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How external wall insulation works
Insulation board on the outside of the wall, a reinforced basecoat over it, and a weatherproof finish on top.
The layers
Existing wall
Checked, cleaned and repaired. Loose render comes off and any damp is dealt with first.
Adhesive
Bonds the board to the wall, applied so there are no gaps for air to move behind it.
Insulation board
Laid in a staggered pattern like brickwork, tight-jointed, with no gaps at the corners.
Mechanical fixings
Fixings through the board into the wall hold it in place for good.
Basecoat and mesh
A reinforced layer that gives strength and resists cracking, with extra mesh at window corners.
Finish
Render or brick slips, in the colour and texture you choose.
Insulation boards compared
Your quote names the board we recommend for your walls, and the reason.
| Board | Strengths | Things to know | Often used on |
|---|---|---|---|
| EPS | Light, easy to shape, good value | Combustible, so not used where rules require non-combustible insulation | Most brick and block houses |
| Mineral wool | Non-combustible, lets moisture pass, helps with noise | Thicker and heavier than EPS for the same performance | Walls near boundaries, older solid walls |
| Phenolic | Most insulation for its thickness | Costs more per square metre | Tight spots where thickness matters |
| Wood fibre | Very breathable, natural material | Thicker for the same performance | Old solid brick and stone that need to dry out |
The details that matter
A thermal camera shows where insulation fails: at the edges. The flat areas are easy. These are the places we take most care over.
- Window and door reveals insulated, not left as cold strips
- Corners wrapped in one continuous layer with no gaps
- A base track above the damp-proof course, with a plinth detail below
- Insulation carried up to the roof line, with eaves extended if needed
- Sills replaced with deeper ones that drip clear of the new face
- Joints sealed where the system meets frames and pipes
How a job runs
Survey
We check walls, damp course, roof edge, sills, flues and boundaries, and talk through finishes.
Quote
A written price naming the board, its thickness, the finish and expected dates.
Scaffold and preparation
Scaffold goes up. Pipes, lights and fittings come off. The wall is cleaned and repaired.
Insulation
Base track, boards and fixings go on, with reveals insulated as we go.
Basecoat and mesh
Applied over the whole wall, with extra mesh at corners of openings.
Finish
Render or brick slips in your chosen colour.
Refit and clear
Sills, pipes and fittings refitted, scaffold down, site cleaned.
Questions about the system
How thick is external wall insulation?
It depends on the board and the performance the wall needs to reach. On a typical house the insulation is often somewhere around 60 to 120 millimetres, plus a few millimetres of basecoat and finish. We confirm the exact thickness on your quote.
Which insulation board is best?
There is no single best. EPS is light and good value. Mineral wool does not burn and lets moisture through. Phenolic gives the most insulation for its thickness. Wood fibre is very breathable and can suit older solid walls. We recommend one after the survey and explain why.
What happens around the windows?
The window reveals get a thinner insulation board so the frames are not swamped. This also stops the reveals becoming cold spots. New, deeper sills go on so rain still drips clear of the thicker wall.
What about the bottom of the wall?
The system starts on a base track set above the damp-proof course. Below that, a moisture-resistant board or a plinth detail is used where needed, so the insulation does not wick up water from the ground.
Will it stop the walls breathing?
Not if the right system is chosen. On old solid walls that need to dry out, we use vapour-open insulation and finishes so moisture can still escape.
Ask about your walls
Tell us a bit about the house.
Want to know which system suits your house?
We will look at the walls and explain the options at a free survey.