This calculator handles two connected tasks: estimating insulation thickness from a target R-value or RSI and converting the project area into complete rolls, batt packs, board packs, kits or bags.
Start with the net area that will actually be insulated
Enter a known area, calculate one or more identical rectangular surfaces, or calculate the four walls of a rectangular room. Doors, windows and other sections that will not receive insulation can be deducted separately.
Three ways to enter the project size
| Mode | Inputs | Useful for |
|---|---|---|
| Known area | One value in ft² or m² | Plans, quotations or several zones already totalled |
| Rectangular surfaces | Length, width and quantity | Roof slopes, ceiling sections, wall panels or floor areas |
| Four room walls | Room length, width and wall height | A simple rectangular room |
R-value and RSI describe resistance to heat flow
R-value is the familiar US measure, while RSI in m²·K/W is widely used with metric specifications. The unit switch converts the target, existing resistance and product resistance rate, allowing the same calculation to be read in US customary or metric units.
The calculator does not choose a code-required R-value or RSI. The required value depends on the country, state, province or territory, climate zone, building element, project type and locally adopted rules.
Use the thermal resistance of the exact product
Enter the product resistance per inch or per 25 mm from its label or technical data sheet. Products in the same broad material family can differ in density, formulation, tested performance and permitted installed thickness.
The target is R-30, the existing layer is R-11 and the new product provides R-3.5 per inch. How much additional thickness is needed?
Answer: The missing resistance is R-19. Estimated thickness is 19 ÷ 3.5 = 5.43 in, or about 138 mm.
Explanation: This estimates the insulation layer itself. Framing, fasteners, air spaces and thermal bridges can change the performance of the complete assembly.
Package coverage must match the selected thickness
Which coverage figure belongs in the form
| Product format | Enter | Check |
|---|---|---|
| Rolls or batts | Coverage of one roll or batt pack | The thickness and stated R-value/RSI must match |
| Rigid boards | Combined board area in one pack | Do not confuse one board with the complete pack |
| Loose-fill or blown-in bags | Maximum net coverage per bag for the selected R-value | Use the correct row from the manufacturer's coverage chart |
| Spray foam or another kit | Area covered by one kit at the required thickness | Do not use a headline coverage figure for a thinner application |
Loose-fill coverage per bag is not constant. It changes with installed thickness, density, settling and target R-value, so a generic square-feet-per-bag figure is not enough.
Why the package quantity rounds up
The selected allowance is added to the net area before the result is divided by package coverage. Because partial unopened retail rolls, packs or bags are not normally available, the purchase quantity is rounded upward.
A project covers 500 ft², the allowance is 10%, and one package covers 100 ft². How many packages are needed?
Answer: The adjusted area is 550 ft². The mathematical result of 5.5 rounds up to 6 packages, providing 600 ft² of purchased coverage.
Explanation: The estimated 50 ft² remainder may consist of separate cuts rather than one reusable piece.
Insulation-layer R-value is not the whole wall or roof result
For planning, the calculator adds the entered existing resistance and the new insulation layer. It does not model wood or metal framing, headers, fixings, joints, moisture, air leakage or thermal bridges, so it is not a complete whole-assembly U-value calculation.
Terminology changes between markets
Common US and international wording
| US-oriented wording | Other wording you may see | Meaning here |
|---|---|---|
| Attic | Loft or roof space | The space below or within the roof |
| Batt | Insulation slab, segment or mat | A pre-cut piece intended to fit a defined space |
| Loose-fill / blown-in | Blown fibre or granulated insulation | Material installed from bags at a specified density and depth |
| R-value | RSI or thermal resistance | Resistance to heat flow in different unit systems |
| Vapor retarder | Vapour control layer | A separate moisture-control component when the assembly requires one |
Before installation, check fire safety, ventilation, airtightness, vapor or vapour control, moisture management, clearances around wiring and recessed lights, and the current instructions for the selected product and assembly.