Installation of Mineral Wool Wall Insulation, Full Adhesion with Anchoring

Façade mineral wool is made mainly from molten basalt and other rock. Its main advantage is that it does not burn. Boards for insulation systems usually have the reaction to fire class A1, so they do not spread flame or melt. This is why mineral wool is often required, or at least strongly recommended, on taller buildings and where houses stand close together.

The second advantage is breathability. Water vapour from inside the house passes through mineral wool more easily than through polystyrene. In older houses with residual moisture in the masonry this matters, because the wall can slowly dry out towards the outside. The third advantage is sound. The fibrous structure dampens street noise, which people living next to a busy road tend to notice.

The downsides are greater weight and a softer surface. That is exactly why mechanical anchoring is a required part of a mineral wool system, not an optional extra.

Preparing the wall

The wall has to be clean and dry, with no grease, salt deposits or loose render. Larger unevenness, usually more than about 10 mm, is levelled with mortar. A very absorbent or dusty surface gets a primer. In practice this step is often rushed, although it decides how long the insulation will stay on the wall.

Starter track and plinth

A starter track (base profile) is fixed along the bottom edge of the insulation. It supports the first row of boards and protects the lower edge of the system. The area near the ground, where rain splashes back, is usually done separately with water-resistant plinth insulation. Mineral wool should not sit in permanent damp.

Bonding the boards

The installer first presses a thin layer of adhesive into the board surface so it grips the fibres, and only then applies the main layer. Lamella boards are bonded over the full surface. Standard boards are bonded as the specific system requires, either fully or with a perimeter bead and dabs covering a set share of the board. The boards go up in a staggered pattern like brickwork, so the vertical joints never line up. At external corners the boards interlock in alternate rows. Around windows, board joints must not meet at the corner of the opening, because cracks tend to start there later.

Anchoring

Once the adhesive has set, usually after one to three days depending on the weather, the installer fits the plate anchors. Mineral wool needs anchors with a larger plate or an extra plate, so the load spreads over the softer surface. The system documentation sets the number and layout of anchors based on building height, wind load and the type of wall. Corners and edges of the façade always get more anchors, because that is where the wind pulls hardest. How deep the anchor goes into the load-bearing wall depends on the anchor type and the masonry. Concrete, hollow brick and aerated concrete blocks each need a different anchor.

Reinforcement layer

A base coat goes onto the insulation and a glass fibre reinforcing mesh is pressed into it. The mesh overlaps at the joints and must be fully embedded, so you can neither see nor feel it. Corners and window reveals get corner beads with mesh, and the corners of openings get diagonal mesh strips to stop diagonal cracks.

Primer and render

When the base coat is dry, the installer applies a primer and a thin-coat render. Silicate or silicone render suits mineral wool well because both stay breathable. Acrylic render lets less vapour through, so it is chosen with more care over mineral wool. All layers should come from one certified system. Mixing products can make the layers incompatible and you lose the system warranty.

The simplest case is a flat, sound façade on a low house with only a few windows. The job takes more work when:

• The old render needs extensive repair or has to be hacked off
• The masonry is damp and the cause of the damp has to be fixed first
• The façade has many windows, cornices, bay windows and balconies
• The house is taller and needs scaffolding that takes longer to put up
• You choose thicker insulation, which is heavier and needs longer anchors
• Downpipes, lights, satellite dishes or outdoor air conditioning units have to be moved
• The windows sit so that the reveals can only take a thin layer of insulation

When you compare quotes, check that they cover the same items: insulation type and thickness, render type, anchoring, scaffolding and wall preparation. Differences between quotes often come from something that is missing.

• Poor wall preparation. Adhesive does not hold on a dusty or greasy wall, and the problem only shows years later.
• Too little adhesive. Air gaps form behind the boards and the system loses strength and thermal performance. You cannot see this from outside, so checks during the work help.
• Skimping on anchors. Fewer anchors than the system specifies, or anchors that are too short, can lead to façade damage in strong wind.
• Missing profiles. Without corner beads, movement joint profiles and window connection profiles, reveals and corners crack and water gets in.
• Bad weather. Adhesives, base coats and renders are usually applied between roughly +5 °C and +25 to +30 °C, not in direct sun, rain or strong wind. The manufacturer gives the exact conditions.
• Mixing brands. Layers from different systems may not work together, and the warranty no longer applies.

The insulation thickness should come from a calculation, not from habit. A designer or energy assessor will propose it based on the energy requirements in your local building regulations and the condition of your house, and will also check the risk of condensation inside the wall. If you are renovating, think about the windows at the same time, so the reveals can be insulated properly afterwards.

External insulation changes how the building looks, so check with your local building authority in advance whether you need permission or a notification. Listed buildings and conservation areas often have extra rules. In a block of flats the owners decide together, and the building management or owners' association organises the work. If you are thinking about a grant or subsidy, check the current conditions directly with the provider before you sign a contract, because some schemes do not support work that started before approval.

Ask the contractor for the technical documentation of the system with its European Technical Assessment, an anchoring plan and a written warranty for both the work and the system.

• The render has an even texture and colour, with no patches or visible joins
• No board outlines, anchor plates or mesh show through the surface
• The façade does not sound hollow anywhere when you tap it
• Corners are straight and reveals are clean and free of cracks
• Connections to windows, sills and the roof are sealed with tapes or profiles
• Window sills project with a drip edge, so water does not run down the façade
• The plinth is separated from the ground and protected from splashing water
• The handover record states the system, the insulation thickness and the anchor type
• You have received the warranty and photos of the anchoring taken before it was covered