Insulation Below vs Above Concrete Slab: Why an Insulated Raft Performs Better
The position of insulation relative to the structural slab has a major impact on thermal bridging, buildability, and long-term performance.
If you're deciding between a traditional raft foundation with insulation above, and an insulated raft with insulation below, the choice has significant implications for thermal performance, construction complexity, and cost. Here's why the insulated raft approach consistently outperforms the alternative.
Quick Answer
Insulation below a concrete raft eliminates thermal bridging at the slab edge, removes the need for additional screed and blockwork, and allows the slab's thermal mass to regulate internal temperatures. Traditional rafts with insulation above create a thermal bridge where the concrete slab meets the external walls.
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