External Wall Insulation (EWI)? External wall insulation is a type of insulation that goes on the outside of the walls of your house. Insulation boards are fixed to the outside of your house and then a lovely new coat of weatherproof render is applied to freshen up the exterior of your home in your choice of colour. This insulation is just like wrapping your home in a blanket!It is a high performance insulation, very effective at warming up the interior of the home and reducing energy bills.