External wall insulation

What EWI actually is

External wall insulation — EWI — is a layer of insulation board fixed to the outside of your walls, then covered with a reinforced basecoat and finished in render. It does two jobs at once: it wraps the building in a continuous thermal layer, and it gives you a brand new external finish in whatever colour and texture you choose.

It's the obvious answer for solid-wall properties built before the 1920s, for non-traditional construction such as concrete or steel-framed houses, and for any home where the walls are cold, damp-prone or simply looking tired. It's also increasingly popular on ordinary post-war semis where the cavity insulation was never up to much.

What it does for the house

• Warmer, and cheaper to heat. Solid walls are the single biggest source of heat loss in an older property. Insulating them makes rooms noticeably quicker to warm up and slower to cool down.
• No more cold spots. A continuous layer over the outside removes the cold bridges where condensation and black mould tend to appear.
• Weather protection. The render system sheds rain and stops the wall itself getting saturated, which is often what's behind persistent internal damp on an exposed elevation.
• A completely new look. The finish is the same through-coloured silicone render we'd use on any rendering job, so the house comes out looking new rather than patched.
• Kerb appeal. Of everything you can do to the outside of a house, this is the change people notice from the road.

How we build it up

1. Survey. We check the wall construction, its condition, and the practical details — roof overhang, sills, thresholds, drainage, meter boxes and any external fittings.
2. Preparation. Downpipes, aerials, lights and vents come off. Anything unsound in the wall face is made good, and a starter track is set level at the base.
3. Insulation boards. Boards are bonded and mechanically fixed to the wall, laid in a staggered bond and cut tight around every opening.
4. Basecoat and mesh. A polymer basecoat with reinforcing mesh embedded through it, with extra diagonal patches at the corners of windows and doors — the places where cracks would otherwise start.
5. Beads and detailing. Corner beads, bell casts, stop beads and movement joints, plus insulated reveals so window openings keep their proportions.
6. Primer and finish. Primer, then the through-coloured silicone top coat applied across full elevations in one pass so the colour runs true.
7. Reinstatement. Sills extended, downpipes and fittings refitted clear of the new wall face, everything washed down and taken away.

The systems we install

We work in manufacturer-tested systems rather than mixing components, because a system is only warranted if it's built as one:

• Johnstone's Stormshield — PPG's EWI and render range. We're an Approved Applier for their external wall insulation system.
• K Rend — silicone thin coat and textured finishes
• Weber (Saint-Gobain) — insulated and traditional render systems

Insulation is usually EPS (expanded polystyrene) for most domestic work, with mineral wool where fire performance or a particular specification calls for it. We'll talk through the options and what each one means for cost and thickness when we come out.

The details that matter

EWI adds around 60mm to 110mm to the face of your walls, and that has knock-on effects that a good installer plans for and a poor one improvises around:

• Window sills have to be extended or replaced so rainwater is still thrown clear of the new face.
• Roof overhang at the eaves and verge needs checking — sometimes the roofline has to be extended.
• Reveals around windows and doors take a thinner insulation board so openings don't end up looking recessed.
• The base detail at DPC level must stop the system wicking moisture up from the ground.
• Boiler flues, vents and extracts need extending through the new build-up.

We work all of this out at the quoting stage and set it out on your written price, so there are no surprises once the scaffold is up.