External Wall Insulation FAQ

EWI is a way of insulating your home by fixing a layer of insulation board to the outside of the walls, then covering it with a protective, decorative render finish. Because the insulation wraps around the whole building on the outside, it deals with heat loss and cold bridging without losing an inch of space inside your rooms.

They both aim to cut heat loss through a solid or poorly insulated wall, but from opposite sides. IWI insulates from inside the room, which eats into floor space and disrupts each room as it’s done. EWI insulates from the outside, so your internal layout, skirting, cornicing and floor space are untouched — but it does change the building’s external appearance, and it needs scaffolding and full access all round the house.

EWI tends to suit whole-house projects best, since it can be done as one continuous wrap rather than room by room, and it doesn’t touch anything inside your home. It’s also a good option where internal period detailing — cornicing, panelling, fireplaces — is worth keeping untouched, or where a tired render or brick finish needs renewing anyway. IWI tends to make more sense on a single elevation, on a listed building where the external appearance can’t change, or where scaffolding the whole house simply isn’t practical.

Most solid brick, stone, or properly-built cavity walls are — but, as with any retrofit measure, it isn’t automatic. Before any board goes up, we check the wall build-up, its condition, and how exposed each elevation is, and rule out anything that needs addressing first.

• Unfilled or previously-insulated cavity walls: EWI shouldn’t go over a wall with an unfilled cavity, or one where cavity insulation has been removed and not replaced, unless the cavity’s been properly sealed first — otherwise warm air can bypass the new insulation altogether.
• Damp: rising or penetrating damp needs sorting before insulating over it, not after.
• Very restricted access: narrow side passages or tight boundaries can limit how much insulation thickness fits, or mean a thinner board is needed in that one spot.
• Listed status or a conservation area: changing the external appearance of the building may need consent first.

A proper site survey — not a five-minute look round. We check the wall construction and condition, note anything fixed to the walls that will need moving or working around (meters, pipes, vents, alarms, lights), measure roof overhangs and window sill projections, and check ground clearance and damp-proofing at the base of the wall.

Usually not for an ordinary house, since EWI is typically treated as permitted development — but there are exceptions: listed buildings, conservation areas, and some flats or maisonettes may need consent, and it’s always worth checking before committing to a design.

The insulation board itself is the easy part. Getting a lasting result is really about everything around the edges — window and door openings, the roofline, ground level, and anything fixed to or passing through the wall.

The insulation and render wrap into the window reveal, and the window sill needs to project a set distance (typically at least 40mm) beyond the new render line, with a drip detail underneath, so rainwater running down the wall is thrown clear rather than tracking back into the wall build-up. Where oversills are added to existing sills, they’re capped and sealed the same way.

Ideally the roof overhangs the new, thicker wall by 30–40mm so the insulation tucks neatly underneath it; where it doesn’t, the gutter, verge or eaves detail sometimes needs extending or adjusting so the whole system is protected from the weather at the top.

The insulation typically stops a short distance above the ground or paving, since it isn’t designed to be in permanent contact with soil or standing water — the exact detail depends on what’s abutting the wall (gravel, paving, a flower bed) and whether there’s a damp-proof course to respect.

Everything fixed to or passing through the wall — downpipes, meter boxes, air vents, boiler flues, external lights and sockets, alarms — gets identified at survey stage and either relocated, extended, or carefully detailed around, so it still works properly once the wall is thicker. Ventilation openings in particular need sleeving through the new insulation so airflow isn’t blocked, in line with PAS 2035 requirements.

A mineral wool EWI system, like the one we install, is non-combustible, so it doesn’t usually need additional fire barriers built in. Power cables on the wall are never boarded over — they’re relocated or left visible and accessible.

Every joint where the render system meets a window, door, roofline or another wall gets sealed properly — usually with a purpose-made sealant and stop profile — so wind and rain can’t get in behind the render, and the wall keeps performing as designed for years, not just for its first winter.

It depends on the wall and the survey findings, but here’s a real-world example — the build-up for a typical EWI Pro mineral wool system, working outward from the existing wall:

• Existing masonry wall, cleaned down and checked for loose or damaged render
• A stabilising primer, or a levelling mortar coat where the substrate is uneven
• Mineral wool insulation board (commonly 100mm, sometimes more or less depending on the target performance), fixed with adhesive and mechanical fixings
• A reinforced base coat with an embedded glass-fibre mesh for strength and crack control
• A thin-coat, decorative render finish — commonly silicone-based, for its combination of breathability and weather resistance

Done properly, a system like this typically brings a solid wall down to a U-value in the region of 0.3 W/m²K — roughly the same performance as a well-built new home’s walls, on a house that may have started out with almost no wall insulation at all.

This is one of the most common worries, especially on a period property — and it’s usually not a problem. The finish that goes over the insulation isn’t limited to a plain render, so the original character of the house can often be recreated rather than lost:

• Render: available in a wide range of colours and textures, from a smooth, modern finish to a more traditional textured coat — a good match for a house that was rendered or pebbledashed to begin with.
• Cladding: timber, composite or other board cladding can be fixed over the insulation instead of a render finish, giving a completely different look and texture where that suits the house better.
• Brick slips: thin brick-effect facings applied over the insulation and finished with real mortar joints, so a house that was originally brick can still look like brick once the wall is insulated — this is a popular choice for keeping a period property looking like itself.

Whichever finish is chosen, it’s worth deciding early — it affects the detailing around windows, doors and the roofline, so it’s part of the design conversation from the start, not an afterthought once the boards are up.

Scaffolding goes up, the wall’s cleaned down and any loose render or debris removed, primer and/or a levelling coat is applied where needed, the insulation boards are fixed on and every junction and reveal detailed, the reinforced base coat goes on with mesh embedded into it, then the final decorative render coat is applied once the base coat has cured.

Every project’s different depending on the size and shape of the house, how many elevations are involved, and the weather — render needs the right conditions to go on and cure properly — but here’s a realistic range for an average semi-detached house:

• Scaffolding: up around the whole house for the duration, which means less natural light at some windows and reduced access to that side of the garden while it’s there.
• Noise and mess outside: cutting boards, mixing render, and general building activity outside your windows.
• Fittings coming off and going back on: downpipes, lights, meter boxes and vents get temporarily removed or relocated.
• Little to no disruption inside: since the work happens entirely outside, your rooms, floors and internal walls aren’t touched.

Yes, in almost every case — because the work is entirely external, it doesn’t stop you using any room in the house, though you may want to keep windows on the working elevation closed while boards are being cut nearby.

Very little. A quality thin-coat render system is designed to be low-maintenance and colourfast for decades, though like any external finish it’s worth an occasional visual check, particularly after severe weather — and any accidental damage (a knock from a ladder or trellis) is best repaired promptly so water can’t get in behind it.