Insulated slab foundation in Adjara for a light house

What an insulated slab foundation is

An insulated slab foundation, sometimes called a Swedish slab, is a shallow cast-in-place slab that rests on a continuous layer of rigid insulation. At the edges the insulation turns up in L-shaped elements, so the concrete is shielded from the ground on every side except the top. Underfloor heating pipes and the sewer runs are cast into the slab from the start, so after the pour you have a finished base with heating, on which the walls can go up straight away.

The system was developed for cold climates, where it matters not to lose heat into frozen ground. Winters in Batumi are mild and the heating saving is smaller, but the advantages remain: underfloor heating without a separate screed, a dry base and a short cycle on a level plot.

Insulated slab foundation in Adjara: where it fits

An insulated slab suits a light house of one or two storeys: timber-frame, SIP panel, light gauge steel or lightweight blocks. It needs a level plot with ground that carries the load without piles, and groundwater below the base. Then the slab on insulation lies shallow, and almost no excavation is needed.

Where we do not offer it:

• a slope, where the slab would need retaining walls and fill underneath;
• high groundwater by the sea, which would leave the insulation standing in water;
• a heavy masonry house in a cast-in-place concrete frame;
• ground where the survey calls for piles.

Seismic loads: why the engineer decides

The main question for an insulated slab in Georgia is earthquakes. The slab rests on insulation, not on the ground, and in an earthquake the horizontal force has to pass through that layer. For a light house an engineer can accept this by calculation; for a heavy one usually not. So we never take a standard Swedish system as it is: the engineer checks the slab against the seismic load for the plot under the Georgian code and, if needed, changes the insulation, the edge reinforcement or chooses an ordinary slab with insulation on top.

What we cast into the slab

Since the slab becomes the floor straight away, everything that goes into the floor is decided before the pour. Underfloor heating pipes are laid in circuits with a manifold, pressure tested and photographed under pressure. The sewer is run to the bathrooms and kitchen to the approved layout, and sleeves for water and cable go in at the entries. Moving a bathroom after an insulated slab is poured is almost impossible without opening the slab, so the room layout is approved before the insulation is laid.

The heat source for the underfloor heating is chosen in advance: a heat pump suits a low-temperature slab well. There is more on this on the page about energy-efficient houses.

Insulation and materials

We have hardly seen complete insulated slab systems with branded elements in Georgia, so we assemble the slab from available materials: extruded polystyrene of the right compressive strength under the whole slab, edge elements of the same material, cross-linked polyethylene pipes, reinforcement and concrete to the calculation. The insulation under the slab must carry the load for years without crushing, so we check its grade against the certificate, not the price.

How much an insulated slab foundation costs

An insulated slab foundation costs from 470 ₾ per m² of house footprint: bed, insulation, underfloor heating and sewer pipes, reinforcement and concrete. For comparison, an ordinary slab without insulation and pipes is from 390 ₾ per m². Plinth insulation from 75 ₾ per m², drainage from 87 ₾ per linear metre, the paved apron from 93 ₾ per m² and the ground survey from 3,000 ₾ are extra, as is the boiler or heat pump. The final cost is set after the engineer's calculation, a site visit and agreement on the estimate.