Foundation and plinth — termPIR® AL

When to use this system

Foundation insulation with termPIR® AL on the external side (before backfilling) is the standard in modern construction for:

• Single-family house with a basement — insulating basement walls reduces heat losses from the lower storey by 40–60%.
• House without a basement, with a slab on grade — plinth insulation eliminates the thermal bridge at the floor–external wall interface (typically ~10% of building losses).
• Passive and energy-efficient buildings — a continuous insulation layer without interruption at the plinth is required.
• Commercial buildings — warehouses, production halls with slab on grade heated to +12°C in winter.

Why PIR rather than XPS in the foundation

Aspect	termPIR® AL	XPS (styrodur)
λ	0.022 W/(m·K)	0.034–0.036 W/(m·K)
Thickness for U ≤ 0.25	90 mm	~140 mm
Water absorption	< 2%	< 1%
Compressive strength	≥120 kPa	200–700 kPa
Rodent resistance	full	full
Price vs. efficiency	higher price/m³, lower thickness	cheaper/m³, greater thickness

PIR is worthwhile when:

• space for the insulation layer is limited (narrow gap when backfilling the excavation)
• you want to unify the insulation material across the whole building (the same termPIR® AL in foundation, walls and roof)
• you are building to passive standard (U below 0.15 required)

Installation requirements

• Waterproofing MUST be placed beneath the insulation — cold-applied bitumen membrane (e.g. Hyperdesmo) or 2-component bituminous coating. PIR does not replace waterproofing.
• Fixing — universal PU adhesives (e.g. Foamglas Adhesive) over the entire surface, alternatively bonding with butyl tapes at corners.
• Protect the insulation from backfilling — geotextile or dimpled membrane on the side facing the soil backfill, protects against mechanical damage.
• Above-ground plinth — above ground level a transition to termPIR® ETX (ETICS) or mosaic plinth render is recommended.

Technical documentation

termPIR® 2025 folder (Gór-Stal, 2025-04-23, pp. 12–13). Waterproofness confirmed by class WL(T)2 per EN 1609 (≤ 2% by weight). Biological resistance confirmed per EN 1604 (dimensional stability).

Layer composition

U-value by insulation thickness

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