Floor Slab Insulation Calculator

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Unit system

Inputs are converted to consistent SI units for calculation.

Area input mode

Choose the quickest way to set slab area.

Waste factor (%)

Adds cutting and fitting allowance to purchased area.

Length (m)

Width (m)

Area (m²)

Length (ft)

Width (ft)

Area (ft²)

Concrete thickness (mm)

Used to estimate concrete thermal resistance.

Concrete conductivity k (W/m·K)

Typical concrete ranges roughly 1.4–2.0.

Other layers R (m²·K/W)

Includes surface resistances and finishes if desired.

Insulation material

Common under-slab board for general loads.

Insulation λ (W/m·K)

Use datasheet λ for realistic results.

Existing insulation (mm)

Set to 0 for new builds.

Existing insulation (in)

Set to 0 for new builds.

Target type

Select the metric your project specifications provide.

Target U-value (W/m²·K)

Lower U-value means better insulation.

Target R-value (m²·K/W)

Higher R-value means better insulation.

ΔT for heat loss

°C°F temperature difference for point heat loss.

Heating Degree Days (HDD)

K·day°F·day for annual energy estimation.

Energy price

USD /kWh

Used to estimate annual savings and payback.

Currency

Only affects display and exports.

Cost input method

Use scaled cost when pricing per thickness is known.

Cost per m² per mm

Example: 0.02 means USD0.02 per m² per mm added.

Cost per m² (for your chosen thickness)

Best when you already have a supplier quote.

Tip: For slabs on ground, consider perimeter edge insulation and thermal breaks at thresholds.

How to Use This Calculator

Choose your unit system and enter slab area.
Set concrete thickness and adjust conductivity if needed.
Select an insulation type, or enter a custom λ value.
Choose a target U-value or a target R-value.
Enter ΔT and HDD to estimate heat loss and annual energy.
Add cost and energy price to estimate savings and payback.
Press Calculate, then export results as CSV or PDF.

Formulas Used

Layer resistance: R = t / λ where t is thickness (m) and λ is conductivity (W/m·K).
Concrete resistance: R_concrete = t_slab / k.
Total resistance: R_total = R_other + R_concrete + R_insulation.
U-value: U = 1 / R_total.
Required insulation: R_ins,req = R_target − (R_other + R_concrete) and t_ins,req = R_ins,req × λ.
Heat loss at ΔT: Q = U × A × ΔT (W).
Annual energy (HDD): E = U × A × HDD × 24 ÷ 1000 (kWh).

Example Data Table

Scenario	Area (m²)	λ (W/m·K)	Target U (W/m²·K)	Required Insulation (mm)	Final U (W/m²·K)	Saved (kWh/yr)
New build, EPS, moderate target	48	0.036	0.25	~130	0.25	~7,000
Upgrade, XPS, tighter target	60	0.030	0.20	~145 (total)	0.20	~10,000
High performance, PIR	80	0.022	0.15	~140	0.15	~18,000

Examples assume 100 mm concrete, R-other 0.17, and HDD around 1500 K·day.

Good practice: Always confirm compressive strength for under-slab boards, detail joints, and consider moisture control layers to protect performance.

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How to Use This Calculator

Formulas Used

Example Data Table

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