External Wall Insulation: Mineral Wool vs EPS

Insulating a facade from outside protects the wall from heat loss and temperature swings. Mineral wool and EPS differ in fire behaviour, vapour permeability and installation.

A large share of a house's heat loss passes through its walls, simply because they make up the biggest area of the building envelope. Insulating walls from the outside reduces the load for heating in winter and cooling in summer, and it also shields the masonry from temperature swings. The question most often debated is the material: mineral wool or expanded polystyrene.

Why insulating from outside makes more sense

When insulation is applied to the outer face, the mass of the wall stays on the warm side. The masonry does not cool down in winter, returns stored heat to the room, and the risk of condensation on the inner surface drops. Insulating from inside reduces floor area, keeps the wall in the cold zone and raises the risk of moisture building up between insulation and wall.

External insulation also covers the edges of floor slabs and beams. These spots usually form thermal bridges, and they are almost impossible to close off with internal insulation.

Mineral wool: non-combustible and vapour-open

Mineral wool, such as stone or basalt wool, is a fibrous material. Its main advantages on a facade are:

• it is classed as non-combustible and reduces the risk of fire spreading along the facade;
• it has high vapour permeability, so moisture in the wall can escape outward;
• it absorbs sound well.

There are drawbacks: it is heavier than polystyrene, needs more anchors, and loses thermal performance when wet, so it must be protected from rain during work. Only dense boards designed for facade systems should be used; soft wool sags on a vertical surface.

Polystyrene: light and moisture-resistant

The polystyrene used on facades is usually expanded polystyrene (EPS) board. It is light, easy to cut and absorbs little water, which makes it a common choice. Extruded polystyrene (XPS) is denser and more moisture-resistant, so it is preferred for plinths, foundations and anything in contact with the ground.

Points needing care: polystyrene is combustible, so fire safety calls for flame-retardant grades and fire breaks made of non-combustible material. Its vapour permeability is low, which can cause trouble on damp walls or in buildings without a vapour control layer inside. Boards left exposed to sunlight for long periods suffer surface damage.

Which to choose depends on the building

The choice depends on building height, wall material and local fire regulations. On tall buildings and above escape routes, non-combustible insulation is often mandatory. On vapour-open wall materials such as aerated blocks, mineral wool keeps the wall able to breathe. On a one- or two-storey private house, EPS with properly designed fire breaks can also be a practical solution.

Combinations are possible too: XPS on the plinth, mineral wool on the main facade, and non-combustible strips above windows and along floor lines.

Besides rendered systems, a ventilated facade is another option. Insulation is fixed to the wall, an air gap is kept in front of it, and the cladding (stone, porcelain stoneware, composite panels) hangs on a separate subframe. Air moving through the gap carries moisture away, so mineral wool is almost always used in this system. Because the metal brackets of the subframe pass through to the wall, isolating pads must go beneath them; otherwise each bracket becomes a small thermal bridge. A ventilated facade needs more preparation and calculation, but the cladding can later be repaired or replaced without touching the insulation.

Installation sequence and typical mistakes

In a rendered insulation system the sequence is: cleaning and levelling the surface, fitting a starter track, bonding the boards with adhesive, fixing them with anchors, applying a thin base coat with reinforcing mesh and finally the finish coat. At each step the manufacturer's system requirements must be followed, since adhesives and renders from different brands may not be compatible.

The most common mistakes are:

• bonding boards with dabs only, leaving air channels around the edges;
• filling open joints between boards with adhesive instead of expanding foam;
• letting board joints line up with the corners of window openings, which later causes cracks;
• leaving window reveals uninsulated;
• not fully embedding the mesh in the base coat.

Zaferoğlu İnşaat builds private houses, residential complexes and commercial premises. Tell us what the walls are built from and how tall the building is; the choice between mineral wool and EPS largely hinges on those two facts.