Cavity Fill Below DPC

150 mm

Where partial or full fill cavity insulation is used, it should terminate at least 150mm below the lowest damp-proof course (DPC)

Every figure for this topic is on one page: Cavity trays and DPC: where are they needed and how high?

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Where partial or full fill cavity insulation is used, it should terminate at least 150mm below the lowest damp-proof course (DPC). This separation prevents any potential moisture ingress through the cavity fill from bypassing the DPC.

Basis in regulation

Part C
Approved Document C
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How to check it on site

Check that any cavity fill material, particularly concrete or insulation, finishes at least 150mm below the DPC level.

Common failure

Cavity fill materials are sometimes extended too high, bridging the DPC and creating a path for damp to bypass the DPC.

Last updated 23 August 2026. This is our editorial summary of publicly available regulation, not building control approval or a warranty determination.

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