RSI to R-Value Calculator

Main conversion

Convert in either direction. A preset may fill the primary value.

Editable preset

Preset values are editable and product-dependent.

Conversion direction

Thermal-resistance value

Decimal precision

Insulation layer assembly

Add construction layers in RSI or R units. Surface films may be included separately.

Layer name

Resistance

Unit

Layer name

Resistance

Unit

Include interior air film

Interior film RSI

Include exterior air film

Exterior film RSI

Include the material result below in the assembly total

Material resistance from thickness

Calculate material RSI using thickness divided by thermal conductivity.

Material thickness

Thickness unit

Thermal conductivity

Conductivity unit

Parallel heat-flow paths

Compare insulated and framing paths. Percentages are normalised when necessary.

Path A resistance

Path A unit

Path A area percent

Path B resistance

Path B unit

Path B area percent

Heat-loss estimator

Estimate steady heat transfer through a selected resistance source.

Resistance source

Custom resistance

Custom unit

Surface area

Area unit

Temperature unit

Indoor temperature

Outdoor temperature

Displayed heat unit

Formula used

R-value = RSI × 5.678263337 RSI = R-value × 0.1761101838 Metric U-value = 1 ÷ RSI Imperial U-factor = 1 ÷ R-value Material RSI = thickness in metres ÷ conductivity Series assembly RSI = sum of all layer RSI values Parallel U-value = Σ(area fraction ÷ path RSI) Effective parallel RSI = 1 ÷ parallel U-value Heat loss = U-value × area × temperature difference

How to use the calculator

Select RSI to R-value or the reverse conversion.
Enter a positive thermal-resistance value.
Choose automatic or fixed decimal precision.
Add insulation layers and optional surface films.
Enter material thickness and conductivity when needed.
Set parallel paths for framing and insulated sections.
Choose a resistance source for the heat-loss estimate.
Press Calculate, then copy or export the results.

Example conversion data

RSI	R-value	Metric U-value	Common interpretation
0.18	1.02	5.56	Low thermal resistance
0.35	1.99	2.86	Typical glazing range
1.00	5.68	1.00	Moderate resistance
2.00	11.36	0.50	Insulated assembly
3.50	19.87	0.29	Higher wall resistance
5.00	28.39	0.20	High thermal resistance
7.00	39.75	0.14	Very high resistance

These examples are illustrative. Product ratings and assembly values can differ.

Frequently asked questions

What does RSI mean?

RSI measures thermal resistance using metric units. Higher RSI means greater resistance to heat flow.

What is an R-value?

R-value measures thermal resistance in imperial units. Higher values usually indicate better insulation performance.

How do I convert RSI to R-value?

Multiply the RSI value by 5.678263337. The calculator performs this step automatically.

How do I convert R-value to RSI?

Multiply the R-value by 0.1761101838. This is the reciprocal conversion factor.

What is a U-value?

U-value describes heat transfer through an assembly. Lower U-values indicate better thermal performance.

Can layer RSI values be added?

Series layer resistances can usually be added. Thermal bridges require a parallel-path method instead.

Why include surface air films?

Interior and exterior films add small thermal resistances. Their values depend on airflow and orientation.

How is material RSI calculated?

Divide material thickness in metres by conductivity. Conductivity must use compatible SI units.

What is a parallel heat-flow path?

Heat may travel through insulation and framing differently. Area-weighted U-values estimate the combined effect.

Is the heat-loss result exact?

The result is a steady-state estimate. Air leakage, moisture, and thermal bridges can change actual performance.

Why can nominal and assembly R-values differ?

Nominal ratings describe individual materials. Whole assemblies include framing, gaps, films, and connections.

Important note

Use manufacturer data for final design decisions. Building regulations may require approved calculation methods.