Perimeter Insulation (External Basement Wall, Below Grade)

Externally insulating the earth-contact basement wall with moisture-resistant XPS or foam glass — usually carried out alongside drainage work.

How does perimeter insulation work?

Perimeter insulation is the external insulation of earth-contact cellar walls. For this, the cellar wall is exposed, waterproofed and fitted with 8 to 14 cm of moisture- and pressure-resistant insulation, usually extruded polystyrene (XPS) or foam glass. The typical build-up is: existing wall, waterproofing, insulation boards, drainage mat, backfill. Because the insulation lies permanently in damp soil, it must not absorb water and must withstand earth pressure; normal facade insulation materials are unsuitable here. The cellar wall's U-value drops from about 1.8 to 0.25 W/(m²·K). Since the ground in winter is considerably warmer than the outside air, the temperature difference at earth-contact surfaces is smaller; this is accounted for in the calculation by a ground factor of 0.5 according to ÖNORM B 8110-6. At the same time, the insulation protects the waterproofing mechanically and prevents the wall from cooling down and forming condensation on the inside.

Which buildings benefit from perimeter insulation?

The measure is worthwhile with a heated cellar or cellars used as living, office or storage space with temperature requirements. It is an option for detached houses as well as apartment buildings. The trigger is decisive for economic viability: if drainage refurbishment, new cellar waterproofing or plinth protection is due anyway and the wall is being exposed, the additional cost of insulation is low. As a standalone measure with its own excavation, however, it rarely pays off. Without the possibility of excavation, for example with boundary construction or closely adjacent services, it is ruled out.

What does perimeter insulation actually deliver?

The saving is 3 to 8 % of heating energy demand. For a detached house with 80 m² of earth-contact cellar wall, the saving is around 5,800 kWh of natural gas per year, equivalent to about 1.2 t of CO2. Costs are 90 to 180 €/m² including excavation and waterproofing, with a payback of 15 to 25 years over a 50-year service life. Bundled with drainage, plinth waterproofing and splash protection, perimeter insulation is a sensible complement to external wall insulation because it closes the thermal bridge at the plinth.

Energy carrier

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

Savings potential

3-8 %

Worked example

In a detached house with a heated cellar, the drainage is being refurbished, so the 80 m² of earth-contact cellar wall has to be exposed anyway. The uninsulated concrete wall has a U-value of 1.8 W/(m²·K). After applying the waterproofing, 12 cm XPS insulation boards and a drainage mat are fitted, reducing the U-value to 0.25 W/(m²·K). The location has 3,500 heating degree days, and heating is by gas at 90 % efficiency. Since the ground in winter is warmer than the outside air, a reduction factor of 0.5 is applied for the ground temperature. The calculation is: 80 × (1.8 − 0.25) × 3,500 × 0.024 / 0.9 × 0.5 = 5,787 kWh/a. The house thus saves around 5,800 kWh of natural gas per year and about 1.2 t of CO2.

Investment & payback

90-180 €/m² inkl. Aushub + Abdichtung · Service life: 50 a · Payback: 15-25 (sinnvoll im Zuge der Drainagesanierung) a

Applicability

Requirements

Exclusion criteria

Typical buildings

Frequently asked questions

How much does perimeter insulation cost per square metre?

Perimeter insulation costs 90 to 180 €/m² of cellar wall area including excavation, waterproofing, insulation boards and drainage mat. The largest cost share is earthworks and waterproofing, not the insulation material itself. If the wall is being exposed anyway for drainage refurbishment, the additional cost of the insulation drops to about 30 to 60 €/m². This is why insulation should always be planned in as soon as excavation is due.

Which insulation material is suitable for perimeter insulation?

For earth-contact components, only moisture-resistant and pressure-resistant insulation materials are permitted: extruded polystyrene (XPS) and foam glass are the standard solutions. XPS is cheaper and easier to work with; foam glass is completely watertight, non-combustible and suitable for high loads. Mineral wool, EPS facade boards or wood fibre absorb water and must not be used in the ground.

Is perimeter insulation worthwhile with an unheated cellar?

With an unheated cellar, cellar ceiling insulation from below is generally the more economical measure, because it requires no excavation and costs 35 to 70 €/m². Perimeter insulation is worthwhile mainly with a heated cellar or as a complement to cellar ceiling insulation when the wall is being exposed anyway for drainage refurbishment. An advantage then is protecting the cellar wall from cooling and condensation.

How thick does perimeter insulation need to be?

8 to 14 cm of XPS or foam glass is recommended. With 12 cm XPS, the U-value of an uninsulated concrete wall drops from about 1.8 to 0.25 W/(m²·K). Because the ground cools the wall less in winter than the outside air, the effect of additional insulation thickness is smaller than on the facade. OIB Guideline 6 specifies a U-value of no more than 0.40 W/(m²·K) for earth-contact walls in refurbishments.

Does perimeter insulation need drainage?

Not necessarily, but in most cases it makes sense. Drainage carries away standing water from the plinth and permanently protects waterproofing and insulation. Since the cellar wall is being exposed for the insulation anyway, the additional cost of drainage pipe and drainage mat is low. In cohesive soils with standing water, drainage according to ÖNORM B 3692 is practically indispensable; with permeable gravel soil, it can be omitted.

Related measures

Standards & sources

Metering points in the EDM Toolbox

Kellerwand-Temperatur (Bestand + Außenoberfläche neu) als Stichprobenmessung möglich, EDM-Effekt aber primär über Heizenergie-Witterungsbereinigung sichtbar.

All measures