Insulation R-Value Converter

The R-value measures thermal resistance, meaning how well a material resists heat flow through it. A higher R-value means better insulation and less heat loss.

Conversion between imperial and metric:

• 1 R-value (ft2 F h/BTU) = 0.176110 RSI (m2 K/W)
• 1 RSI (m2 K/W) = 5.678263 R-value

The formulas are simply:

• RSI = R-value x 0.176110
• R-value = RSI x 5.678263

Common insulation materials and their R-values per inch (per 25.4 mm):

Recommended total R-values by climate zone (USA):

How thick does that make the batt?

This is the question the two tables above exist to answer, and it is worth doing in that order rather than the other way round. Start from the R-value your climate zone asks for, divide by the R per inch of the material you are using, and you have the depth of the cavity you need. R-49 in an attic is 14 to 16 inches of fiberglass, or 7 to 9 inches of closed-cell spray foam or rigid polyiso. That gap is why an attic gets loose fill and a wall cavity gets foam: in the attic, depth costs almost nothing, so you buy the cheap material and use more of it.

The reverse question matters too. A standard 2x4 stud wall gives you 3.5 inches of cavity. Fiberglass fills that with about R-11, and cellulose with R-11 to R-13. Neither reaches R-20 no matter how hard you pack it, and packing it harder makes it worse rather than better. Closed-cell foam does reach it, at R-21 to R-24 in the same 3.5 inches, which is most of the reason people pay for foam in a wall they cannot make deeper.

Practical tips:

• R-values are additive: two layers of R-13 insulation stacked together give R-26 total.
• The R-value per inch varies by material, so thickness alone does not determine insulation performance.
• In Canada and most of Europe, RSI is the standard unit in building codes. Australia’s metric R-values are numerically the same as RSI, just labeled “R”.
• Gaps, compression, and moisture all reduce the effective R-value significantly, so proper installation is as important as the material rating itself.
• The wood studs themselves are a thermal bridge at roughly R-1.25 per inch, so a wall’s whole-assembly R-value always comes in below its cavity insulation rating. That is what continuous exterior foam is for.