What is EPS foundation formwork?
EPS foundation formwork is permanent (lost) formwork made of expanded polystyrene, used to cast ground beams and strip foundations. The elements are set out on the blinding or on the pile caps, the reinforcement is placed inside and the concrete is poured. The formwork is then not struck: it stays where it is and serves as insulation around the foundation.
The product therefore combines two functions: mould during the pour, insulation afterwards.
Build-up and how it works
- The elements consist of EPS boards, often profiled or fitted with plastic or steel spacers that hold the faces at the correct centres
- They are supplied in standard heights and widths and can be cut to size with a hand saw
- On the outer face the EPS remains as perimeter insulation, reducing the thermal bridge between floor slab, wall and foundation
- At the top the system connects to the floor insulation and the wall build-up
Advantages
- Thermal bridge interrupted. The foundation edge is a classic cold bridge; a permanent layer of insulation around the beam reduces heat loss and the risk of condensation and mould on the internal floor edge.
- Speed and weight. The elements are light, can be set and cut by hand, and there is no striking, cleaning or removal of formwork.
- Fewer defect costs. No striking damage to green concrete arrises.
- Dimensionally accurate. Standard elements give a straight, clean beam.
Points to watch
- Pouring and flotation forces. EPS is light; wet concrete is not. Anchor and prop the elements properly and pour in layers. Pouring too fast, or over-vibrating, pushes the formwork apart or lifts it.
- Compaction. Use the poker carefully; knocking it against the EPS face damages the formwork.
- Above-ground finish. EPS is neither impact-resistant nor UV-stable. Finish the section above ground level with a reinforced render, a plinth board or a cladding, and avoid mechanical damage during backfill.
- Fire safety. EPS is combustible. No hot works (cutting, welding, torching) near unprotected formwork during construction.
- Compare with XPS. Where the insulation sits permanently below the water table or under high pressure, XPS is the better choice: lower water absorption and higher compressive strength.
Always check the compressive strength of the EPS grade where the insulation sits under a loaded floor edge.
Related terms
- EPS
- Ground beam
- Formwork
- Perimeter insulation
- Cold bridge
Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.