Calculate Rockwool R-value from insulation thickness using a typical R-4.2 per inch or your product’s rating, or find the thickness for a target R-value.

Estimate resistance from thickness and the product property.

R-4.2 per inch matches Comfortbatt R-15 at 3.5 in and ComfortBoard 80. Check your product label for an exact rating.

Optional settings
For heat flow, enter both area and temperature difference; otherwise leave both blank.

Related Calculators

Rockwool R-Value Formula

The following formula estimates a mineral-wool layer R-value using the applicable productโ€™s US R-value per inch and installed thickness. Use the manufacturerโ€™s rating for the relevant test and installation conditions; a material name or density alone does not establish R-value.

Rโ‚œโ‚’โ‚œโ‚โ‚— = Rper in ร— t

Variables:

  • Rtotal is the layerโ€™s US R-value in hยทftยฒยทยฐF/Btu. Multiply US R by 0.17611018368230588 to obtain RSI in mยฒยทK/W; these are different numerical unit systems.
  • Rper in is the Rockwool R-value per inch (ftยฒยทยฐFยทhr/Btu per in), which varies by product and conditions
  • t is the insulation thickness (inches)

Note: For a given material layer, the R-value depends primarily on thickness and the materialโ€™s thermal conductivity (and can vary somewhat with temperature and density). It does not depend on area or temperature difference. Area and temperature difference are used to estimate steady heat flow through the modeled layer: Q (W) = A (mยฒ) ยท ฮ”T (K) / RSI (mยฒยทK/W). This calculator omits surface films, other layers, framing and air leakage, so its layer U-factor and heat-flow estimate are not whole-assembly ratings.

What is a Rockwool R-Value?

The R-value of Rockwool (mineral wool insulation) describes its resistance to heat flow. A higher R-value means better insulation performance (less heat flow for the same area and temperature difference). For Rockwool products, R-value is typically reported for a specific thickness, and it generally increases roughly in proportion to thickness.

How to Calculate Rockwool R-Value?

The following steps outline how to estimate the Rockwool total R-value, and (optionally) estimate heat flow using that R-value:

  1. Choose the typical ROCKWOOL rating (โ‰ˆ R-4.2 per inch), or enter your product’s US R-value per inch or thermal conductivity ฮป. For conductivity, RSI = thickness (m) / ฮป (W/mยทK).
  2. Measure the installed insulation thickness (t) in inches (convert units if needed).
  3. Calculate total R-value: Rtotal = Rper in ยท t.
  4. For optional heat flow, enter both insulated area and temperature difference in Optional settings; leave both blank to omit that estimate. A zero temperature difference gives zero heat flow.
  5. Select Calculate. For a target layer R-value, choose Thickness for a target R-value; the result is unrounded, with a separate optional round-up step. The default half-inch increment is a mathematical setting, not a stocked-product recommendation.

Example Problem:

Use the following variables as an example problem to test your knowledge:

Rockwool thickness (t) = 2 inches

Product R-value per inch (Rper in) = 4.2 hยทftยฒยทยฐF/Btu per in, the Comfortboard 80 July 2025 datasheet rating at 75 ยฐF. This is a product-specific example, not a universal mineral-wool value.

Area (A) = 100 square feet

Temperature difference (ฮ”T) = 20 ยฐF

Total R-value: Rtotal = 4.2 ร— 2 = 8.4 ftยฒยทยฐFยทhr/Btu.

Estimated heat flow: Q = (100 ร— 20) / 8.4 โ‰ˆ 238 Btu/hr (โ‰ˆ 69.8 W).