Building insulation: cost per m² 2026 and process

Insulation makes the biggest difference in houses that never had any. In many countries that means a large share of older family houses and blocks of flats built before modern energy rules came in. Walls of solid brick, blocks or concrete panels without insulation lose heat across their whole surface. In winter they feel cold to the touch, and moisture easily condenses in the corners of rooms. That is where mould starts.

The walls are not the only way heat escapes, though. In older houses a lot of heat goes straight up through an uninsulated ceiling into the loft. Insulating the loft floor is one of the cheaper measures, and you notice the effect on your heating quickly. Before you decide anything, have the house looked at as a whole. An energy assessment or energy performance certificate shows you where the big losses are and where they are small, so you can plan a sensible order of work.

The order really does matter. Replace the windows before the walls are insulated or at the same time, never afterwards. With a new window, the insulation can be joined neatly to the frame, or the window can be fitted further out, in line with the insulation. If you change the windows once the facade is finished, the reveals get damaged and the whole junction has to be done a second time.

For an ordinary masonry family house, the usual choice is facade-grade expanded polystyrene, known as EPS. It is light, easy to work with and insulates reliably when it is thick enough. Grey graphite EPS insulates better than white. At the same thickness you get a better result, or you can use a thinner board for the same result. The catch is that grey boards must not heat up in direct sun during installation. They warp and gaps open up between them, which is why the scaffolding is covered with shade netting.

Mineral wool does not burn and lets water vapour through more easily. It is used on taller buildings, where fire rules call for non-combustible fire barriers or a facade made entirely of wool. It also suits historic houses and walls that need to dry out naturally. It is heavier and slower to install, so a mineral wool facade costs more.

The plinth and anything below ground need extruded polystyrene (XPS) or special plinth boards. They do not soak up water and they can take the pressure of the soil. Ordinary facade EPS should never be in contact with the ground or with splashing water.

Does a house under polystyrene really stop "breathing"?

This worry comes up again and again. In reality only a small part of the water vapour in a home leaves through the walls. Moisture from cooking, showering and breathing goes out mainly through ventilation. After insulation and new windows the house is more airtight, so you need to air it more often and more regularly, or consider mechanical ventilation with heat recovery. If the walls are already damp from the foundations up, deal with the cause of the damp first and only then insulate.

Thickness is not chosen by eye. It comes from the U-value the wall should reach, which is a measure of how much heat passes through it. The target is set by the national energy performance requirements for buildings. Put simply, the less heat the wall lets through, the better. The result depends on the existing wall and on whether you are aiming for a standard house or a low-energy one. For older masonry houses, EPS around 15 to 20 cm thick is common in practice, and a little less with grey EPS.

Thickness also affects things that are easy to forget:

• Window reveals get deeper and let in less daylight, so windows are often moved out into the insulation layer
• External window sills become too short and need replacing with new ones that project far enough
• The roof has to cover the insulation, otherwise the overhang or flashing must be extended
• Downpipes, lights, the letterbox and other items on the wall have to be moved

Why is it not worth saving on thickness? The insulation board itself is only a smaller part of the total cost. Labour, scaffolding, adhesive, mesh and render cost almost the same whether the board is thin or thick. Saving a few centimetres rarely pays back in practice.

Whether the insulation lasts for decades depends on how it is bonded and anchored. Once the render is finished, you can no longer see underneath. If you can, be there during this stage, or ask the contractor for photos.

• Adhesive: On a flat wall the boards are bonded over their full surface. On an uneven wall the adhesive goes around the edge of the board and in several dabs in the middle, and as a rule it must cover at least 40% of the board. It goes on the board only, not on the wall, and must not squeeze into the joints between boards.
• Bond and joints: Boards are laid like bricks with staggered joints, and they interlock alternately at the building corners. Joints wider than a few millimetres are filled with strips of insulation or low-expansion foam, never with adhesive.
• Anchors: How many anchors go in, and where, is set out in an anchoring plan based on building height, wind exposure and wall type. If nobody knows what the wall is made of, or it is hollow, pull-out tests are done on site to check how much an anchor can hold.
• Recessed anchors: An anchor sunk into the insulation and covered with an insulating cap does not leave light dots on the facade. Surface-mounted anchors can show up after a few years as spots the shape of the anchor heads, especially with morning dew and on north-facing walls.

Bonded EPS boards should not stay bare for long. Sunlight breaks down the surface, the board turns yellow and the reinforcing layer sticks to it less well. Manufacturers therefore state how long EPS can stay uncovered. A yellowed surface has to be sanded before the next layer goes on.

Most problems do not appear on the flat wall but where the insulation meets something else. Before you sign a contract, check that the quotation covers these points.

Windows, reveals and sills

Reveals are insulated with thin boards so that no cold strip is left around the window. The insulation is joined to the frame with a mesh-backed window bead. It takes up small movements of the frame and stops a crack forming along the window. The external sill is fitted sloping away from the window, with end caps at the sides, so water drips off clear of the wall. Badly fitted sills are a common reason why water gets behind the insulation.

Plinth and ground

The bottom edge of the insulation either sits on a base rail, or the plinth is insulated down below ground with board that does not absorb water. The facade render should not run down to the ground, where rain splashes it. A paved strip or a gravel strip around the house helps lead water away from the plinth.

What you will hang on the facade

Do not fix an awning, external blinds, lights, a letterbox, an air-conditioning outdoor unit or railings into the polystyrene alone. For heavier items, mounting blocks or brackets are fitted while the insulation goes up. They carry the weight into the wall without creating a cold spot. In practice they are often forgotten, and a few months later someone is drilling and cutting holes in the new facade. So think ahead about everything that will end up on the house.

The reinforcing layer with fibreglass mesh holds the whole system together. The mesh is pressed into the base coat so it sits in the middle or upper third of the layer, not on top and not against the insulation. Strips overlap by roughly 10 cm. Extra diagonal strips go at the corners of windows and doors to take up the stress that would otherwise crack the render.

When the reinforcing layer has dried, a primer and a thin-coat render are applied. You can choose between these types:

• Silicone: Very good at shedding water and dirt, suits exposed and north-facing walls
• Silicate: Lets vapour through, naturally resists algae and works well with mineral wool
• Acrylic: Cheaper and flexible, but gets dirty faster and algae grow on it more easily
• Mineral: Very vapour-open, usually finished with a coat of facade paint

When you pick a colour, think about how much light it reflects. Dark shades get very hot in the sun and put the whole system under strain. System manufacturers therefore set a minimum light reflectance and offer special renders and paints for dark facades. A coarser grain hides small irregularities, while a fine grain shows them up.

The whole system should come from one manufacturer as a single certified kit. If adhesive, mesh and render from different brands are mixed, you lose the system warranty.

Adhesives, base coats and renders are usually applied at air and wall temperatures of roughly +5 °C to +30 °C. Not in rain, strong wind or direct sun. In summer, the sunny side is rendered in the morning or late afternoon, or the scaffolding is covered with netting. Render that dries too fast ends up patchy and uneven in colour.

Insulating an ordinary family house usually takes several weeks. It depends on the weather, how many corners and recesses the facade has and the size of the crew. The waiting times between layers given in the technical data sheet must be respected. Late spring and early autumn are good periods for the work. Reliable contractors are often booked months ahead, so it pays to contact them early.

In a block of flats, the facade is a common part of the building. The owners decide on insulation by voting under the rules that apply to the owners' association, and the building management usually handles the preparation. The work follows a design that also covers fire safety, for example non-combustible barriers above windows on taller buildings.

For a family house, what the authorities require depends on the scope of the work and on whether the look of the building changes. Local building regulations vary, and some places need only a notification while others need a permit. Do not assume nothing is needed. Check with your local building authority before work starts. If the house is listed or stands in a conservation area, any change to the facade has to be agreed with the heritage authority first.

Many countries run public schemes that support energy renovation of homes, including wall and loft insulation. Conditions, eligible work, contractor requirements and the amount of support change from one round to the next. Read the current conditions before you sign with a contractor. Some schemes require you not to start any work before your application has been submitted.

External wall insulation needs scaffolding, an experienced crew and strict adherence to the rules of a certified system, so it is not a job for DIY. Insulating the ceiling below a loft that is not used or walked on, however, is well within reach of a handy homeowner and pays back fairly quickly. If you are unsure about the wiring, call an electrician, and talk to a chimney sweep about the area around the chimney.

1. Inspect the loft: make sure the roof does not leak, the timber shows no rot or insect damage and the wiring is in good order. Do not bury cables in insulation until you know they are rated for it.
2. Clear out rubble, dust and old materials.
3. Lay a vapour control layer over the ceiling, overlap and tape the joints and turn it up at the external walls so moist indoor air cannot get into the insulation.
4. Lay mineral wool in two layers with staggered joints, butted tightly together with no gaps, so no cold spots form.
5. Keep combustible materials at the required distance from the chimney and use non-combustible insulation around it.
6. If you will need to walk in the loft, build walkways on the joists, not directly on the insulation.
7. Leave space for roof ventilation and keep the vents in the roof space clear.
8. Wear gloves, safety glasses, a respirator and long sleeves while you work.

Check the facade in low, raking light, in the morning or early evening. That is when you see unevenness that disappears in midday light. Things to look at:

• The wall is flat, with no waves and no board joints showing through
• Corners and reveal edges are straight and fitted with corner beads
• There are no cracks around the windows and the frame junction is made with a window bead
• Sills slope away from the window, have side end caps and project beyond the render
• The render has an even colour and texture, with no patches or visible joins
• Junctions at the roof, plinth and around pipes and cables are sealed
• Windows, paving and the surroundings are clean of adhesive and render
• You have received documents for the insulation system used, the warranty and ideally photos of bonding and anchoring

Write any defects into the handover report before the contractor takes down the scaffolding. Once it has gone, many details are out of reach, and fixing them costs considerably more.