About this calculator
This calculator works out the materials to insulate a building's plinth (cokolis) and foundation walls from outside with XPS or EPS-P boards: the boards, the adhesive, the anchors above ground, the dimpled membrane that protects the insulation below ground, and the profiles.
Enter the perimeter — a rectangle, or a list of walls with the number of outside corners — and how far the insulation goes above and below ground.
Formula
outer perimeter = wall perimeter + 2 × board thickness × outside corners
area above = outer perimeter × height above ground
area below = outer perimeter × depth below ground
boards = ceil((above + below) × (1 + waste %) / board area)
foam cans = ceil(above / coverage above + below / coverage below)
mortar bags = ceil((above + below) × kg/m² / bag)
corner bands = min(outer perimeter, corners × 2 × band width)
anchors = ceil(field area × field rate + band area × corner rate) (above ground)
membrane courses = ceil((depth − overlap) / (roll width − overlap))
strips per roll = floor(roll width / depth) when one course is enough, else 1
(a wide roll is ripped lengthwise)
membrane rolls = strips laid round the perimeter one after another, a new
strip overlapping the last; rolls = ceil(strips / strips per roll)
profiles = Σ walls ceil((wall + 2 × thickness) / profile length)
Worked example
A 10 × 8 m house, 100 mm boards 1250 × 600 mm, 0.5 m above and 1.0 m below ground, 10 % waste, foam adhesive:
outer perimeter = 36 + 2 × 0.1 × 4 = 36.8 m
area = 36.8 × 0.5 + 36.8 × 1.0 = 18.4 + 36.8 = 55.2 m²
boards = ceil(55.2 × 1.1 / 0.75) = ceil(80.96) = 81
foam = 18.4 / 12 + 36.8 / 15 = 3.99 → 4 cans
anchors = corner bands 16 m × 0.5 = 8 m² × 8 + 10.4 m² × 4 = 105.6 → 106
membrane = 2 m roll ripped into two 1 m strips; 36.8 m: 20 m + 16.8 m (+0.2 m) → 2 strips = 1 roll
FAQ
Why are anchors counted above ground only? Below ground the backfill presses the boards to the wall; anchors there would also pierce the waterproofing. Above ground the plinth is part of the facade insulation, where Ceresit's adhesive data sheet asks for at least 4 anchors per m² and at least 8 per m² in bands about 2 m wide at the building's edges. The system's approval and the design decide the actual number.
Which adhesive? Foam adhesive (Ceresit CT 84: about 12 m² per 850 ml can on facades, about 15 m² on foundation insulation) or an adhesive mortar (Ceresit CT 83: about 5 kg/m²). Check that the adhesive is approved for your boards (XPS) and for use below ground.
How is the dimpled membrane laid? Horizontally round the building, dimples to the insulation, each sheet overlapping the previous one by about 20 cm (DELTA-MS installation instructions). A roll much wider than the depth is ripped lengthwise into strips, as builders do. It is taken up past the top of the insulation below ground and trimmed level with the ground after backfilling; its top edge is closed with a finishing profile.
Is the board's R the wall's thermal resistance? No — it is R = d / λ of the boards alone (LST EN ISO 6946). Heat loss through the ground needs the ground method (EN ISO 13370), which this calculator does not apply.
Which standard applies? LST EN ISO 6946 for the boards' R. EN 13164 and EN 13163 are XPS and EPS product standards and give no quantities; designing frost-protected shallow foundations (EN ISO 13793) is outside this calculator.
Assumptions and limits
Standard: LST EN ISO 6946 for R = d / λ only.
Defaults to replace: adhesive coverage (Ceresit CT 84, CT 83), anchor rates (Ceresit ZS minimums), membrane rolls 2.0 × 20 m and 200 mm overlap (DELTA-MS) are product figures; the board size, heights, 2.5 m profiles and 10 % waste are examples.
Geometry: the insulation is one height above and one depth below ground all round (no sloping ground); the area is measured on the boards' outer face. Inside corners are not deducted. Board offcuts are covered by the waste. Anchors at openings, the plinth render, mesh, waterproofing and drainage are not counted. Profiles are cut wall by wall.