Thermally efficient foundations

[Nick Grant]

Following on from Andy's definitive timber frame thread I am interested in solutions to thermally efficient ground floor and foundation details. This often seems to be a weak point WRT cold bridging.

For our own timber frame house we used a flat reinforced concrete slab floating on 100mm HD polystyrene with 200mm of edge insulation. The ffl is about 300mm above ground level.

Site prep' included removing topsoil and digging a trench all around to get below frost and into solid subsoil and provide a drain. This was filled with 70mm stone and the whole area leveled with scalpings and crusher dust to for a flat stable base for the HD poly.

Timber frame is 4×2 studs in from slab edge with structural ridge supported by posts. All clad in 9mm ply as VB and racking. Insulation is then on outside and continues from the slab edge up and over roof with minimal bridging, rather like the block timber hybrids.

We powerfloated the slab (no screed) and then polished with a floor grinder (£40/day +£12 for blocks). We are very happy with the result, sealed with a clear stone wax finish. Wish I had put in a single coil of underfloor heating pipe whilst we were at it as that would have neen a useful place to dump heat.

I know concrete is seen as bad by some but I think in this construction it works hard for its living – also very cheap.

[David Olivier]

I'm also puzzled why concrete is so widely seen as “bad” in the UK. In the USA it's widely seen by eco-house enthusiasts as a longer-lived alternative to timber structures and has much lower embodied energy than clay brick (only 10-15% of concrete – the cement – is fired, the rest is made up of local stone and sand).

Before my house was built we experimented with replacing part of the cement in the concrete by limestone dust. This research is unfinished and couldn't be incorporated into that project but in my view the boundary between concrete and materials such as rammed earth or rammed chalk is a very grey area, not black and white.

[Anonymous]

Nick,

I was interested to hear that you had constructed a floating floor over polystyrene. I am thinking of using 200 mm of extruded (or epanded) polystyrene bewteen the slab and screed. Do you know how much static weight of screed can be supported by extruded polystyrene? Also I wonder if anyone knows if a screed of 65 mm thicknes and standard mix would be strong enough without any steel reinforcement? (given that the screed weight would be evenly distributed and the polystyrene would hopefully settle to a fixed compression).

[Nick Grant]

MMM Limecrete!!! Now there's a thread.

Julian, I avoided a screed. Single pour slab on insulation. Insulation was specified to take the load but all the figures such as % strain are out of my head now.

Even 200mm will get fine cracks but I think these add character, think Tate Modern Turbine Hall!

I have been told that slab+screed is a UK thing. In Australia it is usual (I am told) to por a slab and power float it rather than leaving it tamped as usually happens here.

[David Olivier]

Replying to Julian … sorry for delay but worth noting that a similar detail was used at Lower Watts House in Oxon (built 1992). The plot had been previously owned by a developer who went bust after laying beam & block ground floors, so a power-floated slab wasn't an option. On top of the beam & block went 150 mm of HD expanded polystyrene, then a reinforced 75 mm screed.

The loading imposed by a house (in N/mm2) is not that great, especially if it's spread over a large area. Many rafts (either at basement or ground level) have been supported on either expanded or extruded polystyrene in the last 15 years. If one's only supporting a floor slab or screed – not a building – rigid insulation materials such as high-density rock fibre can be used – a lower compressive strength than polystyrene but still sufficient.

HTH

David.

[Eco Design]

Just an update on the limecrete slab metioned a few months ago.

We ended up laying the base as previously mentioned and a 150mm thick slab (no screed) and then clay tiles as a finished surface.

No heating has been put into the building so UFH was removed.

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