed a 200mm slab of hydraulic lime mixed with pumice and LECA – limecrete – to form a breathable and highly insulated base to the new kitchen floor. This was thickened at the edges to 500mm where the loading of the external walls would bear down. Alan proved to the building control officer that it was completely reliable and the design was passed.

Once the limecrete slab had set, Alan laid a series of hot water pipes on top, held in place by plastic runners, to form the underfloor heating. These were protected by a final screed made of, you’ve guessed it, lime. But instead of mixing the lime with sand, Alan used crushed recycled glass, just to add to the eco credentials of the build.

Lessons learned
The only real problem they had was torrential rain, which slowed down the drying process. Alan admits that it would have been better to have installed the underfloor heating and screed after the building had gone up, but that the schedule was too strict to allow this. Once the extension was windtight and watertight, travertine tiles were laid, with lime joints to allow the floor to breathe.

The atmosphere in the finished kitchen is fantastic. Not only does the underfloor heating keep it toasty warm, Alan feels happy knowing that the whole structure is breathable and will do no harm to his beautiful old cottage.

Laying the kitchen foundations

1 The ground is prepared with a trench around the edge to thicken the sides of the raft, or area to be built on.
2 LECA loose insulation material is laid directly over the earth.

Laying the kitchen foundations

3 The breathable separation membrane is cut to fit.
4 Limecrete, a mix of hydraulic lime, pumice and LECA, is poured over the membrane to make a highly efficient insulating slab.

Laying the kitchen foundations

5 Once the underfloor heating pipes have been laid over the limecrete slab, they are protected with a layer of lime screed on top.
6 The travertine floor tiles are laid after the walls and roof of the extension are up.
(The finished kitchen can be seen in the main image at the top of the page).

Reno expertise

Layng underfloor heating to a suspended timber floor Laying underfloor heating to a suspended timber floor

If you are renewing a floor, then underfloor heating is an ideal solution for an old house. But think twice before lifting an original floor.

At Alan’s house, rotten suspended timber joists supported modern boards, which he decided to replace. He repaired the joists, stapled a breathable membrane between and filled them with Warmcel insulation (left). He then laid a 2mm ply board over the top and ran a wet underfloor heating circuit across it. He used leftover crushed glass from the limecrete to lay between the pipes to increase the thermal mass. Engineered oak boards were fitted on top.

Marianne's mini-guide to eco alternatives

LIME Limecrete can be substituted for concrete in most low-rise construction projects. Always use ‘hydraulic’ lime rather than lime putty, which comes in a range of compressive strengths: NHL2, NHL3.5, NHL5, with NHLZ the strongest lime available – it contains blast furnace slag (another waste product) to improve its crushing strength. Ask your lime supplier for advice on the appropriate type for the job in hand.

RECYCLED GLASS Consider using crushed recycled glass instead of sand within a lime mix as dredging for sand production can be damaging to the environment. Tŷ-Mawr makes ‘Glaster’, a lime plaster with crushed coloured glass, producing an exceptionally attractive sparkly finish if you want a modern take on lime plaster.

NEWSPAPER, HEMP AND SHEEP’S WOOL Warmcel (excelfibre.com) is a loose insulation product made from recycled newspaper. It has a fireproof coating, and is great for spraying into tricky spaces. For roof insulation, hemp (hemcore.co.uk) and sheep’s wool (secondnatureuk.com) are great alternatives to conventional glass quilts.