About this calculator
This calculator estimates how much rigid board insulation to buy for a slab-on-ground floor — one or two layers, plus the damp-proof membrane that goes with it and the compressible edge strip that runs around the slab's perimeter. Enter the floor shape, the board size and thickness, and how many layers you're laying.
Formula
floor area (with waste) = floor area x (1 + waste%)
boards per layer = ceil(floor area (with waste) / board area)
total boards = boards per layer x number of layers
R-value = (board thickness x number of layers) / lambda [ISO 6946]
membrane area = floor area x (1 + overlap%)
membrane rolls = ceil(membrane area / roll area)
edge strip length = floor perimeter
edge strip rolls = ceil(edge strip length / roll length)
Each layer covers the full floor area independently — unlike an attic floor laid between joists, a slab-on-ground has no framing to work around, so there is no area deduction for a first layer.
Worked example
Defaults: an 8 x 6 m floor, one 100 mm layer of 1200 x 600 mm boards, λ = 0.035 W/m·K, 10% waste, a 50 m² membrane roll with 10% overlap, 25 m edge strip rolls.
floor area = 48 m²; with waste = 48 x 1.10 = 52.8 m²
boards = ceil(52.8 / (1.2 x 0.6)) = ceil(52.8 / 0.72) = 74
R = 0.1 / 0.035 = 2.86 m²·K/W
membrane area = 48 x 1.10 = 52.8 m² -> rolls = ceil(52.8/50) = 2
edge strip length = 2x(8+6) = 28 m -> rolls = ceil(28/25) = 2
FAQ
Why does a second layer need the same number of boards as the first? A slab-on-ground insulation layer has no joists or framing to fit between — every layer, whichever course it belongs to, covers the entire floor area, so each layer's board count is independent of the others.
What is the edge strip for? A compressible strip run around the slab's perimeter, between the insulation/screed and the walls, to absorb thermal and moisture movement so the floor build-up isn't rigidly locked to the walls. Its length always equals the floor's own perimeter.
Which standard does this follow? ISO 6946 for the R = thickness / lambda formula. Board size, membrane roll size and overlap, and edge strip roll length are all product-specific figures with no single governing standard — check the product's own data sheet.
Assumptions and limits
Board size/thickness, membrane roll size and overlap, and edge strip roll length are all user inputs with example defaults, not data for a specific product. No drawing is produced — the result is a materials list only.