Knowledge Centre / What Is an Insulated Raft? / Why Use an Insulated Raft Instead of a Traditional Raft with Insulation Above?

Why Use an Insulated Raft Instead of a Traditional Raft with Insulation Above?

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Insulation below the slab does more work, with less effort. When insulation sits above a concrete raft, the concrete sits outside the building's thermal envelope and becomes a thermal bridge. With insulation below — as in an insulated raft system — the concrete is brought inside the thermal envelope, the thermal bridge at the slab edge is eliminated, and the concrete's thermal mass works for you rather than against you.

Comparison of 100mm, 150mm and 200mm insulated raft slab thicknesses and their fire-resistance performance
Comparison of slab depths and the illustrated effect on fire-resistance performance.

Four things that change when the insulation moves below the slab 1. The insulated raft system is its own shuttering A traditional raft needs temporary formwork to be erected before the concrete can be poured, then struck and removed afterwards. The insulated raft system — the shaped EPS modules that form the base and edge of the slab — is the formwork. It goes down, the steel goes in, the concrete goes in, and it stays. No separate shuttering to hire, erect, or strip. 2. No additional...

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