How to use the insulation calculator
Insulation is specified by thermal resistance, the R-value in the US or RSI in metric countries, but it is bought by the pack or the bag. This calculator connects the two: it turns a target R-value into a thickness, takes off whatever is already there, and tells you how many batts, rolls or bags of loose fill cover your area.
- Choose units and type. US mode uses square feet, inches and R-values; metric mode uses square metres, millimetres and RSI with the material's thermal conductivity (λ). Pick batts and rolls, or blown-in loose fill.
- Pick the material. This fills in a typical R-value per inch or λ. Overwrite it with the figure on your product label for a closer answer.
- Enter the area, the target and any existing insulation. For a top-up, enter the R-value already there. If you only know the depth, multiply it by about R-3 per inch for old fibreglass or rock wool batts (in metric, divide the thickness in metres by its λ).
- Enter the product details. For batts and rolls, give the thickness and the coverage per pack. For loose fill, enter the settling allowance and either the bag chart's coverage at your R-value or a density and bag weight for an estimate.
Insulation formulas
Thickness needed = target R ÷ R per inch (metric: thickness = RSI × λ)
Existing R ≈ existing thickness × its R per inch (metric: thickness ÷ λ)
Layers = ⌈(target − existing R) ÷ R of one layer⌉
Packs = ⌈area × layers × (1 + extra %) ÷ coverage per pack⌉
Installed loose-fill depth = settled depth × (1 + settling %)
Bags = area × settled depth × density ÷ bag weight × (1 + extra %), or area ÷ chart coverage
Typical values are about R-3.2 per inch for fibreglass batts, R-4.0 for mineral wool batts, R-3.5 for blown cellulose and R-2.5 for loose-fill fibreglass. One RSI equals R-5.678. Densities for loose fill vary a lot by product, which is why the bag chart is the better source when you have it.
| IECC 2021 climate zone | Ceiling | Wood-frame wall |
|---|---|---|
| 0–1 | R-30 | R-13 or R-0 + 10 ci |
| 2 | R-49 | R-13 or R-0 + 10 ci |
| 3 | R-49 | R-20, R-13 + 5 ci or R-0 + 15 ci |
| 4–8 (and 4C) | R-60 | R-20 + 5 ci, R-13 + 10 ci or R-0 + 15 ci |
These are the prescriptive values in IECC 2021 Table R402.1.3, where "ci" means continuous insulation. Jurisdictions adopt different editions and amendments, so confirm the numbers locally. For a full wall or roof build-up, use the R-value and U-value calculator.
Worked examples
Topping up an attic with blown cellulose
A 1,000 ft² attic has 3.5 in of old fibreglass batts, worth about R-11.2 at R-3.2 per inch. Bringing it up to R-49 needs another R-37.8. At R-3.5 per inch, that is 10.8 in of settled cellulose, or about 12.4 in when installed with a 15% settling allowance. At a settled density of 1.5 lb/ft³, the 900 ft³ of cellulose weighs 1,350 lb: 54 bags of 25 lb, or 57 with 5% extra. If the bag chart says one bag covers 16 ft² at R-49, use that instead: 1,000 ÷ 16 = 62.5, or 66 bags with extra.
A metric loft top-up with rolls
A 50 m² loft has 100 mm of glass mineral wool between the joists. With λ = 0.044 W/m·K that is RSI 2.27, which you enter as the existing RSI. To reach the recommended 270 mm (about RSI 6.1), lay one layer of 170 mm roll across the joists, worth RSI 3.86, for a total of RSI 6.14. If one roll covers 6 m², you need 50 × 1.05 ÷ 6 = 8.75, so 9 rolls.
Who this calculator helps
- Architects and energy assessors turning a specified R-value into a thickness and a quantity for a spec or cost plan.
- Builders and insulation contractors pricing a retrofit or new-build job quickly.
- Interior designers and renovators checking how much depth an upgrade will take up in a loft or wall.
- Homeowners working out how many bags to put in the car for a weekend attic top-up.
Practical tips
- Air-seal gaps around pipes, wires and light fittings before insulating; insulation doesn't stop air leaks.
- Keep the eaves ventilated with baffles, and keep insulation the required distance from recessed lights and flues unless they are rated for contact.
- Fix depth markers or rulers across the attic so you can check the blown-in depth as you go.
- Work out the area with the square footage calculator, and size heating or cooling with the BTU calculator once the insulation is upgraded.
Planning a loft conversion after the upgrade? Show the client the finished room with Room Redesign.
