How many packs do I need for a 24 m² ceiling with 100 mm insulation?

With 10% waste the area becomes 26.4 m². If one pack covers 3.6 m², you need ⌈26.4 ÷ 3.6⌉ = 8 packs.

How thick should the insulation be?

Typical thicknesses: 45 mm (internal walls), 95–145 mm (external walls), 100–220 mm (ceilings, often cross-laid), 300 mm (new-build ceilings per code). Ceilings and external walls have the highest code requirements.

What is cross-laid insulation?

Cross-laid insulation is 2 layers installed perpendicular to each other — e.g. 2 × 100 mm. It eliminates thermal bridges between joists and delivers better insulation performance than a single layer of the same total thickness.

How much waste should I budget for?

5–10% for an open ceiling or a simple wall. 10–15% for many penetrations, pipes, angled surfaces or dormers. Keep in mind that pack rounding naturally creates extra surplus.

How much insulation do you need per m²?

Q: How much insulation do you need per m²?

Per m² of surface you always need at least 1 m² of insulation plus an allowance for waste. The actual amount depends on thickness and pack size — one pack typically covers 2–6 m² depending on product and thickness.

Insulation is measured in m² (flat area) or m³ (volume, when thickness is included). Per m² of surface you always need at least 1 m² of insulation — plus an allowance for cutting and waste.

The actual quantity depends on insulation thickness, number of layers and package size. A package of insulation typically covers 2–6 m² depending on the product and thickness.

Example: 24 m² ceiling with 100 mm insulation

A ceiling measuring 6 × 4 metres (24 m²) insulated with 100 mm mineral wool. One package covers 3.6 m².

Ceiling area6 × 4 = 24 m²

Insulation thickness100 mm

With 10% waste26.4 m²

Package coverage3.6 m²/package

Number of packages⌈26.4 ÷ 3.6⌉ = 8 packages

Common insulation thicknesses

The thickness depends on where you're insulating and the requirements of your local building regulations.

ThicknessTypical usePackage coverage*

45 mmInterior walls, partition wallsapprox. 6 m²

95 mmExterior wall, light facadeapprox. 4.5 m²

100 mmExterior wall, ceiling (1st layer)approx. 3.6 m²

145 mmExterior wall, ceilingapprox. 2.5 m²

195 mmCeiling, cross-laidapprox. 2 m²

220 mmCeiling, cross-laidapprox. 1.8 m²

300 mmCeiling (building code, new build)varies

*Package coverage is approximate and varies between manufacturers and products. Always check the packaging.

Don't forget waste

Insulation batts need to be cut at edges, corners, around pipes and electrical boxes. Rounding up to whole packages also naturally creates surplus.

•5–10% — an open ceiling or a simple wall
•10–15% — many penetrations, pipes, sloped surfaces or dormers

For cross-laid insulation (2 layers at right angles) the area doubles — but the waste percentage applies to the total area, not per layer.

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Use MyPlanDIY's free insulation calculator — choose the surface, thickness and package coverage, and get the number of packages instantly.

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