Internal Wall Insulation

Applying the insulation layer on the room side — the option where external insulation is ruled out by heritage protection, lack of space or an articulated historic facade.

How does internal insulation of external walls work?

With internal insulation, the insulation layer is applied to the room side of the external wall, usually in thicknesses of 8 to 10 cm. Unlike external insulation, the masonry remains cold because it lies outside the insulation layer. This makes the measure demanding in terms of building physics: warm, humid room air must not diffuse into the cold wall and condense there. Capillary-active systems made of calcium silicate or mineral foam, which absorb and release moisture, and vapour-permeable wood fibre build-ups have therefore proven effective. Alternatively, a vapour barrier – a membrane that blocks the passage of water vapour – is installed on the room side without gaps. Thermal bridges at adjoining internal walls, floor slabs and timber beam ends have a stronger effect with internal insulation than with external insulation and must be mitigated with insulation wedges or flanking insulation. With correct planning, the U-value drops from about 1.5 to about 0.35 W/(m²·K).

Which buildings benefit from internal insulation?

Internal insulation is the solution when external insulation is ruled out. Typical cases are listed buildings and articulated historic facades, buildings on the property boundary with no space outside, and individual flats in apartment buildings where the owners' association does not approve a facade refurbishment. Requirements are a building physics assessment of the wall construction, moisture management and locating existing services in the wall. Wherever external insulation is technically possible, it should always be preferred, as it is more effective and lower-risk.

What does internal insulation actually deliver?

Heating energy savings are 15 to 30 %, relative to the insulated wall areas. The thermal bridge factor is set at 0.70, considerably less favourable than for external insulation, because flanking losses remain greater. For an 80 m² flat with 50 m² of internally insulated external wall, the saving is around 3,600 kWh of natural gas per year, equivalent to about 730 kg of CO2. The investment is 120 to 280 €/m², with calcium silicate at the upper end. Over a 35-year service life, the measure pays back in 15 to 25 years. A side benefit is the significantly higher surface temperature of the wall, which improves comfort and protection against mould.

Energy carrier

Erdgas, Heizöl, Pellet, Fernwärme, Strom → Erdgas, Heizöl, Pellet, Fernwärme, Strom

Savings potential

15-30 %

Worked example

In a late-19th-century (Gründerzeit) apartment building with a protected facade, an 80 m² flat is to be energetically improved. As external insulation is not permitted, the flat's 50 m² of external wall area is insulated on the room side. The existing wall has a U-value of 1.4 W/(m²·K); after internal insulation, 0.35 W/(m²·K) is achieved. The location has 3,500 heating degree days, and heating is by gas at 85 % efficiency. Because thermal bridges at floor slabs and internal walls have a stronger effect with internal insulation, a correction factor of 0.70 is applied. The calculation is: 50 × (1.4 − 0.35) × 3,500 × 0.024 / 0.85 × 0.70 = 3,633 kWh/a. The flat thus saves around 3,600 kWh of natural gas per year, corresponding to about 730 kg of CO2.

Investment & payback

120-280 €/m² (Kalziumsilikat höchstpreisig) · Service life: 35 a · Payback: 15-25 a

Applicability

Requirements

Exclusion criteria

Typical buildings

Frequently asked questions

How much does internal wall insulation cost per square metre?

Internal insulation costs 120 to 280 €/m² of wall area including installation and surface finish. Calcium silicate boards are at the upper end of the range, mineral foam and wood fibre systems below. Additional costs arise for building physics planning, relocating radiators, sockets and services, and connection details at window reveals. The loss of floor space of 8 to 10 cm per external wall should be factored into the economic assessment.

Does internal insulation need a vapour barrier?

That depends on the system. Capillary-active insulation materials such as calcium silicate or mineral foam manage without a vapour barrier because they absorb any condensation and release it again when the room air is dry. With mineral wool or rigid foam, a room-side vapour barrier or vapour retarder is essential and must be airtightly taped at all junctions. Condensation protection is verified according to ÖNORM B 8110-2 or DIN 4108-3 and WTA Guideline 6-4.

Is internal insulation permitted in listed buildings?

Yes, internal insulation is the standard solution for listed buildings because the external appearance remains unchanged. In Austria, consultation with the Federal Monuments Office (Bundesdenkmalamt) is nevertheless required, particularly if historic interior features such as stucco or wood panelling are affected. With timber beam floors, the embedding of the beam ends in the cold wall is a critical detail that justifies moisture measurement or sensor monitoring.

What is the difference between internal and external insulation?

With external insulation, the masonry lies within the warm insulation envelope, stores heat and stays dry; thermal bridges are easy to manage. With internal insulation, the wall remains cold, the thermal mass is lost, and junctions with floor slabs and internal walls form intensified thermal bridges. This is why only U-values of around 0.35 W/(m²·K) are sensibly achievable with internal insulation, compared with 0.15 to 0.20 W/(m²·K) with external insulation. External insulation is therefore always preferable where possible.

What is the maximum thickness for internal insulation?

In practice, 8 to 10 cm is recommended, depending on the system even just 5 to 6 cm. The thicker the internal insulation, the colder the masonry behind it becomes and the deeper the condensation plane moves into the wall. Thicker build-ups increase the moisture risk but add only little further energy benefit. The resulting U-value is therefore usually between 0.25 and 0.50 W/(m²·K).

Related measures

Standards & sources

Metering points in the EDM Toolbox

Oberflächentemperatur- und Feuchtesensoren in kritischen Bereichen (Balkenköpfe, Innenecken) helfen, ein Tauwasserrisiko frühzeitig zu erkennen. Empfohlen: kombinierte T/rH-Sensoren mit Funkanbindung.

All measures