Insulation Calculator

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Project surface:

Climate target:

Custom added R target:

Surface area:

GrossOpenings

Package coverage:

Thermal resistance:

AddedExisting

Insulation material:

Custom material rate:

R / in

Design temperature difference:

Waste allowance:

Optional price, load hours, and shared surface resistance are zero by default, so opening Advanced does not change the takeoff or thermal result.

Package price:

$/ package

Seasonal load hours:

hours

Shared surface resistance:

Insulation takeoff with quantity, thermal context, optional cost, and estimating basis.
Item	Quantity	Thermal context	Estimated cost	Basis	Copy
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Steady-state conductive heat flow through the entered net area; air leakage, thermal bridging, moisture, and equipment efficiency are outside this comparison.

An insulation plan joins two questions that are easy to confuse. The purchasing question asks how many bags, rolls, batt packs, boards, or kits cover the measured surface after openings and waste. The thermal question asks how added resistance changes conductive heat flow through that same surface.

R-value measures resistance to heat flow. A larger R-value means more resistance, while U-value is its inverse and describes heat transmittance. Adding insulation raises total resistance and lowers transmittance, but installed performance still depends on continuity, thickness, density, moisture, compression, air sealing, and thermal bridges through framing.

Insulation planning quantities and their roles
Quantity	Purpose	Important boundary
Net area	Surface that actually receives insulation	Gross area minus openings; openings must remain smaller than gross area
Adjusted area	Coverage to order before package rounding	Net area plus 0% to 35% waste
Package count	Whole purchasable units	Always rounds up from adjusted area divided by label coverage
Added resistance	Thermal improvement being installed	Compared with a selected added-R reference, not automatically with total assembly R
Modeled heat loss	Before-and-after steady conductive flow	Excludes air leakage, thermal bridging, solar gain, moisture, and dynamic weather

Package coverage must come from the exact product label at the intended installed R-value or thickness. A package that covers 50 m² at one depth will cover less area at a deeper loose-fill application. Typical R-per-inch values can help estimate depth, but manufacturer instructions control installed thickness, density, and coverage.

Climate-zone reference values are planning context for existing U.S. wood-framed buildings. They vary by attic condition, floor, wall sheathing, and basement or crawlspace wall. Some surface-and-zone combinations have no listed added-R target, and local energy code or a project specification may require a different value.

A package estimate is not an installation design. Air sealing, vapor control, fire protection, ventilation, recessed fixtures, electrical hazards, combustion appliances, roof moisture, and safe access may need professional or code-specific review before material is purchased.

How to Use This Tool:

Measure the intended insulated surface and use product-label coverage. Treat climate references and material depth as checks, not replacements for local code or manufacturer data.

Select the Project surface and broad Climate target. Choose Custom added R target when a specification supplies the governing value.
Enter gross surface area and the total area of openings or excluded sections. Both values share one area unit, and openings must be smaller than gross area.
Enter the printed coverage of one package in square metres or square feet, then set the added and existing resistance in either RSI or U.S. R-value.
Select the insulation material for an approximate depth estimate. Use Custom R per inch when the product label gives a different nominal rate.
Enter a design temperature difference, not an indoor or outdoor absolute temperature, and choose a waste allowance from 0% through 35%.
Review Material takeoff for whole packages and overage, then check Installation brief for target status and model limits. Compare the before-and-after curve only after the resistance inputs are verified.

Interpreting Results:

Package count is the ceiling of the exact package requirement. Rounded-order overage is coverage left after waste and whole-package rounding; it is not necessarily reusable material because cuts, cavities, settling, and label-specific coverage can consume it.

Target met means the entered added resistance is equal to or greater than the selected added-R reference. Below reference reports the remaining difference. No listed target means the selected reference table has no value for that surface and climate combination; it does not mean that insulation is unnecessary.

The percentage reduction applies only to modeled steady conductive heat flow through the net area. It should not be presented as a predicted percentage reduction in the whole building's energy bill, HVAC load, or emissions.

Technical Details:

Area is normalized to square metres, resistance to RSI in m²·K/W, and temperature difference to kelvins. U.S. R-value converts to RSI with 1 R = 0.1761101838 m²·K/W. A Fahrenheit temperature difference converts to kelvins by multiplying by 5/9; temperature differences have no offset term.

Formula Core:

The material path rounds coverage to whole packages. The thermal path adds resistance and applies the steady-state conduction equation to the same net area.

\begin{array}{l} A_{net} = A_{gross} - A_{openings} \\ A_{adjusted} = A_{net} \times (1 + \frac{w}{100}) \\ N = ⌈ \frac{A_{adjusted}}{A_{package}} ⌉ \\ R_{upgraded} = R_{existing} + R_{shared} + R_{added} \\ Q = \frac{A \times ΔT}{R} = U \times A \times ΔT \end{array}

N is the whole package count, w is waste percent, R is total RSI, U is 1/R in W/(m²·K), and Q is modeled heat flow in watts. Seasonal reduction, when requested, multiplies the before-and-after watt difference by equivalent load hours and divides by 1,000 to report kWh.

Approximate added depth divides the entered U.S. R-value by the selected R-per-inch rate. Built-in nominal rates are 3.2 for fiberglass batt, 2.8 for blown fiberglass, 3.7 for cellulose, 4.0 for mineral wool, 3.7 for open-cell spray foam, and 5.6 for polyiso board. These values are planning assumptions; an exact product data sheet takes precedence.

Lookup Core:

The built-in table stores added U.S. R-value references. Zero means the source table does not list an added value for that combination; it is returned as No listed target, not as a met zero target.

Built-in added R-value references by surface and climate band
Surface	Zone 1	Zone 2	Zone 3	4A–4B	4C–6	7–8
Uninsulated attic or ceiling	R30	R49	R49	R60	R60	R60
Attic with 3–4 in existing	R25	R38	R38	R49	R49	R49
Floor over unconditioned space	R13	R13	R19	R19	R30	R38
Exterior wall sheathing add-on	—	—	R5	R8	R10	R10
Basement or crawlspace wall	—	—	R13	R13	R19	R19

The target is met when added RSI is at least the converted target, with a tolerance of 10⁻¹². Package rounding subtracts the same tiny tolerance before applying the ceiling so an exact package boundary does not become one extra package because of floating-point noise.

The heat-loss curve evaluates temperature differences at 0.5×, 0.75×, 1×, 1.25×, and 1.5× the entered design difference. Resistance and area stay fixed, so all three heat-flow series change linearly with temperature difference.

Accuracy Notes:

Steady conductive heat flow is a controlled comparison, not a building energy simulation. Framing bridges, air leakage, ventilation, solar gain, thermal mass, moisture, wind, equipment efficiency, and changing weather are not included.

Use product-label coverage and installed R-value for the intended thickness, orientation, and cavity.
Verify local code, fire and vapor-control requirements, attic ventilation, and safe clearances before installation.
Shared surface resistance affects both before and after cases. Add it only when a known layer or film resistance belongs in both.
Seasonal load hours are equivalent hours at the entered temperature difference, not ordinary calendar hours.

Worked Examples:

Existing attic in Zones 4A and 4B

A gross area of 89 m² minus 11 m² of openings leaves 78 m². Adding 10% waste gives 85.8 m². At 8.2 m² per package, the exact requirement is about 10.46, so the order is 11 packages covering 90.2 m² and leaving 4.4 m² above adjusted area. With existing RSI 1.9 and added RSI 6.7, upgraded resistance is RSI 8.6. The built-in R49 added target converts to about RSI 8.63, so the added insulation remains about RSI 1.93 below that reference. At a 22.2 K difference, modeled conduction falls from about 911 W to 201 W through the net area.

References:

Recommended Home Insulation R-Values, ENERGY STAR.
Energy Renovations: Insulation, U.S. Department of Energy.