Which EPS insulation suits the foundations of a house without a basement?

The design should show how the façade insulation transitions into the plinth and how it connects with the floor insulation. In a new house it is easier to plan this detail before the layers are installed. In a renovation, the existing solution must be identified. Simply increasing the thickness of the vertical EPS does not always remove a gap hidden beneath the floor slab.

The material at the foundation is exposed to moisture and ground pressure. Rainwater, periodic dampness and permanent hydrostatic pressure are different conditions. A board approved for one of them does not automatically become suitable for the others. Data on the ground, drainage and water level are therefore needed, not just the catalogue name ‘foundation’.

In the EPS documentation, check the water absorption, mechanical properties and restrictions on use. Some manufacturers specify a maximum depth or exclude water pressure on the boards. These conditions must not be overlooked when comparing offers. Colour does not determine water resistance; there are both white and graphite grades with reduced water absorption.

Lambda allows the thermal protection to be assessed for a given thickness. WL(T) describes water absorption during a long-term total immersion test. A value of 2% or 3.5% does not mean that the wall will be waterproof. It is information about the behaviour of the material itself in a specific test, needed for its selection.

CS(10) specifies the stress at 10% deformation. It is not a ready-made service load-bearing capacity. In vertical insulation, the effect of the ground is assessed, and beneath a structural foundation slab also long-term deformation under the weight of the building. These two locations require separate assessment. The designation EPS 100 does not entitle any board to be used interchangeably.

Thermal insulation reduces heat flow, while waterproofing protects the building element from water. Both layers may be needed. Their position and connections are specified by the design. Boards with low water absorption will not repair a broken horizontal insulation or a leaking service penetration. Before backfilling, these places should remain accessible for inspection.

The adhesive must be suitable for both the EPS and the waterproof coating. Some solvents damage polystyrene. The required drying time of the waterproofing must also be taken into account. It should not be penetrated by fasteners merely because that fixing method is used higher up on the façade. The underground zone has different rules.

There is no single thickness suitable for all foundations. The building layout, floor level, thermal conditions and connection with the remaining insulation are decisive. Even a well-insulating board can leave a thermal bridge if it ends in the wrong place. The depth to which the insulation is carried should not be determined solely on the basis of popular information about ground freezing.

Before backfilling, the EPS and waterproofing must be protected from damage. The type of protection depends on the system, the ground and the function of any drainage layer. Dimpled sheeting is not an automatic substitute for waterproofing. Nor does every system require the same orientation of its dimples. The specific solution should be followed.

The order should state the application, thickness, lambda, water and mechanical requirements. Describe separately the material for the foundation wall, the plinth and, if applicable, beneath the structural slab. A common item ‘hard polystyrene’ is not sufficient. Before delivery, check the format and coverage area, especially when the boards have rebates.

Polystyrene intended for selected foundation applications is offered by, among others, Yetico, Swisspor, Styropmin and Termo Organika. The range of use of individual grades differs and requires verification.

Carry out the acceptance before backfilling. Inspect the continuity of the boards, the connection to the plinth, the condition of the waterproofing and the pipe penetrations. It is worth keeping photographs with a tape measure and the material labels. Once the ground has been placed, repairing a minor oversight may mean digging the wall out again, so this stage is of great importance.