Welcome to the AECB Forums Building Refurbishment and Retrofit DPM position to raft slab: above or below insulation?

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    Anonymous
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    We're building a rear extension with a raft slab and insulating under the slab with polystyrene (continuing around the edge of slab and up the walls with EWI).

    Every detail I've seen, including AECB's gold standard, shows the DPM above the floor insulation, below the slab.

    Wouldn't it be better to run the DPM under the insulation (and have a separate separating layer between the slab and insulation)?

    As you can see from the attached section sketch we also have 1x1m x 2.5m deep mass concrete foundation supporting the slab (due to a big poplar close by), so if we did the above we'd have to lay the DPM under this too.

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