EPS vs XPS Insulated Raft Foundations
ISOQUICK (EPS) set beside Jackodur Atlas and GreenRaft (XPS), using the certificates rather than the marketing.
Who wrote this: ISOQUICK, which uses EPS, so we have a stake in the answer. To keep it fair, this page sets out what the two certificates say and what the suppliers claim, and it does not declare a winner. ISOQUICK statements come from our Kiwa certificate and published tables. Jackodur statements come from its BBA certificate and its UK installer's page. GreenRaft statements come from its own website. We have not tested either material. Part of a wider comparison of UK insulated foundation systems.
The short version
EPS (expanded polystyrene) and XPS (extruded polystyrene) are both used in UK insulated raft systems. ISOQUICK, KORE and Future Found use EPS. GreenRaft and Jackodur Atlas use XPS. Suppliers of each make claims about their material, but a supplier's claim is not a like-for-like test. What you can compare is what independent certificates state, and the certificates below use different methods, so the differences matter as much as the numbers.
What each supplier says about its material
- GreenRaft (XPS or cellular glass): says XPS is "virtually waterproof due to its closed cell structure" with "very high compressive strength", and that it is the "ideal material for supporting large structural loads in potentially wet conditions". It gives no figures on the page reviewed.
- Jackodur Atlas (XPS), via its UK installer: lists "high compressive strengths", "moisture resistant" and "retains properties over time", also without figures.
- ISOQUICK (EPS): the Kiwa certificate assesses moisture control (condensation risk, water vapour resistance and resistance to water absorption), and states that the system's performance does not reduce over its lifetime and that its service life is equivalent to the building's, which it takes to be at least 60 years.
What the two certificates document
| ISOQUICK (EPS) Kiwa BDA Agrément BAF-20-179-S-A-UK | Jackodur Atlas (XPS) BBA Agrément 20/5812, Product Sheet 1 | GreenRaft (XPS / cellular glass) | |
|---|---|---|---|
| Certificate history | First issued February 2022; reissued July 2025 after a three-year renewal. | Originally certificated 19 October 2020; second issue 27 July 2021; amended 22 May 2024. The BBA tells readers to check the current status with it. | Says its insulation carries full product certifications; no certificate numbers on the pages reviewed. |
| Compressive strength (at 10% deformation) | IQ-60e: 200 kPa; IQ-90: 300 kPa; IQ-210: 700 kPa | XPS grade 300: 300 kPa | Not stated |
| Long-term stress limit | Creep stays within 2% strain after 50 years under a permanent stress of 0.3 × σ10: 60, 90 and 210 kPa for IQ-60e, IQ-90 and IQ-210. | Strain stays within 2% after 60 years if the pressure on XPS 300 stays below 83 kPa. | Not stated |
| Thermal conductivity | Declared λD: IQ-60e 0.031; IQ-90 0.033; IQ-210 0.034 W/mK | Design λu: 0.037 W/mK (100–180 mm thick) and 0.038 W/mK (200–320 mm) | Not stated (online U-value calculator) |
| Concrete | ISOQUICK's U-value tables assume a 250 mm slab with 100 mm below ground level. ISOQUICK's standard specification is a minimum of 200 mm of concrete. | Concrete class C32/40; typical depth 200 or 250 mm; minimum A393 mesh top and bottom. | Says it uses ordinary RC35 concrete, because warranty providers require it. |
| What the certificate covers | Insulated concrete raft foundations in new dwellings. Also a Passive House Institute certificate (0379fs03) for ICF, external-insulation and lightweight-timber connection details. | Ground-supported raft foundations used with timber frame, ICF walls and non-load-bearing blockwork internal walls. | Timber frame and SIPs upstands are shown on its page. |
| Junction Psi-values in the certificate | None. The Kiwa certificate points to BRE guidance for calculating them. | Only conservative SAP 2012 defaults: 0.32 W/mK (external wall) and 0.16 W/mK (party wall). | Not stated |
Sources: Kiwa BDA Agrément BAF-20-179-S-A-UK; BBA Agrément Certificate 20/5812 Product Sheet 1 (JACKON Insulation GmbH); GreenRaft product information page; ISOQUICK U-value tables TD-0002 and TD-0004 (May 2025).
Why the numbers can't simply be ranked
- Different quantities. Kiwa gives a declared conductivity (λD). The BBA gives a design conductivity (λu). They are not the same kind of figure, so the two sets of values should not be compared directly.
- Different periods and methods. The creep limits are stated for 50 years by one scheme and 60 years by the other, and are expressed differently.
- Different scope. The BBA certificate names the wall types it covers. Check that your wall build-up is inside the scope of whichever certificate you rely on.
- No product-specific Psi-value in either. The junction figure is where two foundations with the same floor U-value can differ, and neither certificate gives one for its own product. ISOQUICK states a PHPP Psi-value below 0.1 W/mK for a typical 200 mm build-up, and its Passive House certificate sets Ψe ≤ 0.01 W/mK for certified details; ask any supplier for the calculation behind its figure.
Cost claims
GreenRaft states its system costs £23/m² less than a trench footing with a beam-and-block floor, in a comparison using a housebuilder's prices, and is "around 20% less expensive" than competitors. These are the supplier's own figures from its own comparisons. This page does not compare price, so ask any supplier for the itemised basis and check it against your own site.
Questions to ask about the insulation
- Which certificate covers my wall type, and when does it expire?
- What permanent stress will the insulation carry under my walls (in kPa), and what does the certificate allow?
- What thermal conductivity is used in the U-value calculation, and is it a declared or a design value?
- What is the Psi-value at my wall-floor junction, and who calculated it?
- How has the material been assessed for moisture in ground contact?
Get ISOQUICK figures for your project
Send us your plans and wall type and we will give you the U-value, the junction detail and the certification that applies.
Get a Quote All systems comparedSources
- ISOQUICK — Kiwa BDA Agrément BAF-20-179-S-A-UK; U-value tables TD-0002 and TD-0004
- JACKON Insulation GmbH — BBA Agrément Certificate 20/5812, JACKODUR Atlas Foundation Insulation System
- ICF Bondmor Group — Jackodur Atlas raft foundations
- GreenRaft — Product information
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