Description
Within the XPS insulation boards range, Atlas Fundament 100 mm is the deepest foundation board in the series — engineered for Passive-leaning new-builds, deep retrofit projects, and any below-DPC zone pushing past standard Part L thermal targets. The board delivers a thermal conductivity of λ 0.038 W/mK in a closed-cell foundation-grade structure, measuring 600 × 1250 mm with rebated edges and covering 0.75 m² per board at R 2.63 m²K/W.
What Atlas Fundament 100 mm Does in a UK EWI System
Atlas Fundament 100 mm is the deepest XPS foundation board in the range, designed for the projects pushing past standard Part L 2021 thresholds (current at time of writing) toward Future Homes Standard 2026 trajectory thermal targets. At R 2.63 m²K/W and λ 0.038 W/mK in a closed-cell foundation board, the 100 mm depth handles deep foundation wraps, basement perimeters on high-performance new-builds, and full-depth plinth specifications on retrofit projects targeting U-values below the typical 0.18 W/m²K threshold.
Each board measures 600 × 1250 mm with rebated edges that lock together to form a continuous insulation plane without thermal bridging at the joints — so the moisture-resistant layer reads as a single membrane from one course to the next, regardless of the deeper face profile.
Why Specifiers Choose Atlas Fundament 100 mm for UK Walls
- Deepest thermal performance in the range: R 2.63 m²K/W is the maximum from the Atlas Fundament series — the specification for projects targeting U-values below standard Part L thresholds.
- Closed-cell moisture protection: Permanent resistance to ground moisture and hydrostatic pressure keeps thermal performance constant for the life of the foundation up to 2 m below grade.
- Rebated edges for joint integrity: Tight overlapping joints reduce the cold-bridge risk that flat-edge boards leave at every horizontal course — particularly important at this depth where any thermal bridge proportionally erodes the high R-value.
- Freeze-thaw stable: Holds full R-value through repeated UK winter cycles in exposed plinth and below-grade positions.
- Future Homes Standard trajectory: Sits at the depth most high-performance new-builds gravitate toward as Part L tightens through the Future Homes Standard 2026 roadmap.
Thermal Specifications — Atlas Fundament 100 mm
| Parameter | Value | Notes |
|---|---|---|
| Thermal conductivity (λ) | 0.038 W/mK | Declared value for foundation board |
| Thickness | 100 mm | Deepest Atlas Fundament thickness |
| Thermal resistance (R) | 2.63 m²K/W | At declared λ × thickness |
| Reaction to fire | Per EN 13501-1 | Confirm via current DoP at order |
| Water absorption | Low (closed-cell) | Suitable for permanent ground contact |
Physical Specifications — Atlas Fundament 100 mm
| Parameter | Value |
|---|---|
| Board dimensions | 600 × 1250 mm |
| Coverage per board | 0.75 m² |
| Edge profile | Rebated (lap joint) |
| Compressive strength | Foundation-grade (confirm via TDS at order) |
| Boards per m² | 1.33 (add 5–10% cutting waste) |
Where Atlas Fundament 100 mm Performs Best — Wall Types & Build-Up
The 100 mm thickness is where Atlas Fundament earns its place on projects pushing past the standard Part L baseline. The depth is engineered for the high-performance end of UK domestic specification — new-builds aiming at the Future Homes Standard 2026 trajectory, deep retrofit projects on solid-wall stock targeting modern U-value standards, and basement perimeters where the thermal target sits noticeably below current minimums.
- Deep foundation wraps on Passive-leaning new-builds: Apply vertically against the external face of the foundation wall on projects targeting U-values below the typical 0.18 W/m²K threshold.
- Basement perimeter on high-performance projects: Use across basement and cellar perimeters up to 2 m below ground level where the thermal target requires the deepest available foundation insulation.
- Deep plinth zones on solid-wall retrofits: Position from ground level up to the DPC line on projects insulating solid-wall stock with deeper facade EPS above — typically 100 mm or thicker.
- Heated cellar conversions: Lay horizontally over the slab in heated basement and cellar conversions where the floor needs to perform thermally as part of a heated envelope rather than a cold buffer zone.
How Atlas Fundament 100 mm Fits Into a Full EWI System
In a full external wall insulation build-up, Atlas Fundament 100 mm handles the plinth and below-DPC foundation zone, while the main facade above is finished with matching-depth graphite EPS boards bonded with adhesive-and-basecoat and finished with mesh-reinforced render. At 100 mm both above and below DPC, the plinth-to-facade joint reads as one continuous insulation plane — and the deeper system gives the project a stronger position against tightening Part L thresholds without further upgrade work.
Bond the 100 mm board with a specialist polyurethane adhesive such as Ceresit CT84 XPS Foam, which cures fast in the cool, damp conditions typical of UK ground-level work. Above ground — particularly on exposed plinth runs — add mechanical fixings from the EWI fixing accessories range at 6–8 plugs per m², stepping up to 8 plugs per m² on corners and edges. At 100 mm depth, the fixings need a longer plug length than 80 mm or 90 mm builds — confirm plug length against the project fixing schedule before ordering.
How Atlas Fundament 100 mm Compares to Sibling Foundation Boards
| Board | Thickness | R-Value (m²K/W) | Best For |
|---|---|---|---|
| Atlas Fundament 30 mm | 30 mm | 0.79 | Screed underlay, thin plinth, reveal zones |
| Atlas Fundament 50 mm | 50 mm | 1.32 | Floor insulation, shallow foundation wrap |
| Atlas Fundament 80 mm | 80 mm | 2.11 | Standard foundation, basement walls, Part L target |
| Atlas Fundament 90 mm | 90 mm | 2.37 | Below-DPC plinth flush with 90 mm facade EPS |
| Atlas Fundament 100 mm | 100 mm | 2.63 | Deep foundation wrap, Passive-leaning, Future Homes trajectory |
For projects weighing closed-cell foundation insulation against graphite EPS for the main facade, the XPS vs EPS comparison guide walks through where each material wins across the build-up.
How Atlas Fundament 100 mm Performs in UK Weather — Moisture, Frost, Wind Loads
The closed-cell structure keeps long-term water absorption at near-zero, so freeze-thaw cycling across a typical UK winter does not progressively reduce R-value the way it can on open-cell boards exposed to ground moisture. In splash-zone positions at the plinth, the 100 mm board holds its thermal performance through saturation, drying, and re-saturation cycles for decades — and the deeper face protects the substrate from temperature swings that thinner boards transmit more readily.
Wind-load exposure is the most demanding of any thickness in the range. The 100 mm face area catches significantly more uplift than 80 mm or 90 mm boards do, so the mechanical fixing layout should follow the project-specific pattern — typically 6 plugs per m² across the field and 8 plugs per m² on corners and edges, with additional fixings around openings. On exposed coastal or high-altitude sites, the fixing schedule may require a denser pattern still.
Handling & Storage on Site
- Keep dry and shaded: Stack boards flat on a clean, level surface. Cover with sheeting if outdoor storage exceeds a few weeks — prolonged UV softens the closed-cell surface.
- Score, don't snap: Cut with a fine-tooth saw or sharp blade rather than snapping the rebated edge. A clean cut keeps the bonding face true and protects the closed-cell surface — especially important at 100 mm where any compromised edge magnifies the cold-bridge risk.
- Brush the rebate before adhesive: Dust on the lap joint breaks the foam adhesive seal; a quick brush gives a full-face bond.
- Confirm plug length before ordering fixings: The 100 mm board needs a longer mechanical fixing than 80 mm or 90 mm builds. Order plug length to suit the board thickness plus the substrate-embed depth specified in the project fixing schedule.
Certifications & Compliance — Atlas Fundament 100 mm
Atlas Fundament boards are manufactured to recognised European foundation insulation standards. Reaction to fire is classified under EN 13501-1, and the declared thermal conductivity (λ 0.038 W/mK) appears on the manufacturer's current Declaration of Performance. For projects requiring specific compressive strength values, water absorption test results, or Approved Document L 2021 calculation inputs (current at time of writing), request the latest DoP at order — exact certified values can vary between production batches and the current Atlas Fundament datasheet is the authoritative reference for compliance submissions, particularly on projects pursuing above-standard thermal targets.
Is Atlas Fundament 100 mm Right for Your Project?
- Choose this thickness if your project targets U-values below the standard Part L baseline — Passive-leaning new-builds, deep retrofit on solid-wall stock, or any specification on the Future Homes Standard 2026 trajectory where the foundation insulation needs to match an above-standard facade build-up.
- Working at standard Part L depth? The Atlas Fundament 80 mm at R 2.11 m²K/W is the volume-runner for most domestic new-builds and EWI retrofits.
- Need plinth-to-facade alignment with a 90 mm EPS elevation? The Atlas Fundament 90 mm matches the most common facade EPS thickness above DPC for a flush plane.
- Detailing the plinth-to-facade transition? The plinth thermal-bridge guide walks through the overlap detail that keeps the DPC line condensation-free regardless of the chosen thickness.
What to Order Next — Pack Sizes, Lead Times, Compatible Components
Atlas Fundament 100 mm is supplied as a single board (0.75 m² coverage). For project calculations, allow 1.33 boards per m² plus 5–10% cutting waste. To complete the foundation build-up, order Ceresit CT84 XPS Foam for bonding and a fixing pattern of 6–8 plugs per m² above grade, stepping up to 8 plugs per m² on exposed corners and edges — with plug length set to match the 100 mm board depth plus the project's substrate-embed requirement.
FAQ — Atlas Fundament 100 mm Coverage, Compatibility, Installation
How many 100 mm Atlas Fundament boards do I need per square metre?
Each board covers 0.75 m² (600 × 1250 mm), so 1.33 boards per m² is the base figure. For deep foundation wraps and full basement perimeters, add 5–10% cutting waste for corners, openings, and rebated overlaps, then round up to the nearest full board.
When is 100 mm the right foundation thickness?
The 100 mm thickness is specified where the project targets U-values below the standard Part L baseline — Passive-leaning new-builds, deep retrofit on solid-wall stock, projects positioned for the Future Homes Standard 2026 trajectory, or heated cellar conversions where the floor and perimeter need to perform thermally as part of a heated envelope. For projects sitting at standard Part L thresholds, 80 mm is typically the more cost-effective specification.
What adhesive bonds Atlas Fundament 100 mm to a foundation wall?
A specialist polyurethane adhesive such as Ceresit CT84 XPS Foam is the standard route — it cures fast in cool, damp UK ground-level conditions and bonds reliably to closed-cell board surfaces. Apply in continuous beads around the perimeter plus a central bead, brush dust from the rebated edge before placement, and press firmly to spread the foam across the full bonding face.
What plug length do I need for 100 mm mechanical fixings?
Plug length must cover the 100 mm board depth plus the substrate-embed depth specified in the project fixing schedule — typically 50–60 mm of embed into the substrate, giving an overall plug length around 150–160 mm for standard masonry conditions. Confirm the exact figure against the project schedule before ordering, since substrate type and condition affect the embed depth requirement.
Can I use 100 mm Atlas Fundament on a facade above DPC?
The closed-cell structure limits vapour permeability, so the main facade above DPC typically benefits from graphite EPS or mineral wool rather than foundation-grade board — vapour-permeable insulation lets moisture migrate outward through the render system. The 100 mm Atlas Fundament earns its place at the plinth and below-DPC zones where its moisture resistance is the deciding factor, with matching-depth graphite EPS continuing the build-up above DPC for a flush 100 mm plane top to bottom.
How does 100 mm compare to 90 mm Atlas Fundament?
At the same declared λ of 0.038 W/mK, the 90 mm Atlas Fundament delivers R 2.37 m²K/W and the 100 mm version delivers R 2.63 — an 11% uplift for an extra 10 mm of depth. The 90 mm board is the alignment specification for projects where the facade EPS sits at 90 mm; the 100 mm board is the right pick where the facade build-up has been set at 100 mm or where the project targets U-values below the standard Part L baseline.