Basecoat and mesh: the step between boards and render

The layer that actually holds the system together

Most of the questions we get about external wall insulation are about the boards - which thickness, how to fix them, EPS or wool. Fewer people ask about the basecoat and mesh layer that goes on afterwards, but it is the part of the system doing the structural work. The boards insulate. The basecoat and embedded mesh is what ties the render system together and stops it cracking along board joints as the wall moves with the seasons. Get this layer wrong and it does not matter how well the boards were fixed.

Step 1: Get the beads on before you touch the basecoat

Every edge, corner and opening needs a bead fitted before basecoating starts, not after.

• A PVC Corner Bead with Mesh 2.5m goes on external corners, keeping the arris straight and giving the corner some impact resistance.
• A PVC White Stop Bead with Mesh 2.5m finishes the render neatly where the insulation meets a different material, such as a cill or a change of finish.
• A Window Drip Bead with Mesh 2.5m goes over window and door openings, throwing water clear of the reveal instead of letting it track back into the render.

All three come with a mesh wing built in, bedded into the basecoat along with the main mesh sheet so there is no seam where the bead meets the flat wall.

Step 2: Basecoat and embed the mesh in one wet pass

This is the step people rush, and it decides whether the render cracks in year two or lasts.

Apply Ceresit ZU 2-in-1 EPS Adhesive & Basecoat White 25kg (or the equivalent Ceresit ZU 2-in-1 EPS Adhesive & Basecoat 25kg) with a notched trowel, then push Fibreglass Reinforcing Render Mesh 160g - 50m2 Roll into the wet basecoat while it is still workable, top to bottom so each strip laps the one below. The mesh has to sit fully within the basecoat, not scraped bare on top of it - if you can see bare fibreglass once it has gone off, there was not enough material behind it.

Overlap every mesh strip by roughly 10cm at joints, and at external corners carry the mesh at least 20-25cm around each face. Around window and door openings, cut a diagonal strip of mesh into each corner of the reveal before the main sheet goes on - that is where cracks concentrate, because it is where the greatest stress builds up around an opening.

Component	Job	Typical coverage
Ceresit ZU 2-in-1 Basecoat (25kg)	Embedding the mesh, building the reinforced base layer	around 4 sqm per bag
Fibreglass Mesh 160g (50m2 roll)	Reinforcement, plus diagonal corner patches at openings	up to 50 sqm per roll, less once overlaps and offcuts are allowed for
Ceresit CT16 Primer (15kg)	Priming the cured basecoat before render	around 30 sqm per tub

Check the datasheet for exact open time, mixing ratio and full cure period for the basecoat - these vary with temperature and site conditions, so we would rather point you to the datasheet than guess a figure here.

Step 3: Let it cure properly before you go any further

Do not prime or render over a basecoat that has only skinned over on the surface. It needs to cure through its full depth, not just look dry, or the primer and render coat on top will trap moisture and eventually blister or delaminate. Cold, damp or shaded elevations take noticeably longer than a wall in full sun, so build that into your programme rather than working to a fixed number of days.

Step 4: Prime before the render goes on

Once the basecoat has cured, Ceresit CT16 Render Primer 15kg goes on before the top coat render. It keys the surface, evens out absorption, and is tinted close to the render colour so thin patches in the final coat do not flash through as a different shade. Skipping the primer is a false economy - render applied onto an unprimed basecoat dries unevenly and can end up patchy. From there you are on to the silicone render finish coat, its own step with its own decisions on colour and texture.

A common mistake worth avoiding

Basecoating a whole elevation on a warm, breezy day, then trying to bed mesh into sections that have already started to skin over. The mesh will not embed properly, it sits proud, and that is where faint mesh lines telegraph through the render months later. Work in sections you can actually keep wet, and have a second person on the mesh if the wall is a big one.

Browse the full External Wall Insulation range for boards, beads, mesh, basecoat and primer together, so the whole system comes from one delivery.

Get your materials list sorted before you start

Send us the elevation dimensions, plus the number of openings and external corners, and we will work out how much basecoat, mesh, bead and primer you need. In-stock orders go out within 7 working days, and delivery is free in Ballincollig and Cork City. Insulation boards are quoted individually. Collection is by appointment only.

Call 085 868 8459 or email sales@all4building.ie and we will sort the list out with you.