01/08/2025
Check your insulation – this isn't a plaster you pay a few quid for and fix later if needed.
This is an investment of several, sometimes over ten, thousand pounds. Let’s start with the fact that this year a record number of people contacted me asking for repairs to botched insulation jobs. Unfortunately, that’s not possible.
A failed insulation starts as early as the stage of gluing the boards to the wall.
First of all, the wall must be primed before applying adhesive.
I repeat: gluing – because many companies “dry-fit” the polystyrene boards. It may be fast, but it definitely won’t result in a flat surface.
Next, a starter track must be installed at DPC level. EPS must not be used below that, to avoid moisture rising from the ground. That’s unacceptable — whether you're doing insulation or rendering.
At the bottom, you must use a waterproof material, such as XPS, or render over it.
All pipes must be extended properly, and under no circumstances should gas pipes be covered — this is extremely dangerous.
Each board must be covered with adhesive around the edges, so air doesn’t blow behind it. This must be done across the entire wall.
Cuts near windows and sills must be in an L-shape. Boards should never be aligned flush with edges — this causes cracks.
Also, you must not use 5 cm offcuts to fill gaps or holes.
Every pipe, lamp, or bracket must have reinforcement underneath, so that any future installation doesn’t damage the insulation.
Then the entire surface must be rubbed down — to level it out and roughen the polystyrene, which improves adhesion.
Every gap, hole, or space near windows and doors must be filled with foam.
Never fill insulation gaps with adhesive — that’s a serious mistake.
Every board must be mechanically fixed with wall plugs.
Techniques vary depending on the type of plug, but you must always use polystyrene caps to avoid the "ladybird effect" in wet weather.
Otherwise, the surface ends up covered in ugly patches. It doesn’t affect insulation performance, but it completely ruins the appearance.
Only after all this can you move on to embedding mesh into adhesive.
And here are some key things to remember:
– Protect everything — windows, doors, roof, driveway, and everything around the house must be covered,
– pipes and other exposed elements must also be shielded.
Next, install corner beads and trims, using a spirit level or laser, and measure distances from windows and doors to keep everything straight and symmetrical.
Once trims are in place, reinforce all corners and window reveals with mesh, to strengthen the most vulnerable areas.
Then install window sills or covers — and each sill must extend beyond the insulation, not sit flush with the window reveal.
Sills should also be shaped in an L-form at the corners to stop water from streaking down and staining the render.
You can then start embedding the main mesh — it must be fully embedded into wet adhesive.
You must not leave the mesh exposed and dry, planning to cover it with a second coat the next day. That’s completely wrong.
After that comes the second adhesive layer.
Once that has fully dried, the entire surface must be primed using a coloured primer that matches the render.
Only then can you begin applying the top coat render.
At this point — no advice will help you anymore.
You either know what you’re doing, or the house will end up looking awful.
Only when everything is completed can you remove all protection and enjoy a job well done.
As you can see — it takes real effort and attention to detail to ensure insulation not only performs well, but also looks good.
