Three systems, one decision
The question "what does insulating a house cost" has an awkward answer: the price of the insulation itself is the least interesting line in the budget. What drives the total is the height and articulation of the facade, meaning scaffolding, then the number of openings and details, the condition of the existing render, and everything that has to be solved alongside the insulation: the plinth, the balconies, the eaves, the sheet metal work. That is why two quotes for the same house can differ by tens of percent while both claim to offer "the same insulation".
In practice the owner of an older family house is choosing between three build-ups: an external thermal insulation composite system, ETICS with expanded polystyrene, the same system with mineral wool, and a ventilated rainscreen facade. The difference between them is not in thermal conductivity, even though that is what most sales arguments lean on. The lambda values sit so close together that one or two centimetres of thickness cancels the gap. Four other things decide: fire behaviour, vapour permeability, robustness and cost.
What actually separates the three
| Criterion | ETICS with EPS | ETICS with mineral wool | Ventilated facade |
|---|---|---|---|
| Insulation lambda W/(m·K) | 0.031–0.040 | 0.034–0.040 | depends on the infill, usually wool at 0.034–0.040 |
| Reaction to fire | class E, combustible, the system is assessed as a whole | class A1 to A2, non-combustible | depends on infill and cladding, the cavity changes fire spread |
| Vapour permeability | low, closed build-up | high | highest, the cavity vents moisture continuously |
| Mechanical robustness | low at ground-floor level | medium | high, cladding takes impact and pressure |
| Installation demands | lowest | higher, heavier material and more precise fixing | highest, needs a designed subframe and a sound substrate |
| Relative system cost | the baseline, cheapest | modestly above EPS | typically a multiple of ETICS |
| Surface maintenance | repainting after some years, algae on north walls | same as EPS | cladding replaceable panel by panel, cavity needs clearing |
| Where it makes sense | dry masonry house, ordinary geometry, tight budget | fire-critical situations, damper masonry | exposed walls, ground floor, combined with ETICS |
The practical reading: ETICS with polystyrene is the standard for a dry masonry house with ordinary geometry, and it is the cheapest. Mineral wool is chosen where fire is a factor or where you want the masonry to keep drying outwards. A ventilated facade is a different discipline altogether, with a load-bearing subframe, fixings into a sound substrate and cladding you also choose aesthetically. On a family house it usually pays off as a combination: stone or panel cladding on the ground floor and plinth of the exposed elevation, ETICS everywhere else.
Thickness: the first centimetres change the house, the last change the budget
Thickness is not designed by what fits. It follows from the U-value of the whole build-up and from what the next layer still buys you. Thermal resistance grows linearly with thickness, but heat loss falls hyperbolically. The first five centimetres do most of the work and every further five add less. A model calculation for a 450 mm solid brick wall, insulation at lambda 0.037 W/(m·K), ignoring thermal bridges:
| Insulation thickness | Wall U-value W/(m²·K) | Loss reduction against the original wall | Gain from that step alone |
|---|---|---|---|
| none | 1.40 | 0 % | starting point |
| 5 cm | 0.48 | 65 % | 65 pp |
| 10 cm | 0.29 | 79 % | 14 pp |
| 15 cm | 0.21 | 85 % | 6 pp |
| 20 cm | 0.16 | 88 % | 3 pp |
| 25 cm | 0.13 | 90 % | 2 pp |
The last column is the whole argument. There is a counter-argument that gets lost in payback discussions, though: thickness is always cheaper today than in fifteen years, when it would mean erecting the scaffolding again and reworking every reveal again. The sensible band therefore sits where insulation is still cheap relative to scaffolding and details, but does not yet cause disproportionate trouble at the reveals and the eaves. The actual figure comes from a designer working with the full wall build-up, not from a table of averages.
Four details that decide the outcome
Insulation is mostly area, yet failures arise almost exclusively in the details. A thermal bridge at a junction can undo a precisely designed thickness across the field, because it enters the balance as a linear thermal bridge coefficient, the Psi-value, counted per running metre of junction rather than per square metre of wall.
- The window reveal. A window sitting in the original plane of the masonry ends up at the bottom of a deep recess. The reveal has to be covered with a thinner layer of insulation, typically 20–40 mm, carried over the frame rather than merely up to it. If the windows are being replaced, they belong in the plane of the insulation or as close to it as possible. An unresolved reveal is the most common reason mould appears in the window corner after a renovation.
- The plinth and the junction to the foundation. Insulation must not stop above ground. It continues into perimeter insulation in extruded polystyrene, taken below grade and protected against moisture and backfill pressure. Skip that strip and you create a continuous thermal bridge around the entire building, and the cold floor stays cold.
- The eaves. Adding 16 to 20 cm to the wall shortens the existing overhang by exactly that much. A short overhang means more water on the facade, more stress on the render and faster soiling. The fix is extending the rafters or building the eaves out, and it has to be priced up front, not improvised from the scaffold.
- Balconies and penetrations. A cantilevered slab passing through the wall is a textbook thermal bridge that insulation wraps rather than removes. Wrapping the slab above, below and along the edge helps, but the full answer is to cut it off or replace it with a free-standing structure. The same goes for canopies, brackets and every fixing that now has to pass through the insulation into the masonry.
What usually goes wrong
- Render cracks running diagonally out of window corners. The cause is usually a missing diagonal mesh reinforcement at the corner of the opening, or boards laid so that the joint between them continues the line of the reveal.
- Debonding and poor adhesion from bonding onto a friable substrate. An adhesion test and, where needed, removal of the old render belong before the quote, not in the complaint.
- Algae and greenish patches on the north elevation. The thin outer layer of an ETICS has little thermal mass, cools below dew point overnight and is wet in the morning. Moisture plus shade equals algae. Wider eaves, water led away from the facade, a render with low water absorption and distance from mature planting all help. Biocidal additives wash out over time and do not solve it on their own.
- Fixings telegraphing through the render. Anchor heads show up after a season as a regular grid. It is a cosmetic rather than a functional fault, avoided with recessed anchors and insulation plugs.
Moisture and internal insulation
Internal insulation is not an option you pick by preference. It is the answer for situations where the facade cannot be touched: a protected street elevation, a plot boundary on the face of the wall, a party wall shared with a neighbour. It is physically worse for a simple reason. The masonry stays outside, in the cold zone, and the dew point moves into the interface between insulation and wall, where condensate has nowhere to dry to.
That is why it is designed with a hygrothermal calculation and along one of two logics. Either the build-up is closed with a vapour retarder of variable diffusion resistance, which keeps moisture out of the wall in winter and lets it dry inwards in summer. Or a capillary-active material is used, for example wood fibre insulation or calcium silicate, which carries moisture back to the surface itself. Both share the same conditions: thickness is deliberately limited, partitions and floor slabs remain thermal bridges, and the junction details have to be drawn. Polystyrene glued on from the inside and covered with plasterboard is the most reliable way to grow mould behind the furniture.
Sequence and cost: what actually pays
Two sequences waste money even when the workmanship is flawless. The first: insulating before replacing the windows. The reveal gets done twice and worse the second time, the scaffolding goes up twice, and the new window still does not end up in the plane of the insulation. The second: insulating before fixing rising damp. Insulation does not remove moisture, it only closes its way out. The full order of measures is covered in a separate piece on the order of steps in a deep renovation; here the rule is enough: windows and damp come before the facade.
Cost can only be managed once you know what makes it. Instead of hunting for a price per square metre online, ask every contractor for answers to these questions:
- Is scaffolding included, with delivery, erection and the number of weeks of hire? On sloping terrain or where public space has to be occupied, this is the line that moves most.
- What is the result of the adhesion test on the existing render, and what happens if it fails?
- How many anchors per square metre, of what type, and who determined that?
- How are the reveals handled, at what thickness and with which profiles?
- Where does the plinth insulation end, and how much digging is involved?
- Do all components come from one certified system, meaning adhesive, boards, anchors, mesh and render? Mixing components voids both the system certificate and the warranty.
- Who prepares the fire safety assessment, and what does it imply for the bands around openings?
Part of the budget can be covered by state support. The conditions change with every call, so they are not repeated here; the current mechanism and the application process are set out in the guide to the Obnov Dom subsidy.