Description
Renders World stocks the blue foundation insulation board 80mm as a single-board line for plinth, foundation and below-DPC work, one of five depths in the XPS plinth and below-DPC board range.
What the 80 mm XPS Board Does Below DPC
The XPS foundation board 80mm carries a declared thermal resistance of 2.20 m²K/W at λD 0.036 W/mK, enough to keep a continuous insulation plane running from footing level up to the damp-proof course.
That resistance is the manufacturer's declared value for the 80 mm thickness rather than thickness divided by lambda, so it is the figure an assessor can drop straight into a build-up calculation. Where a plinth has to line through with a rendered facade above, 80 mm also matches the most common graphite facade thickness, so the two planes meet flush instead of stepping at the DPC line.
The manufacturer's data sheet sets the intended use, and it reads as a ground-contact and ground-adjacent board rather than a facade board:
- Foundation and basement walls below ground level, in normally loaded positions, where backfill gives the wrap permanent restraint.
- Foundation walls and plinths above ground level, carrying the render build-up down into the splash zone.
- Terraces, sitting under the waterproofing and the finish layers.
- Ground-bearing floors in normally loaded positions, laid over a level slab ahead of screed.
Why Specifiers Choose the 80 mm Depth for Foundation and Plinth Zones
Five declared figures do the work at this thickness, and each one changes something about how the board is ordered or fitted.
- 2.20 m²K/W declared resistance — a real contribution to a wall or floor U-value from only 80 mm of build-up depth, so reveals, thresholds and paving levels stay workable.
- λD 0.036 W/mK — a low conductivity for a moisture-tolerant board, so the plinth reaches its target without a deeper step below the facade.
- 100 kPa compressive strength at 10% deformation, so the board takes backfill pressure and paving loads at the plinth as part of the build-up.
- No more than 3% water absorption under long-term total immersion, which is what qualifies it for the wet and periodically saturated positions the sheet names.
- 1000 × 500 mm boards — two boards to the square metre, so take-off arithmetic stays simple and a 500 mm plinth run is one uncut course.
Declared Specifications for the 80 mm Board
Every figure in the table for the XPS foundation board 80mm was read from the current manufacturer technical data sheet during this session, while the board format was read from the live product listing rather than from the sheet.
| Property | Declared value | What it means on site |
|---|---|---|
| Thickness | 80 mm | Plinth and foundation depth that lines through with an 80 mm facade board |
| Board format | 1000 × 500 mm | Two boards per square metre; sold as a single board |
| Thermal conductivity | λD 0.036 W/mK | Low conductivity for a moisture-tolerant board |
| Thermal resistance | 2.20 m²K/W | Declared for 80 mm; the value a U-value calculation takes |
| Compressive strength | 100 kPa at 10% deformation | Takes backfill and paving loads at the plinth |
| Water absorption | 3% maximum, long-term total immersion | Suits wet and periodically saturated positions |
| Reaction to fire | Class E | The board's own class; a rendered build-up is classified separately |
One detail is worth settling before the order goes in. The sheet declares this product in both a smooth-edged and a rebated form and does not say which one reaches this shelf, so ask at order and detail the joints as plain butt joints until the rebate is confirmed — a point the datasheet leaves open and the site drawing has to close.
How to Install the 80 mm Board at the Plinth and Below Ground
Installation follows the sheet's own sequence, and the separating layer between board and soil is the step that repays the most attention.
- Set the datum first. Strike a level line at the DPC and work down, so the plinth courses arrive at footing level without a tapered cut.
- Bond with a compatible adhesive. The sheet names cement-based adhesives, polyurethane foams and water-dispersion bituminous compounds; the cement-based route sits on the EPS adhesives basecoats shelf. Solvent-based bituminous products are outside what the sheet allows.
- Separate the board from the ground. Below ground level the sheet asks for a separating layer — a reinforced layer with damp-proofing or waterproofing over it, or a dimpled membrane — before anything is backfilled.
- Backfill in even lifts. Compacting in stages keeps pressure balanced along the wrap and gives the 100 kPa board the restraint it is specified for.
- Fix mechanically above grade. Above ground the plinth carries wind load, and both plug length and pattern come from the system holder's fixing table for that substrate; the plugs, discs and base tracks sit in the insulation fixing accessories range.
- Continue above the DPC in facade board. Graphite EPS insulation boards take the elevation above the DPC line, where vapour behaviour and render compatibility matter more than immersion performance.
Store the boards flat and protected from mechanical damage, away from ignition sources and organic solvents, as the sheet instructs. Renders World supplies the same 1000 × 500 mm format across the depths in this range, so a mixed-thickness plinth still sets out on one grid, and a mosaic render finish for plinths is the usual decorative close for the zone above ground.
How the 80 mm Compares With the Other Foundation Depths
| Depth | Declared resistance | Where it fits |
|---|---|---|
| 30 mm board | 0.80 m²K/W | Tight perimeter runs and shallow returns |
| 50 mm board | 1.35 m²K/W | Floor build-ups and shallow foundation wraps |
| 80 mm board | 2.20 m²K/W | Plinth lining through with an 80 mm facade board |
| 90 mm board | 2.50 m²K/W | Below-DPC work matched to a 90 mm facade |
| 100 mm board | 2.75 m²K/W | Deepest wrap and the plinth-to-wall transition |
Every resistance above is the declared value for that thickness, which is why the steps are not perfectly even. For the wider question of which board belongs where in a build-up, the guide to XPS and EPS boards compared for UK walls works through the split, and the plinth thermal bridge and mould prevention guide covers the overlap detail at the DPC.
Ordering the 80 mm Board — Price, Quantity and Companions
The XPS foundation board 80mm is sold as a single board at £8.30 per board, as displayed on the product page in August 2026, and was listed as available at the time of that fetch. At 1000 × 500 mm each board covers 0.5 m², so 1 ÷ 0.5 = 2 boards per square metre, and £8.30 ÷ 0.5 m² = £16.60 per m² for the board alone, before adhesive, fixings and the render build-up.
For a 30 m² plinth run that is 30 × 2 = 60 boards, plus a cutting allowance for corners and returns. Renders World lists the adhesive, the fixings and the plinth finish as separate lines rather than a bundled rate, so the take-off stays visible on the quotation.
FAQ — Quantity, Compatibility and Plinth Detailing
How many 80 mm boards are needed per square metre?
Two. Each 1000 × 500 mm board covers 0.5 m², so 1 ÷ 0.5 = 2 boards, and a plinth 30 m in length at 500 mm high is 15 m², or 30 boards before any allowance for cuts at corners and returns.
Which adhesive suits this board below ground?
The data sheet names three routes — cement-based adhesives, polyurethane foams and bituminous compounds based on water dispersion — and rules out solvent-based bituminous products. Below ground the adhesive is only part of the detail: the separating layer and the backfill are what hold the wrap in position long term.
Can this board be used on the facade above the DPC?
Its declared strengths are immersion and load performance, which belong to the plinth, foundation and basement perimeter. Above the DPC the facade is better served by a graphite board designed for adhesive-and-basecoat bonding and a mesh-reinforced render, with the 80 mm depth carried through so the two planes stay flush.
How does the 80 mm compare with the 100 mm board?
Declared resistance rises from 2.20 to 2.75 m²K/W, a gain of 0.55 m²K/W for 20 mm of extra depth. Where paving levels, thresholds or a matching facade thickness fix the plinth depth, 80 mm is the working choice; where the wrap is free to grow, the 100 mm foundation board takes the deeper target.
What does reaction to fire class E mean for a rendered plinth?
Class E is declared for the board on its own, and it stays a board property. The rendered build-up over it is a different assembly with its own test evidence, so a plinth specification takes its performance from the system holder rather than from this figure.
Technical Documentation
- HYDROSTYR 100 technical data sheet (GENDERKA, PDF) — declared conductivity, resistance by thickness, compressive strength, water absorption and reaction to fire.
- The declaration of performance is confirmed at order, as the supplying plant determines which declaration applies.