Full-Surface Polystyrene Wall Insulation Installation

The job is a chain of stages that build on each other. Each one needs suitable weather and time to dry. If one stage is rushed, the problem often only shows after the first winter.

Preparing the wall

The wall must be firm, dry and clean. The installer removes dust, grease, moss, algae and any render that crumbles or sounds hollow when tapped. Cracks and broken areas are repaired and left to dry. Absorbent surfaces, such as aerated concrete blocks or old lime render, get a primer so the adhesive does not dry too quickly. Flatness matters a lot for full-surface bonding. The notched adhesive layer is thin and cannot take up larger bumps, so an uneven wall has to be levelled first. If that is not practical, the perimeter strip and dab method is used instead.

Base profile and bonding the boards

A base profile, sometimes called a starter track, is fixed along the bottom edge. The first row of boards sits on it, and it also protects the lower edge of the insulation. The adhesive is mixed according to the manufacturer's instructions, spread on the board with a notched trowel and the board is pressed onto the wall straight away. Adhesive does not belong on the board edges. Boards are laid in a staggered bond like bricks, so the vertical joints of neighbouring rows never line up. At the corners of the house, the boards interlock in alternate rows. Around windows, boards are cut into an L shape so that no joint runs out exactly from the corner of the opening.

Anchoring, reinforcement and render

Once the adhesive has hardened, usually after one to three days depending on the weather, the insulation is fixed with façade anchors. Their number and layout come from the design or the manufacturer's fixing plan. Building height, the type of masonry and wind exposure all play a part, and edges and corners usually need more anchors. The board surface is then sanded flat. Glass fibre mesh is pressed into the fresh base coat so that it is fully embedded, with neighbouring strips overlapping by roughly 10 cm. Extra diagonal mesh patches go at the corners of windows, and corner beads with mesh go on the edges. When everything is dry, a primer is applied, followed by a thin-coat render in the chosen texture and colour.

• White façade EPS is a common, proven material for most houses.
• Grey (graphite) EPS insulates better at the same thickness, so a thinner board can do the same job. It heats up more in the sun, so the scaffolding needs shade netting during installation.
• XPS or plinth EPS goes on the plinth and below ground level, where there is more water and more risk of knocks.
• Mineral wool is the alternative where fire safety requires it, or where you want a more vapour-open wall build-up. On taller buildings, fire regulations often require at least mineral wool fire barriers, and in some cases non-combustible insulation throughout.

Do not guess the insulation thickness. Your local building regulations set the thermal performance walls must reach, and a designer or energy assessor can work out the thickness your wall needs. On houses in cooler climates, something in the range of roughly 15 to 20 cm is common today. The extra material for a thicker board is usually modest, the labour hardly changes, and you cannot add more later.

It pays to use adhesive, anchors, mesh, primer and render as one system from one manufacturer. The components are tested together. If you mix brands and something fails, you may be left without a warranty.

How broken up the façade is matters most. A large flat wall goes quickly. Every window, corner, bay, balcony or ledge means cutting, extra trims and more time. An older house with loose render or uneven walls needs repair and levelling before the first board goes on. The job also takes longer with:

• Scaffolding on a tall or hard-to-reach façade
• Taking down and refitting window sills, gutters, lights, satellite dishes or an air conditioning outdoor unit
• Gas pipes or a gas meter box with the main valve on the façade
• Thicker insulation that needs new, wider external window sills

Only a qualified, registered gas engineer may alter gas installations on the façade, and work on the supply connection is arranged with the gas network operator. If you smell gas during the works, leave the area and call your local emergency number or 112.

Whether insulating your house needs only a notification or a full permit depends on the building, its location and local building regulations. Listed buildings and conservation areas often have extra rules on how the façade may look. Check with your local building authority before you hire a contractor. In a block of flats, the owners decide on insulation together, and the work is organised by the building management or owners' association.

If you want to apply for an energy efficiency grant, find out the conditions of the current scheme in advance. Schemes usually require documents and a qualified contractor before work starts. Insulation that is already installed generally does not qualify.

• Bonding onto a dusty or loose surface. The adhesive sticks to the dust, not the wall, and the boards come loose over time.
• Full-surface bonding on an uneven wall. The board only touches the high spots and leaves cavities behind it. Air moves through them, the insulation loses its effect and strong wind can tear it off.
• Boards without a staggered bond and gaps filled with offcuts. Cracks then appear in the render along the joints. Narrow gaps should only be filled with low-expansion foam made for the purpose, wider ones with a strip of polystyrene.
• Working in unsuitable weather. Below about +5 °C the adhesive does not cure properly. In direct sun and heat it dries before the board is pressed on. The manufacturer gives the upper limit, usually around +25 to +30 °C. Rain and strong wind also cause damage.
• Too few anchors or badly placed ones. The problem only shows after a storm, especially in exposed spots.
• Missing trims and reinforcement around windows. Without diagonal mesh patches and window connection trims, the render cracks right at the window corners.
• Leaving polystyrene unrendered for a long time. Sunlight damages the board surface and the base coat then bonds less well.

When you take over the work, walk around the façade in daylight, ideally when the sun is low and shines across the wall, because that shows up unevenness. Check in particular:

• The render is flat, with no waves, bubbles or visible outlines of boards or anchors
• There are no cracks, especially at the corners of windows and doors
• The corners of the house and the window reveals are straight and have corner beads
• The connection to windows is made with tape or a connection trim, not just a filled joint
• The plinth is made of a water-resistant material and the base profile is straight
• Window sills project far enough, have a drip edge and shed water clear of the façade
• The render colour is even, without patches or visible joins between sections
• You have a written record from the contractor of the system used, the insulation thickness, the warranty terms and any documents for the building authority or a grant