XPS INSULATION BOARDS

Ground-contact insulation is specified on a different set of figures from facade insulation, and this shelf sits at the bottom of a full external wall insulation build-up rather than on the elevation. The five boards here are GENDERKA HYDROSTYR 100 foundation boards, held in trade stock at Renders World.

Where XPS Insulation Boards Belong in a UK Ground-Contact Build-Up

XPS insulation boards in this range are declared at a thermal conductivity of λ 0.036 W/mK, which is the figure that sizes a plinth or a foundation wrap against a project's U-value target. Thinner boards reach the same target than a poorer conductivity would allow.

The manufacturer's own sheet sets out where the boards are intended to work, and that list is narrower and more useful than a general moisture claim.

• Foundation and cellar walls below ground level, in normally loaded conditions — the position the board was designed around.
• Foundation walls and plinths above ground level — so the same board carries the plinth up to the damp-proof course, not only the buried section.
• Terraces — flat, trafficked build-ups where load rather than exposure sets the specification.
• Floors on the ground, normally loaded — cellar and ground-floor slabs beneath screed.

Above the damp-proof course the priority changes from load and wet to vapour movement, which is where the graphite EPS insulation boards take over on the main facade.

Why Specifiers Fit These Boards Below the Damp-Proof Course

Four declared figures carry the decision at the plinth, and each one has a practical consequence on site.

• 100 kPa compressive strength — so the boards take backfill pressure and paving loads at the plinth without deforming under the finished surface.
• Water absorption of 3% under long-term total immersion — a board that spends its service life in wet ground keeps its thickness, and with it the thermal figure the calculation was based on.
• Declared thermal resistance published per thickness — the value handed to an assessor comes off the declaration rather than from dividing thickness by conductivity, and the two are not identical.
• Reaction to fire declared class E for the board — which is why the boards are specified behind a reinforced render above ground and under cover below it, in every position on the list.
• One board format across all five thicknesses — course-setting and take-off arithmetic stay the same whether the wrap is thin or deep.

Choose Your Board Thickness by Zone and Depth

Every board in the XPS insulation boards range at Renders World is declared to EN 13163, so the only variable in the table below is thickness and the thermal resistance that comes with it. Read the zone, then check the resistance figure against the target in your calculation.

Where the plinth meets the facade, the thickness chosen here is read together with the board going on above it, and the XPS and EPS material comparison works through that junction across a whole build-up.

How to Install Foundation Boards at the Plinth and Below Ground

The manufacturer's sheet is specific about two things installers often decide by habit: what the boards are bonded with, and what has to sit between the board and the soil.

1. Waterproof and level the substrate first — the wall face is tanked and made good before any board goes on, because nothing in the build-up above corrects a wet substrate below.
2. Bond with a material the sheet names — a cement-based adhesive, a polyurethane foam adhesive, or a water-dispersion bitumen compound, chosen for the substrate rather than for what is open on the van.
3. Separate the boards from the ground — below ground level the sheet calls for a separating layer, either a reinforced layer with waterproofing over it or a dimpled membrane, so the boards are never in direct contact with soil.
4. Work upward in staggered courses — start off the footing and offset the vertical joints course to course, keeping joints tight so the layer reads as one plane to the damp-proof course.
5. Add mechanical restraint above ground — backfill restrains the buried section, while an exposed plinth takes fixings from the insulation fixing accessories range, to the layout the system specifies.

Getting the separating layer in is the step that keeps the rest of the detail honest, and it is the one most often value-engineered out of a plinth.

Installer Tips for Wet Ground, Backfill and Site Storage

• Backfill in even lifts — filling one side of a wrap to full height in a single go pushes boards out of plane, and lifts of a few hundred millimetres each side keep the line true.
• Keep stock away from solvents and ignition sources — the sheet is explicit on this, alongside protecting boards from mechanical damage, so a covered stack away from cutting and hot works keeps a delivery usable.
• Plan the plinth-to-facade transition before the first course — the junction is where a cold path forms if the two insulation layers stop short of each other, and the plinth thermal bridge and mould prevention guide details how to lap them.
• Finish the visible plinth for the traffic it takes — a plinth at path level earns a hard-wearing decorative finish, which is where the mosaic plinth renders come in above the boards.

Deciding Between XPS, Graphite EPS and Mineral Wool

• Specify the XPS insulation boards here for foundation walls, plinths and below-DPC zones, and for normally loaded floors on the ground — the four positions the manufacturer's sheet names.
• Move to graphite EPS above the DPC — the EPS insulation boards range carries the facade, where thickness and vapour behaviour drive the choice rather than ground load.
• Ask the system designer about wool — slabs from the mineral wool insulation range answer a fire strategy question, and the class that governs it belongs to the slab's own declaration rather than to this page.
• Confirm the separating layer and the adhesive together — both are set by the sheet, and settling them at order stage is quicker than reworking a wrap.

Renders World stocks the five thicknesses as single boards, so a plinth run can be ordered to the metre rather than by the pack.

FAQ: Ordering, Thickness and Compatibility

How many boards make a square metre?

Two. The boards are listed at 1000 × 500 mm, so 1.0 m × 0.5 m = 0.5 m² per board, and 1 m² ÷ 0.5 m² = 2 boards, plus an allowance for cuts at corners and openings. Pack quantities are not published here, because the figures on the manufacturer's sheet are tabulated against a different board format and are confirmed at order instead.

Can these boards be used above the damp-proof course?

Yes for the plinth, which the sheet names among the intended uses along with foundation walls, terraces and normally loaded floors on the ground. The main facade is a different question, and the graphite boards handle it — take that decision through the material comparison for UK build-ups rather than carrying one board type all the way up.

What can the boards be bonded with?

The sheet names cement-based adhesives, polyurethane foam adhesives and water-dispersion bitumen compounds, so the choice follows the substrate and the tanking already in place. Below ground the boards also need a separating layer between them and the soil, which is a specification requirement rather than an optional extra.

Which thickness meets my U-value target?

That is decided by a calculation across the whole build-up, not by a board on its own, and the figure to give your assessor is the declared thermal resistance for the thickness you choose — the values in the table above. Existing construction, ground conditions and the layers above the boards all move the answer, so the calculation comes first and the thickness follows it.