Sunday 25 October 2020  

Thermal insulation of foundation slab in practice
Temeljna plošča omogoča izvedbo odličnega energijskega ščita pod objektom. Toplotno izolacijo kontinuirano namestimo pod temeljno ploščo, s čimer se izognemo toplotnim mostovom in toplotnim izgubam, hkrati pa dobimo masiven element, ki služi akumulaciji notranje toplote.
V nadaljevanju na kratko predstavljamo postopek gradnje temeljne plošče na toplotni izolaciji FIBRANxps 400-L.

The process of construction of energy shield in the ground:


  • Installing services below the foundation slab

Services below grade are installed below the level of the foundation slab. If the construction is built with earthquake protection, laying the services should also be carried out according to the earthquake protection regulations.

  • Preparation of the ground

After having laid the service channels (sewage, water, gas, electricity…) the ground surface is leveled out and strengthened with evenly compacted gravel as intermediary layer. The requirements of a geotechnical expert should be taken into account. To level out the surface even more precisely, fine sand or dry concrete is used. After the base has been prepared in this way, the first layer of thermal insulation is laid over it.

  • Thermal insulation

According to the calculated loading of the building, the type of insulation is selected by the statics expert. In wooden or other type one-family dwellings, the thermal insulation boards, e.g. FIBRANxps 400-L, are laid with edges butted tightly together, thus preventing the formation of un-necessary thermal bridges.

The first layer of insulation should extend over the total surface area of the foundation, thus providing thermal protection of the soil below the foundation slab. Why? If the building has no basement, the foundation slab usually is not deep enough to reach the level of freezing, therefore the horizontal insulation surface is made larger, causing a longer cold path, with the same effect as if the foundation was deeper.

Unfortunately, thermal bridges are formed on areas with vertical channels, as service pipes have to run through insulation. In case of a building with earthquake protection, the services must be executed according to the principles of earthquake protection construction (flexible pipes, channels),so that in case of a higher shear strength the installation ducts are not damaged.

Thermal insulation boards FIBRANxps 400-L serve as thermal insulation below foundation slabs of residential buildings (e.g. cellar+ground floor+1st floor). 2.5m long boards enable quick installation and produce smaller number of junctions; due to the shiplap (L) edges the boards are suitable for installation in a single layer. For low-energy houses the recommended thickness of thermal insulation is at least 12cm, but for a high-quality low-energy house and a passive house the recommended total thickness is approx. 24 cm, applied in two layers. Whenever the double-layer thermal insulation is selected, we use the given advantage of protecting the waterproofing membrane with the second layer of insulation.

  • Waterproofing layer

The waterproofing layer (self-adhesive bitumen waterproofing, polyolefin foil, EPDM membrane, or other) is laid over the first layer of thermal insulation, and then protected with the second layer of thermal insulation FIBRANxps 400-L. In the same way the good-quality earthquake protection insulation of lower buildings is carried out.


It is strictly forbidden to apply waterproofing membrane to polystyrene thermal insulation, such as FIBRANxps, with naked flame, and therefore in case of using bitumen waterproofing, self-adhesive waterproofing has to be chosen. The chlorides present in PVC membranes also have a damaging effect on polystyrene, therefore when applying PVC waterproofing membranes, an additional separating layer needs to be applied.

  • Formwork of foundation slab

Before installing the second layer of thermal insulation, a formwork around the foundation slab is made, and the second layer of FIBRANxps 400-L insulation is placed inside it, thus protecting the waterproofing from damaging.

The second layer of thermal insulation FIBRANxps 400-L, placed inside the formwork of the foundation slab, serves as a protection for the waterproofing membrane. The first layer of thermal insulation is larger than the surface area of the foundation slab.

  • Sealing membrane

Before pouring the concrete on steel reinforcement, it is recommended to apply a sealing membrane, which prevents leaking of cement milk through the joints between thermal insulation boards.

  • Reinforced concrete foundation slab

The foundation slab is executed as steel reinforced concrete slab. The type, quantity and positioning of the reinforcement is determined by a statics expert, as well as the thickness of the slab and the quality (brand) of concrete.

  • External wall

After having built the walls, the wall self-adhesive waterproofing is applied at the level of plinth and linked with the horizontal waterproofing. As it is not recommended to bend the waterproofing at right or acute angles, the joint between the vertical and horizontal waterproofing is smoothed by applying an angle fillet made of FIBRANxps that is being manufactured specially for this purpose. If waterproofing is bent at right angle, it could be damaged. We cannot afford any mistakes when working under the ground, as later this area will not be accessible anymore.

The vertical waterproofing is then linked, by using the FIBRANxps angle fillets, to the horizontal waterproofing layer.

  • Floor above the foundation slab

In case of foundation slab which is insulated from below, there is no need for a conventional floor construction. What we have invested in making the foundation slab, is showing its advantages also in this case. Waterproofing that is usually applied on the floor slab, has already been applied, and the thermal insulation as well. The foundation slab (massive concrete) acts like a huge accumulator of heat and provides a constant temperature of the space. During the first days some more heat will have to be introduced in the building for heating purposes, but with time the heat will accumulate in the concrete mass. The concrete foundation slab will later act as a passive heat source.

A hint: when building floors, the acoustic insulation must be ensured, to prevent the spreading of impact sound.  For this purpose a closed cell polyethylene foam FIBRANxpe can be selected, and applied before the installation of floating floor.

Earthquake protection (earthquake insulation)

Thermal insulation boards FIBRANxps have been tested also for loadings during earthquakes. FIBRANxps, applied below the entire surface of the foundation slab of a building, reduces the effects of earthquake shocks. In case of a more powerful earthquake, the earthquake insulation enables a slight movement of the building, as if its foundation was laid on a sand base, which significantly reduces the effect of devastating forces on the construction of the building.

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