Insulation with a rendered finish
External wall insulation in Scarborough
External wall insulation (EWI) fixes insulation boards to the outside of a wall and covers them with a reinforced render system. It is most useful on solid walls that have no cavity to fill. It also moves the outside face of the house outward, which changes every window, sill, roof edge and pipe on that wall.
On this page
What the system is made of
From the wall outward: insulation boards, commonly expanded polystyrene, phenolic foam or mineral wool, are fixed to the existing wall. A base coat designed for insulation is applied over them with alkali-resistant mesh bedded in it; K Rend's HP14, for example, is made for polystyrene, phenolic and mineral wool boards. A primer and a thin-coat finish such as a silicone render go on top.
The Energy Saving Trust describes EWI simply as a layer of insulation on the outside of the wall covered with render or cladding, and says it should be fitted by a specialist installer trained by an approved system designer. Ask for the named system and its designer rather than a general description of boards and render.
- Existing wall Prepared; fittings removed and the base track fixed above ground level.
- Insulation boards Bonded with adhesive and, in most systems, mechanically fixed to the wall.
- Basecoat with mesh Reinforcement mesh embedded across the board joints and around openings.
- Primer Where the system requires it.
- Finish render A thin-coat finish that weatherproofs the insulation.
Wall on the left, finish on the right. Not to scale: coats, thicknesses and primers follow the chosen manufacturer’s system and the condition of the wall.
Which walls suit it
EWI is aimed at solid walls. The Energy Saving Trust notes that homes built before the 1920s often have solid walls, and suggests measuring wall thickness at a window or door: a brick wall narrower than about 260mm is probably solid. A rendered wall needs a professional to check, because the render hides the brickwork pattern.
It is not a cure for a damp or failing wall. The Trust says EWI cannot be installed until existing damp problems are resolved, or if the walls are not structurally sound, and that roofs, overhangs, gutters and downpipes should be repaired first. On older buildings, Historic England's guidance on whether and how to apply EWI, and how to detail it at openings and roof edges, is the place to start.
What changes around the house
Insulation adds depth to the wall, so the details all move. The Trust notes that EWI often means extending the roof at the eaves and verges, moving rainwater pipes and relocating meter boxes. Windows usually stay where they are, so the reveals get deeper and need insulating too, and sills need extending or replacing so they still throw water clear of the new face.
At the base, the system stops above the damp-proof course and must not bridge it. Air leakage also falls once cracks in the old wall are covered, so the Trust advises that the installer assesses whether extra ventilation is needed indoors.
Rules that apply here
Government guidance on householder permitted development says that in National Parks and conservation areas, cladding any part of a house is not permitted development. Inside the North York Moors National Park, or in a Scarborough conservation area, EWI therefore needs a planning application. Elsewhere it may be permitted, but because it changes the look of the house it is sensible to ask North Yorkshire Council first.
Building regulations apply too. Approved Document L expects an insulated wall in an existing home to reach 0.30 W/m²K, subject to feasibility. Where a building with a residential use has a storey 11m or more above ground, Approved Document B says insulation in the external wall should be class A2-s1, d0 or better. The four- and five-storey terraces the design guide describes on South Cliff are the kind of building where that height needs checking.
Installation and aftercare
Installation follows the order of any render system, with insulation added first: scaffold, fittings off, wall repaired, boards fixed, base coat with mesh, then primer and finish. K Rend's data sheet for its silicone thin coat asks for at least 14 days between base coat and finish, and both need dry weather above 5°C, so expect the scaffold to stay up for a while.
Treat the finished surface with care. It is a thin render over softer insulation, so ladders should have protective ends, as K Rend advises for its finishes, and anything fixed to the wall later needs a fixing the system allows. Clean it gently, as for any silicone finish, and check the sealant at windows and pipe penetrations.
Survey wall type, damp and structure; repair roof and rainwater goods
Planning and building regulations checks
Scaffold; remove pipes, meters and fittings
Fix insulation boards; detail reveals, sills and eaves
Base coat with mesh; at least 14 days before the finish (K Rend thin coat)
Primer, finish coat; refix and extend fittings
Common questions
Will EWI cure damp in a solid wall?
No. The Energy Saving Trust is clear that existing damp has to be resolved before EWI goes on. Insulation can reduce condensation on cold inside surfaces, but it does not stop rain getting in or water rising from the ground.
Can I insulate just the rear of the house?
Yes, but the end of the insulation needs a designed stop where it meets uninsulated wall, and the depth change shows at corners and on the roofline. Decide early, ideally when the rear is being [re-rendered](/house/) anyway.
See housing and planning information for Scarborough and nearby places, including the source and geography behind each figure.
