Thermal insulation calculator
Estimate insulation quantity and thermal resistance: area, thickness, lambda, waste, packs, volume, R-value and budget.
This choice applies to every calculator; amounts are not converted.
Insulation calculation
Estimate insulation to buy and, when lambda is known, the thermal resistance R of the layer.
Existing R-value & budget
Purchased area
Surplus
Indicative total R-value
Budget
Calculation details
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How does this tool work?
Insulating a loft, a wall or a floor raises two separate questions: how many packs to buy, and what thermal performance will the installed layer give? This calculator answers both. From the area, allowance, thickness and the lambda conductivity of the product, it returns the number of packs or rolls, the area actually purchased, the installed volume, the thermal resistance R of the layer and, if you enter a price, the budget.
The area comes from the dimensions of the zone or from a known area. The cutting allowance is applied, then the area to plan is divided by one pack’s coverage and rounded up to a whole pack. The surplus compares the purchased area with the area that really needs insulating.
The thermal resistance of a uniform layer is its thickness in metres divided by its conductivity λ in W/(m·K). If existing insulation is kept, its R can be added to get an indicative total R for the insulating layers.
Quantity starts from the area to insulate and the real coverage of rolls or boards. It does not select thermal performance: keep the manufacturer-specified thickness and installation method instead of compressing insulation to make it fit.
How to use this tool
Enter the area
Give the length and width of the zone or the known area directly, without openings.
Describe the product
Enter the thickness in millimetres and the declared lambda printed on the pack or data sheet.
Add allowance and pack size
Choose the cutting allowance and the area one pack covers at that thickness.
Complete if needed
Add the R of existing insulation that stays and the price per pack to get a total R and a budget.
Thermal resistance and quantities
The thicker the layer and the lower its lambda, the higher its resistance R.
Layer resistance
R = d ÷ λ
d in metres, λ in W/(m·K), R in m²·K/W.
Layers in series
R total = R1 + R2 + …
Valid for stacked uniform layers.
Packs
N = ⌈A × (1 + allowance) ÷ C⌉
C is the area covered by one pack.
- d
- insulation thickness (m)
- λ
- thermal conductivity (W/(m·K))
- A
- area to insulate (m²)
- C
- coverage of one pack (m²)
The calculated R ignores thermal bridging through framing, air layers, surface resistances and installation defects. Whole-element performance is calculated under the applicable energy rules. US R-values are in ft²·°F·h/BTU: multiply an SI value by about 5.68 to compare.
Use cases
Loft insulation rolls
Count the rolls for a second cross-laid layer and check the total R with the old wool left in place.
Wall lining with boards
Compare two products with different lambda values at the same thickness before choosing.
Suspended timber floor
Estimate the boards fixed between or below the joists and the resistance achieved.
Examples
8 × 6 m loft at 200 mm
0.200 ÷ 0.035 = 5.71. Reaching R = 7 would take about 245 mm of the same product, or a second layer.
Input
8 m × 6 m, 5% allowance, 200 mm, λ 0.035, 2.5 m² packs, €30 per packResult
48 m², 50.4 m² to plan, 21 packs, 52.5 m² bought, 10.08 m³, R ≈ 5.71 m²·K/W, budget €63042 m² wall with existing insulation
0.120 ÷ 0.032 = 3.75, plus the R of 1.5 from the insulation kept in place.
Input
Area 42 m², 8% allowance, 120 mm, λ 0.032, 3.6 m² packs, existing R 1.5Result
45.36 m² to plan, 13 packs, 46.8 m² bought, 5.44 m³, layer R 3.75, total R 5.25Tips and best practices
- Never compress fibrous insulation to make it fit: losing thickness means losing thermal resistance.
- Check that the coverage on the pack matches the chosen thickness; the same product often comes in several thicknesses.
- For two cross-laid layers, calculate each layer with its own thickness and add the R values.
- For blown insulation, use the manufacturer’s table, which includes settlement and bags per m² for each thickness.
Sources and references
Frequently asked questions
How do I calculate thermal resistance R?
Divide the thickness in metres by lambda. A 100 mm mineral wool at λ 0.035 gives 0.100 ÷ 0.035 ≈ 2.86 m²·K/W; at 0.032 it reaches 3.13.
What R value should I aim for?
It depends on the national energy rules and the element: lofts usually receive the highest values. In the UK, Approved Document L sets target U-values; in France, renovation grants commonly require R ≥ 7 for lofts. Check the current local requirements.
Where do I find a product’s lambda?
On the pack label and the data sheet, usually as a declared value under the CE marking. Prefer that declared figure to a generic value for the material.
Can I add the R of old insulation?
Yes, if the layers are uniform and stacked and the old insulation is dry, uncompressed and in good condition. Crushed or damp insulation no longer has its original resistance.
Why is the purchased area larger than the area to insulate?
The cutting allowance is added first, then the pack count is rounded up. The surplus shown is the difference between the area bought and the real area.
Is the insulation R the R of the wall?
No. A real wall includes framing, linings, sometimes an air layer, and thermal bridges. Its U-value is calculated with all those layers plus surface resistances.
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