Home Insulation Guide: U-Values, Costs and Payback by Measure

Heat flows from warm to cold, through every surface of a building and every gap in it. Insulation slows the flow, and airtightness stops warm air from leaking out. Between them they set how much energy a house needs before you even choose a boiler or heat pump. The European Commission puts buildings at roughly 40% of EU final energy consumption and about a third of energy-related emissions, and most of the buildings that will stand in 2050 are already built. This guide covers the physics, puts each measure into kWh and euros, and shows where your money goes furthest.

The physics: U-values and heat loss

How well a material conducts heat is given by its thermal conductivity λ (W/mK). In a layered building element, each layer has a thermal resistance R = thickness / λ. The U-value is the inverse of the total resistance, surface resistances included. The lower the U-value, the better the insulation.

Heating degree days measure how cold a climate is: about 2,000–2,500 in London and Paris, 3,000–3,500 in Berlin, Warsaw or Bucharest, 3,500–4,000 in Stockholm or Toronto, and 4,500+ in Helsinki or Minneapolis (base 15.5–18 °C depending on the source).

Typical U-values

Common insulation materials

Where the heat goes

In an uninsulated 1960s detached house, losses typically split like this: walls 25–35%, roof 20–25%, ventilation and air leakage 15–25%, windows and doors 10–20%, floor 10–15%. Flats lose much less through the roof and floor because the neighbours act as a buffer. In poorly detailed buildings, thermal bridges (balconies, lintels, wall–floor junctions) add another 5–15%.

Measures ranked by cost-effectiveness

Costs are indicative for Western Europe in 2024–2026; Eastern Europe is often 30–50% cheaper. Savings assume a gas-heated house paying about €0.11/kWh for delivered fuel in a 3,000 HDD climate.

On energy savings alone, walls and windows look like poor value. In practice they are usually done alongside façade repairs or window replacements that were due anyway. Then only the marginal cost of the efficiency upgrade counts, and payback gets much shorter. The comfort gains are real too: no cold walls, no draughts, no condensation. Model your own house in the insulation savings calculator.

Worked example: insulating a loft

Take a 1970s bungalow in central Germany with 80 m² of loft floor and 60 mm of old mineral wool (U ≈ 0.55 W/m²K). Climate: 3,300 HDD. Heating: gas boiler with 88% seasonal efficiency at €0.11/kWh.

At €22/m² installed (€1,760), it pays back in about 5.4 years and keeps saving for 40+ years after that. CO₂ saved: 2,952 kWh × 0.202 kg/kWh ≈ 0.6 tonnes/year. With a heat pump at SPF 3.2 and €0.30/kWh instead, the same work would save 812 kWh of electricity, about €244/year.

The degree-day method overstates savings somewhat when internal and solar gains are high. Monitored homes usually achieve 60–90% of the modelled figure, partly because people spend some of the saving on warmer rooms (the “comfort take-back” or rebound effect).

Ventilation and airtightness

The better the fabric, the larger the share of heat lost through air leakage. A blower-door test measures airtightness as n50 (air changes per hour at 50 Pa) or q50 (m³/h·m²). Old houses often score n50 = 6–15. Modern codes ask for about 3–5 without mechanical ventilation, and passive houses for ≤0.6.

Sealing a 400 m³ house from n50 = 10 to n50 = 4 cuts infiltration from about 0.5 to 0.2 ACH, which saves roughly 0.33 × 0.3 × 400 × 3,300 × 24 / 1,000 ≈ 3,100 kWh/year. The rule is “build tight, ventilate right”. An airtight house needs controlled ventilation: trickle vents and extract fans at the very least, or mechanical ventilation with heat recovery (MVHR), which recovers 75–90% of the heat in the exhaust air.

Moisture, mould and thermal bridges

Insulation moves the dew point inside a wall. Get it wrong and you get condensation, rot and mould. Some ground rules:

• Vapour control on the warm side. Internal insulation needs either a vapour control layer or a capillary-active, vapour-open system (calcium silicate, wood fibre). Never leave a gap behind it.
• Keep solid walls breathable. Historic solid masonry often needs lime-based, vapour-open systems.
• Check exposure before cavity fill. In walls facing driving rain, water can wick across the fill. A survey should check for this.
• Treat junctions. Carry insulation around window reveals and floor junctions so mould has no cold spots to start from.
• Keep ventilation. Do not block air bricks under suspended floors or ventilation in cold roofs.

Insulation and heat pumps

The two work together. Cut a house’s design heat loss from 12 kW to 8 kW and you can fit a smaller unit, often €1,500–3,000 cheaper, and run the radiators at a lower flow temperature, which lifts seasonal efficiency by 10–20%. You don’t need passive-house standard, though. A deep retrofit costing €50,000+ saves less per euro than a heat pump combined with the cheap insulation measures. Compare scenarios in the heat pump calculator and read Heat Pumps Explained.

Subsidies and policy

Most European countries subsidise insulation. Examples include Germany’s BEG programme, France’s MaPrimeRénov’, the UK’s ECO and Great British Insulation Scheme, Italy’s Ecobonus, and national Recovery and Resilience Plan schemes across the EU. The revised EU Energy Performance of Buildings Directive (2024) requires member states to cut average residential primary energy use by 16% by 2030 and 20–22% by 2035, starting with the worst-performing buildings. See what applies in your country on our country pages.

Common mistakes to avoid

• Compressing loft insulation. Mineral wool works by trapping air. Squash it under boards to make storage space and you can halve its performance. Raise the boards on loft legs or use rigid boards above the joists instead.
• Leaving gaps. Heat takes the path of least resistance, so a 5% gap in an insulation layer can raise heat loss through that element by 20–30%. Fit batts tightly, stagger the joints and seal around services.
• Forgetting the loft hatch and pipes. An uninsulated, unsealed hatch acts as a chimney for warm air. Once the loft turns cold, its pipes and tanks need insulating against frost.
• Insulating without ventilation. Blocking trickle vents, air bricks or eaves ventilation to “keep the heat in” traps moisture. Plan the ventilation; don’t leave it to chance.
• Ignoring summer overheating. Well-insulated homes with big south- or west-facing windows can overheat. External shading, night ventilation and roof insulation with high heat capacity (wood fibre, cellulose) keep summer temperatures down.
• Choosing on λ alone. A thin, high-performance board helps where space is tight. Usually, though, the cheapest way to hit a target U-value is a thicker layer of a cheaper material. Compare cost per m² at the target U-value, not price per board.
• Skipping quality checks. Ask for photos before surfaces are closed up, and consider a thermal imaging survey when the job is done. Poor workmanship is the most common reason measured savings fall short of the forecast.

A sensible order of works

1. Survey. An energy audit with a blower-door test and thermal imaging (on a cold day) finds the leaks and weak spots.
2. Cheap and fast. Draught-proofing, loft insulation, hot water tank and pipe insulation, radiator reflector panels on external walls.
3. Medium. Cavity wall fill, suspended floor insulation, secondary glazing.
4. Combine with renovations. External wall insulation when the render needs repair, internal insulation when you replaster, new windows when the old ones fail (with careful detailing at the reveals).
5. Then the heating system. Size the heat pump to the improved building.

Bottom line

Insulation keeps paying for decades and has no moving parts. Start with airtightness and the loft, which often pay back within five years, and deal with walls and windows when they need work anyway. Take the U-value change, multiply by area and degree days, and you have a decent savings estimate. Or let the insulation savings calculator and heating cost comparison do the maths for your home.

Figures in this guide are typical ranges drawn from the sources above; your own numbers depend on location, prices and equipment. Use the linked calculators for site-specific estimates.