XPS or Foam Glass Under Foundations?

10 September 2026

For most foundation and ground-slab applications in Central and Eastern Europe, XPS — specifically PNP XPS 300 or XPS 500 — offers compressive strength up to 500 kPa, a declared thermal conductivity (λD) of 0.034 W/(m·K), and long-term water absorption rated WL(T) 0.7 (≤ 0.7 Vol.-%), all at significantly lower installed thickness than foam glass for the same thermal resistance. Foam glass can be preferable where fire performance above Euroclass E is a regulatory requirement or where point loads exceed the capacity of available XPS grades. A structural engineer and a thermal designer must confirm the final material choice for each project.

Direct Answer

For the vast majority of residential and commercial foundation details — strip foundations, slab-on-grade, Swedish slab, perimeter insulation — XPS is the technically sound and material-efficient choice. PNP XPS 300 (boards above 30 mm) carries a compressive strength of 300 kPa at 10 % deformation or yield (CS(10\Y)300), a λD of 0.034 W/(m·K), and long-term water absorption by immersion of WL(T) 0.7 (≤ 0.7 Vol.-%). PNP XPS 500 raises the compressive ceiling to 500 kPa where heavier loads apply. Foam glass is worth considering when the project specification demands a reaction-to-fire class better than Euroclass E, or when local regulations explicitly require an inorganic insulation material — this alone does not prove XPS is unsuitable, but it is a decision that requires confirmation by a qualified designer.

Why the Question Matters

Insulation beneath a foundation slab or along a perimeter wall lives in one of the most demanding environments in a building: permanent compressive load, contact with ground moisture, possible freeze–thaw cycling, and zero access for remediation once the structure is built. Getting the material choice wrong costs far more to correct than to specify correctly at the outset.

The two materials most commonly compared for this position are extruded polystyrene (XPS) boards to EN 13164 and foam glass boards (cellular glass) to EN 13167. Both are rigid, load-bearing, and moisture-resistant. Their differences matter in specific scenarios.

Key Properties Compared

The table below uses declared values from PNP XPS technical data sheets (EN 13164:2012+A1:2015). Foam glass values are shown as typical published ranges — confirm with the foam glass manufacturer's DoP for any specific project.

Property	PNP XPS 300 (>30 mm)	PNP XPS 500	Foam glass (typical range)
Compressive strength (CS at 10 % / yield)	300 kPa	500 kPa	400–1 600 kPa (grade-dependent)
λD, W/(m·K)	0.034–0.035	0.034–0.035	~0.038–0.045 (see TDS)
Long-term water absorption (immersion)	WL(T)0.7 (≤ 0.7 Vol.-%)	WL(T)0.7 (≤ 0.7 Vol.-%)	Virtually zero (closed cells)
Water absorption by diffusion	WD(V)3 (≤ 3.0 Vol.-%)	WD(V)3 (≤ 3.0 Vol.-%)	Negligible
Freeze–thaw resistance	FTCD1 (≤ 1 Vol.-%)	FTCD1 (≤ 1 Vol.-%)	Inherently resistant
Reaction to fire (Euroclass)	E	E	A1 (non-combustible)
Compressive creep (CC at 2%/1.5%/50 yr)	CC(2/1.5/50)130	CC(2/1.5/50)150	See manufacturer TDS
Governing product standard	EN 13164:2012+A1:2015	EN 13164:2012+A1:2015	EN 13167

Foam glass typical ranges are indicative only. Always use the manufacturer's current DoP for design.

Where XPS Has a Clear Advantage

Thermal efficiency per millimetre. PNP XPS 300 achieves λD 0.034 W/(m·K). Because foam glass typically has a higher λD, reaching the same thermal resistance (RD) requires greater thickness — relevant where floor-to-ceiling height or foundation geometry is constrained.
Load-bearing range of 300–500 kPa. For residential and most commercial slabs this covers typical design loads. PNP XPS 500 (CS(10\Y)500) and XPS 700 (CS(10\Y)700) extend coverage to industrial floors and heavy civil infrastructure.
Freeze–thaw durability. The test showed FTCD1 (≤ 1 Vol.-% change after freeze–thaw cycling following diffusion water absorption) for PNP XPS 300 and XPS 500 — this supports long-term performance in unheated perimeter and below-grade positions in Central and Eastern European climates.
Proven foundation system integration. PNP XPS is used in shallow foundations, Swedish slab, slab-on-grade, and perimeter/plinth details — see the PNP solutions catalogue.
Dimensional stability. Both products are rated DS(70,90), meaning volume change is controlled under sustained temperature and humidity.

Where Foam Glass May Be Preferred

Fire class requirement above Euroclass E. XPS 300 and XPS 500 are both classified Euroclass E. If the authority having jurisdiction or the project specification requires Euroclass A1 or A2 (non-combustible), foam glass satisfies that requirement; XPS does not. A fire engineer must confirm applicability to the specific subgrade installation.
Inorganic material requirement. Some project briefs or local standards explicitly require inorganic insulation in contact with soil or concrete; foam glass is inorganic. This alone does not prove XPS is non-compliant in all jurisdictions — verify against the applicable national building code.
Extremely high point loads. High-grade foam glass can reach compressive strengths well above 700 kPa. Where point loads exceed the capacity of PNP XPS 700 (CS(10\Y)700), foam glass or engineered load-distribution layers should be reviewed with a structural engineer.

Selecting the Right PNP XPS Grade for Foundations

The correct grade depends on the permanent compressive load, the required thermal resistance, and the exposure class. A structural engineer must calculate the design load at insulation level. As a starting reference:

XPS 300 — foundations and ground slabs under standard residential and commercial loads (≥ 300 kPa for boards above 30 mm). XPS 300 product page
XPS 500 — industrial floors, parking decks, cold-store floors, road construction (≥ 500 kPa). XPS 500 product page
XPS 700 — extreme civil engineering and infrastructure loads (≥ 700 kPa). XPS 700 product page
XPS 300 W — where the insulation face must bond to waterproofing membranes or adhesive compounds (waffled surface, same compressive class). XPS 300 W product page

For application guidance see Which PNP XPS Should I Choose? and What Does '300 kPa' Compressive Strength Mean?. Current TDS and DoP documents are available from the documentation page.

Sources

XPS 300 technical data sheet (EN): pnp_xps300-300TB_600x1250_techcard_en(20.07.2026).pdf (updated 2026-07-22)
XPS 300 Declaration of Performance: DoP PNP XPS 300 EMI_en (U).pdf (updated 2025-10-14)
XPS 300 W technical data sheet (EN): pnp_xps300W-300WTB_600x1250_techcard_en(20.07.2026).pdf (updated 2026-07-22)
XPS 300 W Declaration of Performance: DoP PNP XPS 300 W EMI_en (U).pdf (updated 2025-10-13)
XPS 500 technical data sheet (EN): pnp_xps500-500TB_600x1250_techcard_en(20.07.2026).pdf (updated 2026-07-22)
XPS 500 Declaration of Performance: DoP PNP XPS 500_en (U).pdf (updated 2025-07-22)
XPS 700 technical data sheet (EN): pnp_xps700_600x1250_techcard_en (21.07.2023).pdf (updated 2025-03-17)
XPS 700 Declaration of Performance: DoP PNP XPS 700_eng.pdf (updated 2025-03-17)
XPS 400 technical data sheet (EN): pnp_xps400-400TB_600x1250_techcard_en(20.07.2026).pdf (updated 2026-07-22)
EN 13164:2012+A1:2015 — Factory made products of extruded polystyrene foam (XPS)
PNP XPS solutions catalogue (foundations, slab-on-grade, shallow foundation, Swedish slab details)

Frequently asked questions

Can XPS be used directly in contact with soil and ground moisture?

Yes. PNP XPS 300 and XPS 500 are rated WL(T)0.7 (≤ 0.7 Vol.-%) for long-term water absorption by immersion and FTCD1 for freeze–thaw resistance, making them suitable for below-grade and perimeter applications in contact with soil and groundwater. Confirm the specific exposure class and drainage design with a qualified designer and the current TDS.

What compressive strength does a typical house foundation slab require?

This depends on the structural load at insulation level, which a structural engineer must calculate for the specific project. As a reference, PNP XPS 300 delivers 300 kPa (CS(10\Y)300) for boards above 30 mm, which is commonly specified for residential slab-on-grade; heavier structures may require XPS 500 (500 kPa) or XPS 700 (700 kPa). Always verify with the current TDS and a structural engineer.

Is the fire class of XPS a problem under a concrete foundation slab?

PNP XPS 300 and XPS 500 carry Euroclass E. In a fully encapsulated subgrade position — sandwiched between concrete and soil — the practical fire risk is very low, but whether Euroclass E meets the requirements of a specific project or national building regulation is a decision for the fire engineer and the authority having jurisdiction, not a determination that should be made from this article alone.

Does XPS creep under permanent load in a foundation?

PNP XPS 300 (boards above 30 mm) is rated CC(2/1.5/50)130, meaning the long-term compressive creep limit at 50 years is 130 kPa at a 2 % deformation criterion. PNP XPS 500 is rated CC(2/1.5/50)150. A structural engineer must verify that the permanent design load at insulation level does not exceed the applicable creep limit declared in the TDS.

Which PNP XPS surface type is correct under a foundation slab?

Smooth-surface boards (XPS 300, XPS 500) are the standard choice under concrete slabs. Waffled-surface boards (XPS 300 W) are used where adhesion of a waterproofing membrane or adhesive compound to the insulation face is required — for example in bonded waterproofing systems on podium slabs. See the XPS 300 W product page for details.

Where can I get the Declaration of Performance and technical data sheet for PNP XPS foundation grades?

DoP and TDS documents for all published grades are available on the PNP XPS documentation page at /documentation/. For project-specific queries, contact the technical team at sales@pnpinsulation.com.