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How external wall insulation works
Board on the outside, a reinforced coat over it, and a finish that keeps the weather out.
Layer by layer
Prepare the wall
Loose render off, cracks repaired, damp dealt with.
Base track
Fixed above the damp-proof course to start the system level and dry.
Board and adhesive
Boards bonded with no gaps behind, laid staggered like brickwork.
Fixings
Mechanical fixings through each board into the wall.
Basecoat and mesh
A reinforced layer over everything, extra mesh at the corners of openings.
Finish
Render or brick slips in your chosen colour.
Board specification
We recommend one after the survey. Your quote names it and gives the reason.
| Board | Pros | Cons | Typical use in London |
|---|---|---|---|
| EPS | Light, easy to cut, good value | Combustible | Rear and flank walls away from boundaries |
| Mineral wool | Non-combustible, vapour open, helps with sound | Thicker and heavier | Walls near boundaries, old solid brick |
| Phenolic | Highest performance for its thickness | Higher cost | Narrow side returns, tight reveals |
| Wood fibre | Very breathable, natural | Thicker for the same performance | Older walls prone to damp |
Where the care goes
Insulation fails at its edges. These details decide how well the whole system works.
- Reveals insulated so window edges do not stay cold
- Corners wrapped continuously, no gaps between boards
- Base track above the damp-proof course, plinth detail below
- Eaves or verges extended if the roof would not cover the new wall
- Deeper sills that drip clear of the new face
- Flues, vents and pipes extended out to the new surface
From survey to scaffold down
Survey
Walls, damp course, roof edge, sills, flues, boundaries, planning and access.
Written quote
Board, thickness, finish, scaffold and expected dates.
Licences and bookings
Pavement scaffold licence, parking suspension and any gas engineer visit for flues.
Scaffold and preparation
Scaffold up, fittings off, wall repaired and protected.
Insulation and basecoat
Track, boards, fixings and reinforced basecoat.
Finish and refit
Finish coat or brick slips, then sills, pipes and fittings back on.
Clear
Scaffold down and the site left clean.
Technical questions
How thick will the insulation be?
That depends on the board and on the performance the wall has to reach under Building Regulations. On a house it is often somewhere around 60 to 120 millimetres, plus the basecoat and finish. The exact figure is on your quote.
Which board will you use?
It depends on the wall. EPS is light and good value. Mineral wool is non-combustible and lets moisture through, which matters near boundaries and on old walls. Phenolic is thinnest for its performance, which helps in tight spots. Wood fibre is very breathable.
Can old solid brick still breathe?
It should. Old solid walls dry out through their faces, so on those we use vapour-open boards and finishes that let moisture escape rather than trapping it.
What about cold spots around windows?
Reveals get a thinner board so frames are not buried, which stops them becoming cold strips. New, deeper sills go on so water still drips clear.
What about taller buildings?
The rules on external wall materials are much stricter for taller residential buildings. Tell us the height of the building at the start so the right materials and route are considered.
Ask about your walls
Tell us about the property.
Want a specification for your house?
It starts with a free survey.