Description
The XPS foundation board 100mm is the deepest board in the Renders World XPS foundation board range: a blue, water-repellent XPS board, 1000 × 500 mm, sold by the board. It is the depth to use where a plinth has to line up with a 100 mm insulated wall above.
What the XPS Foundation Board 100mm Is For
A wall insulated with 100 mm boards needs a plinth of the same depth, or the render steps in at the damp-proof course and leaves a ledge for water and a weak spot for cold. This board fills that gap from the DPC down into the ground:
- Plinths under a 100 mm facade, so the mesh and render run over the joint in one plane.
- Deep foundation wraps, where the excavation is open anyway and the extra depth costs little.
- Below-ground foundation walls, continuing the same 100 mm plane down towards the footing.
Why Choose the 100 mm XPS Foundation Board
- The highest thermal resistance in the range — about 2.60 m²K/W. 0.100 ÷ 0.038 = 2.63, rounded down to 2.60.
- λD 0.038 W/mK, the value on the board's CE marking, the same as every other depth.
- 100 kPa compressive and 150 kPa bending strength, for backfill and paving loads at the plinth and for handling a deep board on site.
- 5 % water absorption under long-term immersion, the figure that matters for a board in damp ground.
Specification — XPS Foundation Board 100mm
| Property | Value |
|---|---|
| Board | XPS foundation board, blue, water-repellent |
| Thickness | 100 mm |
| Board size | 1000 × 500 mm (0.5 m²) |
| Sold as | 1 board |
| Thermal conductivity λD | 0.038 W/mK |
| Thermal resistance | 2.60 m²K/W (0.100 ÷ 0.038 = 2.63, rounded down) |
| Compressive strength | 100 kPa |
| Bending strength | 150 kPa |
| Water absorption, long-term immersion | 5 % |
| Reaction to fire (the board alone) | Class E |
Before You Order 100 mm
The XPS foundation board 100mm stands further off the wall than any other depth here. Check on the drawing where paving falls, door thresholds, gullies and air bricks meet the plinth before the first board goes on — moving an air brick is easier on paper than on site.
Above the DPC, the 100 mm graphite EPS facade board continues the same depth up the wall. Bonding products are in the adhesives and basecoats for insulation boards, starter tracks and anchors in insulation fixing accessories, and a hard-wearing plinth finish in the mosaic plinth renders.
100 mm Against the Other Depths
| Board | Thermal resistance (thickness ÷ 0.038) | Typical place |
|---|---|---|
| 30 mm | 0.75 m²K/W | Thin plinths where little depth is free |
| 50 mm | 1.30 m²K/W | Shallow plinths and foundation wraps |
| 80 mm | 2.10 m²K/W | Plinths under an 80 mm facade |
| 90 mm | 2.35 m²K/W | Plinths under a 90 mm facade |
| 100 mm (this board) | 2.60 m²K/W | Deepest wrap, 100 mm facades |
Whether the last 10 mm over the 90 mm board is worth having depends on the whole wall — the U-value and insulation thickness guide shows how to work it out, and the plinth thermal bridge guide covers the joint at the DPC.
How Many Boards
Two boards make a square metre, because each covers 0.5 m². Work out the plinth area — length times height — double it, round up, and add a few boards for cuts at corners and openings.
FAQ — 100 mm XPS Foundation Board
How many 100 mm boards make a square metre?
Two — each board is 1000 × 500 mm, or 0.5 m². The board is sold singly, so order the exact count plus a few for cuts.
Can I use it on the wall above the DPC?
Above the DPC the wall is normally insulated with a facade board made for render. Use this board for the plinth and below ground, and carry the same 100 mm depth up the wall in a graphite EPS facade board so the two planes stay flush.
100 mm or 90 mm?
100 mm gives about 2.60 m²K/W, 90 mm about 2.35. Match the plinth to the facade board above; where the facade is 90 mm, take the 90 mm foundation board.
Which adhesive do I use for the 100 mm board?
An adhesive for insulation boards, from the adhesives and basecoats range. Check that the adhesive's own data sheet covers XPS boards and, below ground, damp conditions.
What does fire class E mean for a rendered plinth?
Class E is the board's own reaction-to-fire class. A rendered plinth is assessed as a whole system, so the finished wall takes its class from that system, not from the board.