Roof Insulation Calculator

Insulation R-value and thickness from energy code.

Inputs

Roof Area (sqft)

Target R-Value

Insulation Type (0-3)

Waste Factor (%)

Tab to move between fields

Results

Required Thickness (in)

5.5

Achieved R-Value31.4

U-Factor (BTU/hr·ft²·°F)0.03

Number of Layers2

Insulation Boards (4×8)198

Cover Boards (4×8)99

Calculated Thickness (in)5.26

Est. Material Cost ($)$15,259.00

Layer Thicknesses3, 2.5

How to Use This Calculator

Enter total roof area (sqft) and target R-value.
Select insulation type (polyiso, EPS, XPS, or mineral wool) and set waste factor (%).
Review insulation thickness (in), achieved R-value, U-factor, number of layers, and board count.
Confirm achieved R-value meets local energy code minimums (ASHRAE 90.1 or IECC requirements).

How the result changes with Target R-Value

Target R-Value	Required Thickness (in)
8.9	2
29	5.5
52	9.5
72	13

What each input means

Roof Area (sqft): Total roof area to insulate in square feet.
Target R-Value: Required R-value per energy code. IECC: Zone 4=R-25, Zone 5-6=R-30, Zone 7-8=R-35.
Insulation Type (0-3): 0=Polyiso (R-5.7/in), 1=EPS (R-3.8/in), 2=XPS (R-5.0/in), 3=Mineral Wool (R-4.2/in).
Waste Factor (%): Extra material for cuts and fitting. 5% typical for flat roofs.

What each result means

Required Thickness (in): Insulation thickness rounded to nearest standard board size.
Achieved R-Value: Actual R-value from the specified thickness.
U-Factor (BTU/hr·ft²·°F): Thermal transmittance (lower = better insulation).
Number of Layers: Layers needed (thick assemblies may require 2 layers with staggered joints).
Insulation Boards (4×8): Total 4 ft × 8 ft insulation boards across all layers.
Cover Boards (4×8): HD fiberboard or gypsum cover boards over insulation.
Calculated Thickness (in): Exact calculated thickness before rounding to standard size.
Est. Material Cost ($): Approximate material cost for insulation, cover board, and adhesive.

How this is calculated

Worked example, using the default values

Identify Input Parameters
4 parameters
Roof Area (sqft) = 3000, Target R-Value = 30, Insulation Type (0-3) = 0, Waste Factor (%) = 5 = 4 input(s) provided
Calculate Required Thickness
Required Thickness
5.5 = 5.5
Calculate Achieved R-Value
Achieved R-Value
31.4 = 31.4
Calculate U-Factor
U-Factor
0.0318 = 0.0318

Engine last updated August 5, 2026. Checked against 1 independently-derived test — how we verify calculators.

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