EPS FOUNDATION 50mm (1 BOARD) 0.038W

Sold as a single 1000 × 500 mm board, the 50 mm thickness is where the XPS insulation boards range at Renders World stops being a depth-limited fix and starts carrying a resistance figure worth putting in a build-up sheet.

Where the 50mm Foundation Board Fits on a UK Plinth

The XPS foundation board 50mm is a water-repellent polystyrene board declared at λ 0.036 W/mK with a declared thermal resistance of 1.35 m²K/W, specified under EN 13163 for plinths, below-ground cellar walls and ground-bearing floors under normal loading. That resistance is the reason this thickness gets specified rather than the 30 mm.

• Foundation and cellar walls below ground level under normal loading, where the board sits behind a separating layer rather than in raw contact with the soil.
• Foundation walls and plinths above ground level, the declared application that covers the visible zone from the ground up to the damp-proof course.
• Terraces and ground-bearing floors under normal loading, where 50 mm still fits inside most domestic screed build-ups.
• Shallow perimeter wraps on lighter domestic work, where a deeper board would need more excavation than the thermal gain justifies.

Where the plinth meets the insulated facade above it, the junction decides whether the detail works, and the plinth thermal-bridge and mould guide works through how the two zones are brought together without a cold line at the transition.

Why Specifiers Choose the 50mm Board

The case for this thickness is a balance between what it returns thermally and how little depth it costs — the points below are what settle it on site.

• Declared thermal resistance of 1.35 m²K/W, taken from the manufacturer's declared table rather than calculated, and a substantial step up from 0.80 m²K/W at 30 mm for 20 mm of extra depth.
• 100 kPa compressive stress at 10% deformation, so the board takes backfill and paving loads at the plinth and screed loads on a floor.
• 30 kN/m² long-term design load with creep strain held inside 2%, which is what makes it a floor board and not only a wall board.
• 3% water absorption under long-term total immersion, the property that puts it below the damp-proof course in the first place.
• First thickness with a lap-jointed option in the manufacturer's pack table, where the smooth-edge and rebated pack areas diverge — 6.00 m² against 5.64 m².
• ±2 mm thickness tolerance, tight enough that a 50 mm course stays flat under a screed or a reinforcing layer.

Declared Specifications for the 50mm Board

Every figure below for the XPS foundation board 50mm is read from the manufacturer's HYDROSTYR 100 technical data card, fetched in August 2026. Each is a property of the board itself, never of a wall or floor the board sits inside.

Property	Declared value	What it means on site
Thermal conductivity	λ 0.036 W/mK	Thinner board for a given resistance target
Thermal resistance at 50mm	1.35 m²K/W	Declared value; use it, do not calculate one
Compressive stress at 10% deformation	100 kPa	Backfill, paving and domestic screed loads
Long-term design load	30 kN/m²	Creep strain stays within 2% under that load
Bending strength	150 kPa	Handles without flexing off at 500 mm width
Water absorption	3%, long-term total immersion	Wet and periodically flooded zones
Reaction to fire	Class E	The board alone, never a rendered build-up
Thickness tolerance	±2 mm	Courses stay flat under screed or mesh

 

These values are declared against EN 13163, the harmonised standard the data card is issued under, and the reaction-to-fire class describes the board on its own. What a finished, rendered wall achieves is a system assessment answered by the system supplier, and no board document settles it in either direction.

How to Install the 50mm Board Below and Above DPC

The manufacturer's installation clause fixes two things — how the board is bonded, and what has to sit between it and the ground. Everything else is site sequence.

1. Set a datum and run the first course level. Above grade the base track that carries it sits in the insulation fixing accessories range; below grade the course is set off the footing line.
2. Bond with a documented adhesive. The card names three routes — cement-based adhesives, polyurethane foams, and water-dispersion bituminous masses. Cement-based options sit in the EPS adhesives basecoats range.
3. Check which edge you have received. At 50 mm the pack table lists both a smooth-edge and a lap-jointed pack, so confirm the edge profile on delivery before planning the joint layout: lap joints close differently from butt joints and the pack area is not the same.
4. Separate the board from the ground. Below ground level the card requires a separating layer — a reinforced layer with damp or waterproofing, or a dimpled membrane — between the board and the soil. This is the step that keeps the 3% absorption figure meaningful for the life of the wall.
5. Finish the exposed zone. Above the damp-proof course a plinth is usually closed with a mosaic plinth render over its mandated key coat, which handles splash exposure the facade render above is not built for.

One point worth carrying into a take-off: because the manufacturer's below-ground clause names adhesive routes rather than mechanical anchors, plug lengths and fixing counts for this board come from the system supplier's own table. Renders World publishes no plug-to-thickness mapping until that table is in hand, which is a more useful answer than a rule of thumb that sends a buyer the wrong length.

How the 50mm Board Compares Across the Foundation Range

Board	Declared resistance	Where it goes
30mm foundation board	0.80 m²K/W	Screed underlay, thin plinth, reveals
50mm, this board	1.35 m²K/W	Shallow wrap, floors with headroom
80mm foundation board	2.20 m²K/W	Standard foundation and cellar walls
90mm foundation board	2.50 m²K/W	Deeper wrap on retrofit plinths
100mm foundation board	2.75 m²K/W	Plinth-to-wall transition thickness

 

All five share the same declared conductivity, so resistance is the only variable and the choice is purely how much depth the detail can give. Where the question is a foundation-grade board at the plinth against a graphite facade board above it, the XPS and EPS comparison guide sets out which board belongs in which zone.

Ordering the 50mm Board — Format, Packs and Price

Renders World sells the XPS foundation board 50mm as a single board, which suits patch work and short runs; larger areas are worth ordering by the pack figure below.

• £6.50 per 1000 × 500 mm board, read from the product page in August 2026, and listed as available at that reading.
• £13.00 per m² — at 0.5 m² per board, one square metre takes two boards, so 2 × £6.50 = £13.00/m².
• A 12 m plinth run 400 mm high = 4.8 m²; 4.8 ÷ 0.5 = 9.6, so ten boards cover it, and a margin for cuts at returns and openings takes it to eleven.
• Manufacturer pack at 50 mm: twelve boards, 6.00 m² smooth-edge or 5.64 m² lap-jointed — the difference is the overlap and it is worth using the right figure in a take-off.

On format there is a divergence inside the manufacturer's own sheet, and it reads as a purchasing instruction rather than a problem. The commercial section declares packs at the standard 50 × 100 cm board size — 1000 × 500 mm, the format sold here — while the available-sizes line beside it lists 100 × 100 cm and 100 × 150 cm with other sizes by prior agreement. In practice the 1000 × 500 mm board is the stock item, and the larger sheets are an agreed-order line rather than something to assume is on the shelf.

Is 50mm the Right Thickness for Your Build-Up?

• Choose 50mm where the detail has 50 mm to give and the job wants a real resistance figure — shallow perimeter wraps, domestic floors with screed headroom, and plinths on new work where the facade line is still being set.
• Drop to the 30mm foundation board where depth is the binding constraint, such as a screed underlay inside a fixed finished level or a reveal return.
• Step up to the 80mm foundation board at 2.20 m²K/W for a full foundation or cellar wrap, which is the more usual domestic specification where excavation allows it.
• Treat thermal targets as a build-up question. A board carries a declared resistance, not a wall U-value, and the U-value and insulation thickness guide shows how the layers are added up properly.

FAQ — Fitting and Ordering the 50mm Foundation Board

How many boards do I need per square metre?

Two. Each board measures 1000 × 500 mm, which is 0.5 m², so 1 m² takes two boards. The manufacturer's 50 mm pack holds twelve boards and covers 6.00 m² with smooth edges or 5.64 m² lap-jointed. Divide the run by 0.5, round up, then add a margin for cuts at corners and openings.

Can the 50mm board go under a domestic screed?

Ground-bearing floors under normal loading are one of the declared applications on the manufacturer's card, and the board is declared at 100 kPa compressive stress at 10% deformation with a 30 kN/m² long-term design load. Lay it over a level slab and confirm the loading against your own floor design, which is the step worth getting right before the pour.

Can it sit in direct contact with the ground?

Below ground level the card requires a separating layer between board and soil — a reinforced layer with damp or waterproofing, or a dimpled membrane. Detailed that way the board is doing exactly the job it is declared for, and the separating layer is the detail that protects the whole plinth long term.

What bonds the 50mm board to a foundation wall?

The card names cement-based adhesives, polyurethane foams and water-dispersion bituminous masses, so substrate and weather drive the choice rather than preference. Cement-based options for insulation boards sit in the EPS adhesives basecoats range.

How does 50mm compare with 80mm thermally?

Both share a declared conductivity of λ 0.036 W/mK, and the declared resistance is 1.35 m²K/W at 50 mm against 2.20 m²K/W at 80 mm. Where excavation and the plinth line allow the deeper board, 80 mm is the stronger specification; where they do not, 50 mm is the sensible fit.

What is the reaction-to-fire class, and does it describe the finished wall?

The board is declared class E on the manufacturer's data card. That class belongs to the board alone — a finished, rendered external wall is assessed as a whole system, so take that figure from the system supplier for the build-up you are specifying.

Technical Documentation

• GENDERKA HYDROSTYR 100 technical data card — declared properties, resistance table by thickness, and pack format.
• The Declaration of Performance is issued per production plant and is confirmed at order rather than quoted here.