Skip to main content

Foundation & Floor Slab XPS Insulation

Foundation & Floor Slab XPS Insulation

PNP XPS boards with a compressive strength of 300 kPa or higher are the correct choice for perimeter and under-slab insulation, where sustained ground pressure, moisture and freeze–thaw cycling demand a board that retains its dimensions and thermal resistance indefinitely. XPS 300 (smooth) and XPS 300 W (waffled, for bonded waterproofing or render layers) are the primary products; XPS 500 and XPS 700 are available where industrial or civil-engineering loads exceed 300 kPa. Required insulation thickness must be determined by a qualified designer in accordance with the national energy and structural codes of the country of use.

Why XPS Is Used Below and Around Foundations

Insulation at foundation level must perform under conditions that would degrade many other materials: permanent compressive load from the structure above, long-term contact with damp or saturated soil, freeze–thaw cycling in the frost-active zone, and decades of service without access for inspection or replacement. Extruded polystyrene (XPS) conforming to EN 13164:2012+A1:2015 is engineered for exactly these conditions.

  • Closed-cell structure: the homogeneous cell network keeps long-term water absorption by total immersion at WL(T) ≤ 0.7 Vol.-% for smooth-surface structural grades, so thermal resistance does not degrade in permanently wet ground.

  • Freeze–thaw resistance: XPS 300, XPS 500 and XPS 700 carry the FTCD 1 rating (≤ 1 Vol.-% change after freeze–thaw cycling per EN 13164).

  • Compressive creep: long-term deformation under sustained load is quantified by the CC(2/1.5/50) class declared in each product's DoP, supporting confident design service lives.

  • Dimensional stability: all structural grades are rated DS(70,90), confirming stability at elevated temperatures and humidity.

Recommended PNP XPS Products

Select the grade whose compressive strength class covers the design load at 10 % deformation (or yield), as calculated by the structural designer. The table below lists the grades relevant to foundation and floor-slab applications.

ProductCompressive strengthλD (W/(m·K))Fire classWL(T)FTCDSurfaceTypical use caseXPS 300300 kPa (>30 mm) / 200 kPa (20–30 mm)0.034–0.035E≤ 0.7 Vol.-%FTCD 1SmoothUnder-slab, perimeter wall, general foundationXPS 300 W300 kPa (>30 mm) / 200 kPa (20–30 mm)0.034–0.035E≤ 0.7 Vol.-%FTCD 1WaffledPerimeter insulation bonded to waterproofing membrane or render; basement walls with adhesive finishXPS 400400 kPa0.034E≤ 0.7 Vol.-%—SmoothCar parks, cold stores, heavily loaded floor slabsXPS 500500 kPa0.034–0.035E≤ 0.7 Vol.-%FTCD 1SmoothIndustrial floors, road-adjacent foundations, high-load slabsXPS 700700 kPa0.034–0.035E≤ 0.7 Vol.-%FTCD 1SmoothExtreme civil-engineering loads, heavily trafficked infrastructure

Surface choice: use smooth-surface boards (XPS 300, XPS 500, XPS 700) where the insulation bears directly against a concrete slab or compacted granular layer. Use the waffled surface (XPS 300 W) where a waterproofing membrane, adhesive compound or render is bonded directly to the insulation face — the waffled texture provides the mechanical key required for reliable adhesion.

For perimeter insulation of plinths and basement walls where a render or finish coat is applied, also consider XPS 300 W or, for lighter residential construction, XPS 150 W X-Grip. Load selection remains the designer's responsibility; consult the current TDS and DoP for each grade.

Typical Layer Build-Ups (Indicative Only)

The sequences below are generic and indicative, derived from the PNP solutions catalogue. Actual specification — materials, thicknesses, fixings, drainage strategy — must be determined by a qualified designer to suit site conditions, structural loads and the national standards of the country of use.

Under-Slab Floor (Slab-on-Grade)

  1. Floor covering

  2. Cement-sand screed

  3. Polyethylene film (slip / vapour layer)

  4. PNP XPS thermal insulation (e.g. XPS 300)

  5. Sand and gravel blinding coat

  6. Geotextile separation layer

  7. Undisturbed or compacted subgrade

Floor on Reinforced Concrete Base (Suspended or Podium Slab)

  1. Floor covering

  2. Cement-sand screed

  3. Vapour barrier

  4. PNP XPS thermal insulation (e.g. XPS 300)

  5. Levelling screed

  6. Reinforced concrete floor slab

Perimeter / Foundation-Wall Insulation (Buried)

  1. Drainage membrane (studded or composite)

  2. PNP XPS thermal insulation (e.g. XPS 300 or XPS 300 W)

  3. PVC rondel (mechanical hold)

  4. Waterproofing PVC membrane

  5. Geotextile protection layer

  6. Foundation slab / wall

  7. Backfilled and compacted soil

Shallow (Strip or Raft) Foundation

  1. Wall structure

  2. Floor finish

  3. Sectional waterproofing

  4. Foundation slab

  5. Blind area around the building

  6. PNP XPS thermal insulation

  7. Coarse or medium sand

  8. Separation layer: geotextile

  9. Undisturbed soil

  10. Drainage

Heat-Insulated Slab Foundation (Swedish Slab)

  1. Wall structure

  2. Floor finish

  3. Sectional waterproofing

  4. Foundation

  5. Blind area around the building

  6. PNP XPS thermal insulation

  7. Coarse or medium sand

  8. Separation layer: geotextile

  9. Undisturbed soil

Cold-Store / Refrigeration Floor

  1. Floor covering

  2. Reinforced concrete floor slab with cooling system

  3. PNP XPS thermal insulation (e.g. XPS 500)

  4. Sand

  5. Cement-sand screed with heat-emitting piping (anti-frost layer)

  6. Undisturbed soil

Installation Notes

  • Substrate: lay boards on a flat, stable and adequately compacted granular layer (typically sand or gravel blinding). Voids beneath boards create stress concentrations under load.

  • Joints: offset board joints in successive layers so that no continuous joint runs through the full insulation thickness. Use L-edge boards where specified to minimise thermal bridging at joints.

  • Protection layer: always place a separation sheet (polyethylene film, geotextile) between the XPS and a wet concrete pour to prevent cement-paste infiltration of joints. The same layer acts as a slip plane to allow differential movement.

  • Perimeter boards: boards against foundation walls and plinths should be held in place during backfilling; PVC rondels or compatible adhesive dabs are commonly used. Backfill in controlled lifts to avoid point loads or impact damage.

  • Waterproofing sequence: for buried perimeter insulation the waterproofing membrane is typically applied to the structure first; the XPS board then protects the membrane mechanically during backfilling. Confirm the sequence with the membrane manufacturer.

  • Adhesive compatibility: only solvent-free, XPS-compatible adhesives or mortars may contact the boards. Solvent-based products can chemically attack polystyrene.

  • Waffled surface (XPS 300 W): when bonding a membrane or render to the waffled face, ensure the adhesive is pressed fully into the texture to achieve the bond area specified by the system supplier.

  • Stacking and storage: store boards flat, off the ground, protected from prolonged UV exposure and mechanical impact. XPS is not classified for permanent UV exposure.

Insulation Thickness Guidance

Insulation thickness for foundations and floor slabs depends on the national energy-performance regulation of the country of use, the local design winter temperature, ground thermal properties, and the overall thermal strategy of the building. PNP XPS boards are available in the following standard thickness ranges for the relevant grades:

  • XPS 300 / XPS 300 W: 20–150 mm (standard); TB variant extends the range — see current TDS for exact availability.

  • XPS 400: 40–100 mm (standard); TB variant available — see current TDS.

  • XPS 500: 40–120 mm (standard); TB variant available — see current TDS.

  • XPS 700: 20–150 mm — see current TDS.

Where a single board thickness is insufficient to meet the thermal target, two layers with staggered joints are routinely used. The declared thermal conductivity (λD) of all grades above is 0.034 W/(m·K) for thicknesses up to 100 mm and 0.035 W/(m·K) for thicknesses of 120 mm and above (XPS 400: 0.034 W/(m·K) up to 100 mm; see TDS). Thermal resistance (RD) for a specific thickness follows directly from RD = thickness / λD.

A qualified designer must calculate and specify the required thickness to satisfy the applicable national thermal regulation and any condensation / dew-point requirement. This page does not constitute a design instruction.

For background on compressive strength class selection, see the knowledge article What Does '300 kPa' Compressive Strength Mean? and the general guide Which PNP XPS Should I Choose?

Applicable Standard

All PNP XPS boards for foundation and floor-slab applications are manufactured and CE-marked in accordance with EN 13164:2012+A1:2015 — Thermal insulation products for buildings — Factory made products of extruded polystyrene foam (XPS) — Specification. Each product is accompanied by a Declaration of Performance (DoP) under CPR 305/2011.

Country-specific application standards, energy codes and structural design standards (e.g. Eurocode 7 for geotechnical design) apply in addition. Verify the requirements of the country of use with a qualified designer.

Documents to Download

All current technical data sheets (TDS), Declarations of Performance (DoP), BIM objects and the Environmental Product Declaration (EPD) are available from the PNP XPS Documentation page.

  • TDS — XPS 300 / XPS 300 TB (EN, updated 2026-07-22)

  • DoP — XPS 300 (EN, updated 2025-10-14)

  • TDS — XPS 300 W / XPS 300 W TB (EN, updated 2026-07-22)

  • DoP — XPS 300 W (EN, updated 2025-10-13)

  • TDS — XPS 400 / XPS 400 TB (EN, updated 2026-07-22)

  • DoP — XPS 400 (EN, updated 2026-04-10)

  • TDS — XPS 500 / XPS 500 TB (EN, updated 2026-07-22)

  • DoP — XPS 500 (EN, updated 2025-07-22)

  • TDS — XPS 700 (EN, updated 2025-03-17)

  • DoP — XPS 700 (EN, updated 2025-03-17)

  • EPD — PNP XPS, Várpalota plant (updated 2025-03-19)

  • PNP BIM Libraries (updated 2025-02-24)

For project enquiries or to request samples, contact PNP XPS sales or email sales@pnpinsulation.com.

Sources

  • XPS 300 technical data sheet (EN), updated 2026-07-22

  • DoP PNP XPS 300 EMI (EN), updated 2025-10-14

  • XPS 300 W technical data sheet (EN), updated 2026-07-22

  • DoP PNP XPS 300 W EMI (EN), updated 2025-10-13

  • XPS 400 technical data sheet (EN), updated 2026-07-22

  • DoP PNP XPS 400 (EN), updated 2026-04-10

  • XPS 500 technical data sheet (EN), updated 2026-07-22

  • DoP PNP XPS 500 (EN), updated 2025-07-22

  • XPS 700 technical data sheet (EN), updated 2025-03-17

  • DoP PNP XPS 700 (EN), updated 2025-03-17

  • XPS 150 W X-Grip technical data sheet (EN), updated 2026-07-22

  • EPD — PNP XPS, Várpalota plant, updated 2025-03-19

  • PNP BIM Libraries, updated 2025-02-24

  • EN 13164:2012+A1:2015 — Thermal insulation products for buildings — Factory made products of extruded polystyrene foam (XPS) — Specification

  • PNP solutions catalogue (foundations and buried structures, shallow foundation, Swedish slab, slab-on-grade floor, floors on reinforced concrete base, floors of refrigerators)

Frequently asked questions

Which PNP XPS grade should I specify under a residential ground-floor slab?

XPS 300 is the standard choice for residential under-slab insulation, providing 300 kPa compressive strength (for boards thicker than 30 mm) and a long-term water absorption rating of WL(T) ≤ 0.7 Vol.-%. The required thickness must be calculated by a qualified designer to meet the applicable national energy code; see the current XPS 300 TDS and DoP for declared values.

When should I use XPS 300 W instead of XPS 300 for perimeter insulation?

Use XPS 300 W when a waterproofing membrane, adhesive compound or render is bonded directly to the face of the insulation board: the waffled surface provides the mechanical key needed for reliable adhesion. For perimeter boards that simply bear against soil or a drainage membrane with no bonded finish layer, smooth XPS 300 is equally suitable. See the knowledge article <a href="/knowledge/xps-300-vs-xps-300-w/">XPS 300 vs XPS 300 W — Smooth or Waffled Surface?</a> for a detailed comparison.

What compressive strength is needed under an industrial floor slab?

This depends on the design loads calculated by the structural engineer. PNP XPS 500 (500 kPa) and XPS 700 (700 kPa) are available for high-load industrial applications; both carry the FTCD 1 freeze–thaw rating and WL(T) ≤ 0.7 Vol.-% water-absorption class. Load selection must be made by a qualified structural designer — consult the relevant TDS and DoP and refer to the applicable national standard.

Can XPS boards be used in permanent contact with damp or waterlogged soil?

Yes. The closed-cell structure of XPS 300, XPS 400, XPS 500 and XPS 700 gives a long-term water absorption by total immersion of WL(T) ≤ 0.7 Vol.-%, meaning thermal resistance remains stable in permanently wet ground. All structural grades are also rated FTCD 1, confirming freeze–thaw durability.

How do I calculate the required insulation thickness?

Divide the target thermal resistance (RD, m²·K/W) by the declared thermal conductivity (λD = 0.034 W/(m·K) for boards up to 100 mm; 0.035 W/(m·K) for 120 mm and above). The required RD value must be derived from the applicable national energy-performance regulation by a qualified designer — this page does not provide a design instruction. Where a single board is insufficient, use two layers with staggered joints.

Are PNP XPS boards suitable for a heat-insulated slab (Swedish slab) foundation?

Yes. The solutions catalogue lists a heat-insulated Swedish slab build-up using PNP XPS boards placed beneath and at the perimeter of the foundation slab, on a sand bed with a geotextile separation layer. The specific grade and thickness must be determined by the structural and energy designer for the project.

What is the freeze–thaw resistance rating of the structural XPS grades?

XPS 300, XPS 300 W, XPS 500 and XPS 700 are all rated FTCD 1, meaning volumetric change after freeze–thaw cycling (tested per EN 13164 after long-term water absorption by diffusion) is ≤ 1 Vol.-%. This supports their use in the frost-active zone alongside foundations.

Where can I find the Declarations of Performance and technical data sheets?

All current DoPs and TDS files are available for download from the PNP XPS Documentation page at /documentation/. BIM objects and the plant EPD are also available there. For project-specific technical support, contact sales@pnpinsulation.com.

Share

Useful articles

All articles

Contact us

Leave your contact details and we will get back to you as soon as possible.

Your request has been sent

We will contact you shortly