Choosing EPS insulation for a house, from the foundations to the roof

On plans and sections, it is worth marking all insulation layers. Joining them at the plinth, windows and eaves is as important as the thickness at the middle of the wall. With such a map, it is easier to divide the order into the appropriate EPS grades. There is no need to look for a single board that will perform without any reservations in every location.

The declared lambda describes heat conduction through the material. A lower value allows the same resistance to be achieved with a smaller thickness. The resistance of a layer is calculated using the formula R = d/λ, where the thickness d is given in metres. The U-value, and after taking account of the required corrections the Uc-value, relates to the entire building element. It is not a property of the pack of polystyrene itself.

Depending on the application, TR, CS(10), water absorption and dimensional stability are also required. Density may be supporting information, but it does not replace these data. The number 100 in the designation EPS 100 does not mean a density of 100 kg/m³. Always compare the description in the offer with the declaration of performance for the product.

In the design, it is also worth marking special locations such as a heavy kitchen island, the threshold of a terrace door or the support for roof equipment. These may require a local solution different from the rest of the building element.

In a rendered façade, the polystyrene is part of the ETICS system. The boards work together with the mortar or adhesive, the fixings specified in the design, the reinforcing layer, the primer and the render. Their selection should fall within the system documentation. A random combination of components makes it difficult to assess adhesion, durability and the behaviour of the façade during a fire.

The important value is TR, i.e. tensile strength perpendicular to the surface. The geometry of the boards and their condition on delivery should also be checked. Graphite EPS requires protection against excessive heating during work. A better lambda alone will not remedy a poorly prepared substrate, open joints or errors around windows.

A foundation wall works under different conditions from a dry façade. The material may come into contact with moisture, and after backfilling it is loaded by the ground. A grade with documented suitability for such a location is needed. Water absorption WL(T) describes the result of a specific test. It does not mean complete resistance to any quantity of water and any pressure.

Thermal insulation does not replace the waterproofing of the wall. An adhesive compatible with the EPS and the sealing layer must also be selected. A board approved for damp ground is not necessarily suitable beneath a structural foundation slab. There, loads, long-term deformation and water conditions are assessed separately.

Beneath the screed, the polystyrene carries loads through a layer that distributes the pressure. CS(10) specifies the stress at a deformation of 10%, not the permissible service pressure. Creep, board support and point loads matter. Acoustic EPS additionally serves to reduce footfall noise. It is not selected by hardness alone.

On roofs there are roof boards, slope wedges and boards with a bitumen felt facing. Selection covers pressure, wind suction, moisture, temperature and the classification of the entire roof covering. Ordinary EPS should not automatically be transferred to an inverted roof. Such an application requires a product and build-up specifically intended for it.

For each part of the house, note the board type, thickness, thermal conductivity (lambda) and the required mechanical or moisture-related properties. Add the area in square metres and specify the edge profile. The nominal dimensions of a board with profiled edges may differ from its effective coverage. Use the relevant manufacturer’s table to work out how many packs to order.

Well-known polystyrene manufacturers include Termo Organika, Austrotherm, Swisspor and Yetico. Each of these companies offers various board grades, so any comparison should concern the properties of a specific product.

On delivery, keep the labels and batch documents. Inspect the corners, flatness and any signs of moisture on the packaging. Only after this check should the material be divided between façade, foundations, floors and roof. This is a simple way to ensure that a good board does not end up in the wrong place.