At a glance: XPS foundation and perimeter insulation
- 🧊 Different job, different material: XPS (extruded polystyrene) is denser and more moisture-resistant than standard EPS, which is why it is specified below ground and not the facade board used for ETICS.
- 💧 Compressive strength matters below slab: XPS grades are rated by compressive strength (kPa), and the correct grade depends on ground bearing and loading conditions.
- 🏗️ Common applications: foundation perimeter insulation, insulation under slabs, inverted (upside-down) roofs and below-grade walls.
- 📋 Check the DoP: compressive strength, water absorption and thermal conductivity should all be confirmed on the Declaration of Performance before ordering.
Why XPS is a different specification decision to facade EPS
Irish self-builds and passive house projects increasingly specify insulation below ground level — under foundation slabs, around the building perimeter, and on inverted roof build-ups. This is a different job to facade insulation, and it calls for a different material: XPS (extruded polystyrene), not the EPS or graphite EPS boards used in ETICS facade systems.
Poland is a significant European manufacturer of XPS board, and Irish buyers can access a wide range of compressive strength grades directly from factory, provided the specification is confirmed correctly before ordering.
1. Compressive strength: the key specification
XPS boards are graded by compressive strength (commonly 300, 500 or 700 kPa), and the correct grade depends on where the board sits in the build-up:
- under slab, low loading: lower compressive strength grades are usually sufficient,
- under slab, heavier loading or car park decks: higher compressive strength grades are required,
- perimeter/foundation wall insulation: moderate compressive strength combined with high water resistance.
2. Moisture resistance and long-term thermal performance
Unlike EPS, XPS has a closed-cell structure that gives it very low long-term water absorption, which is essential for a material installed below ground or under a waterproofing membrane. Confirm the declared water absorption value and long-term thermal conductivity (not just the initial lambda value) on the technical datasheet.
The fix: always request the Declaration of Performance (DoP) alongside the technical datasheet, and check that the compressive strength grade matches your structural engineer's specification, not just the insulation supplier's default recommendation.
3. Applications and thickness
| Application | Typical compressive strength | Key consideration |
|---|---|---|
| Under slab (residential) | 300 kPa | Ground bearing conditions |
| Under slab (heavier loading) | 500–700 kPa | Structural engineer's specification |
| Perimeter/foundation wall | 300 kPa | Water resistance, drainage detailing |
| Inverted roof | 300–500 kPa | UV-stable protection layer required above board |
Sourcing XPS for your foundation or perimeter insulation?
Actitrade sources XPS insulation boards from established Polish manufacturers, checks compressive strength and moisture resistance against your specification, and arranges delivery to your Irish site.
Start with our building materials sourcing proposal or contact Actitrade to discuss your project.