External wall insulation, sometimes called external insulation or EWI, is an insulation layer fixed to the outside face of an existing wall and finished with render or cladding. It is widely used to upgrade solid wall and older cavity wall houses without losing internal floor area.
EWI wraps the outside of the building, so it reduces thermal bridging at junctions and keeps the existing masonry within the warm envelope, which helps reduce condensation risk internally. It also avoids disruption inside the house, since the work is carried out from scaffolding outside.
The main considerations are appearance and detailing. The wall thickness increases, so eaves, sills, reveals, thresholds and rainwater goods must all be adapted. Where a building is a protected structure or in an architectural conservation area, planning permission and conservation advice are likely to be required.