External wall insulation
External Wall Insulation Doncaster: Insulated Render
External wall insulation, or EWI, wraps the outside of the house in insulation boards and finishes them with a reinforced render. The house looks rendered afterwards, but the job is closer to re-skinning the walls: sills, pipes, meter boxes and roof edges must be checked against the new wall depth, with extensions or relocation where the design requires them.
On this page
What goes on the wall
The insulation trade body INCA describes the components as insulation boards, adhesive, mechanical anchors, a reinforcement basecoat with glass-fibre mesh, a finish, and the beads, trims and flashings that close the edges. Boards are fixed with adhesive, anchors or both. The anchors are specified by the system designer, and INCA recommends pull-out tests on the actual wall so the type and number of fixings suit it; washers should be at least 60mm across.
Over the boards, INCA's general guidance is a basecoat of at least 6mm overall, built in two layers with the mesh in the first. The finish can be a thin silicone or acrylic coat, a mineral render or a dash. Board choices include EPS, mineral wool, PIR, phenolic and wood fibre, and the system designer should calculate the existing and proposed U-values to show the result meets building regulations.
- 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.
Is your wall a candidate?
EWI is mostly used on solid walls, which have no cavity to fill. INCA's pre-installation survey list is a useful guide to what has to be checked first: rising damp and its causes, existing movement joints, salts on the surface, defects in the wall, pull-out strength, whether the wall is straight enough or needs a levelling coat, and features that must be kept or reproduced. A wet or failing wall has to be put right before boards go over it.
Approved Document L sets 0.30 W/m²K as the standard for a wall upgraded with internal or external insulation, and treats re-rendering most of a wall as a renovation that should include an upgrade where it is feasible; building control can confirm what applies. Planning matters too. Government guidance says cladding a house in a conservation area is not permitted development, and INCA advises checking with the council even elsewhere. On an older stone or soft-brick wall, ask for a moisture assessment before agreeing to any system.
The edges that make or break it
At the base, INCA's details show a starter track fixed at DPC level and trims set 150mm above the ground to reduce splash-back, with the DPC left unbridged. At windows, new sills should overhang the finished face by at least 40mm, so existing sills often need over-sills. Reveals get stop beads and sealant. At the top, the roofline may need extending so the eaves still overhang the thicker wall, and INCA notes that the eaves zone needs insulation to avoid a cold bridge.
Services are the part people underestimate. Gas and electricity meters, pipes, lights, satellite dishes, cables and rainwater goods all have to be moved, extended or refixed. INCA stresses that the system should be continuous, because gaps create cold spots and weak points where water gets in.
Survey the walls, test fixings and check planning and building control.
Move or extend services, downpipes and fittings; prepare the wall.
Fix the starter track at DPC level and bond the first row of boards.
Fix the boards with adhesive and anchors in the specified pattern.
Apply the basecoat with mesh, adding extra mesh at opening corners.
Fit beads, over-sills and trims, then apply the primer and finish coat.
Refit services and fittings and keep the system records.
Weather on site
INCA advises applying basecoats only in fine, dry weather, checking the forecast for 48 hours ahead, and keeping finishing coats out of direct sun and strong wind so they do not cure too quickly and crack. Unfinished boards need protecting during breaks. Keep the full system name and drawings afterwards; anyone fixing a new light or bracket later needs to know what is behind the render.
Common questions
Does EWI need planning permission in Doncaster?
In a conservation area, yes: national guidance says rendering or cladding a house there is not permitted development. Elsewhere it depends on the change in appearance and whether an Article 4 direction applies, so check with the council before ordering materials.
Will the house look different afterwards?
Usually a little. The walls move outwards by the thickness of the boards plus render, so window reveals get deeper, sills project further and the eaves overhang less unless the roofline is extended. Brick details such as soldier courses or bay surrounds disappear under the new face unless they are reproduced in the render, which INCA lists as a decision to make at survey stage.
See housing and planning information for Doncaster and nearby places, including the source and geography behind each figure.
