Insulation R-Value Thickness Calculator
Estimate the required insulation thickness to reach a target R-value based on material R-value per unit thickness.
What the insulation R-value thickness calculator does
The insulation R-value thickness calculator tells you how thick a layer of insulation has to be to reach a target R-value. Pick a material from the preset list or enter the R-value per inch yourself, type the total R-value you want, choose inches or centimetres, and press Calculate. The result is the required thickness in the chosen unit with the metric or imperial equivalent alongside.
The formula
thickness = target R-value ÷ R-value per unit thickness
R-values add in proportion to thickness, so the calculation is a simple division. With the default target of R-30 and 3.5 per inch: 30 ÷ 3.5 = 8.6 inches, or 21.8 cm.
R-value per inch of common materials
| Material | R per inch (imperial) | Inches for R-30 | Notes |
|---|---|---|---|
| Fiberglass batt | 3.2 | 9.4 | Cheapest; loses value if compressed |
| High-density fiberglass | 3.7 | 8.1 | Used where cavity depth is limited |
| Cellulose loose-fill | 3.8 | 7.9 | Recycled paper; settles 10 to 20% over time |
| Mineral wool | 4.0 | 7.5 | Fire resistant, holds shape |
| Rigid foam board (XPS / polyiso) | 5.0 | 6.0 | Polyiso is higher when new, lower when cold |
| Closed-cell spray foam | 6.0 | 5.0 | Also an air and vapour barrier |
The presets set the R-per-inch field automatically. Product labels give the declared figure for the actual batch, which should replace the preset when you have it.
Typical target R-values
| Location | Cold climate (IECC zones 5 to 7) | Mild climate (zones 2 to 3) |
|---|---|---|
| Attic / ceiling | R-49 to R-60 | R-38 |
| Wood-frame wall | R-20 to R-21 cavity plus R-5 continuous | R-13 to R-15 |
| Floor over unheated space | R-30 | R-13 to R-19 |
| Basement wall | R-15 continuous | R-5 or none |
For an attic at R-49 with blown cellulose: 49 ÷ 3.8 = 12.9 inches. With loose-fill, add the manufacturer’s settled-thickness allowance so the value holds after a few years.
When the cavity is too shallow
A 2×4 wall is 3.5 inches deep, which holds R-11 to R-13 of fiberglass or R-21 of closed-cell spray foam. A 2×6 wall (5.5 inches) holds R-19 to R-21 in fiberglass or R-23 in mineral wool. When the target exceeds what the cavity can hold, add continuous rigid foam on the outside of the studs; this also cuts thermal bridging through the framing, which otherwise reduces the whole-wall value by 20 to 30% below the cavity figure.
Imperial and metric R-values
North American R-values are in ft²·°F·h/BTU. The SI equivalent, m²·K/W, is 5.678 times smaller: R-30 imperial is RSI 5.3. European products quote thermal conductivity λ in W/m·K instead; R per metre is 1/λ, so mineral wool at λ = 0.035 gives RSI 2.86 per 100 mm, which converts to R-16 imperial per 100 mm, or about R-4.1 per inch. The thermal resistance calculator works from λ and thickness directly, and the U-value calculator turns a total R into the U-value used in European regulations.
Common mistakes
- Entering RSI as an imperial R-value. A target of “5” meant as RSI needs to be entered as 28.4.
- Compressing batts to fit. An R-19 batt squeezed into a 3.5 inch cavity delivers about R-13.
- Counting nominal rather than settled thickness for loose-fill.
- Ignoring air leakage. Insulation slows conduction; gaps around it let air carry heat straight through. Air sealing usually matters as much as thickness.
Related tools
The Fourier heat transfer calculator converts the finished R-value into heat loss in watts, and the heat flux converter switches units. For the rest of the job, the drywall sheets calculator and paint calculator estimate finishing materials, and the roof rafter length calculator helps size an attic retrofit.
External references
- U.S. Department of Energy: insulation and recommended R-values by zone
- ENERGY STAR: recommended home insulation R-values
Frequently asked questions
Does doubling thickness double the R-value?
Yes for the insulation layer itself. Whole-assembly gains are smaller because framing, air films and cladding do not change.
Which material gives the most R-value per inch?
Vacuum insulated panels (R-25 to R-40 per inch) and aerogel (R-10) top the list but are expensive. Among common products, closed-cell spray foam and polyiso board lead at R-6 to R-6.5 per inch.
Is there a point where more insulation stops paying off?
Yes. Heat loss is proportional to 1/R, so going from R-10 to R-20 halves the loss, but R-40 to R-50 cuts only a further 20%. Code minimums are set near the economic optimum.
How do I convert R-value per inch to lambda?
λ (W/m·K) = 0.1442 ÷ R per inch. Fiberglass at R-3.2 per inch is λ = 0.045.
Does the calculator include air films or existing layers?
No. It sizes the insulation layer alone. Subtract the R-value of existing materials and surface films from the target before entering it.