Insulation Calculator

What Is an Insulation Calculator?

An insulation calculator finds the total R-value of a multi-layer wall, attic, or floor assembly. Add each material layer with its thickness to get combined thermal resistance, RSI in metric units, and the U-value used for heat transfer calculations.

How Total R-Value Is Calculated

Each layer contributes resistance based on its R-value per inch and thickness:

$$ R_{total} = R_1 t_1 + R_2 t_2 + R_3 t_3 + \ldots + R_n t_n $$

Where $R_n$ is the material R-value per inch and $t_n$ is thickness in inches. The U-value is the reciprocal of total R-value: $U = 1 / R_{total}$.

To convert R-value to RSI (metric), divide by 5.6785917. For heating load estimates using U-values, try the Heat Loss Calculator.

Frequently Asked Questions

What is R-value?

R-value measures thermal resistance in °F·ft²·hr/BTU per inch of thickness. Higher R-values mean better insulation. Total R-value increases when you add more insulation or thicker layers.

What is the difference between R-value and U-value?

R-value measures resistance to heat flow. U-value measures conductance (how easily heat passes through). U-value equals 1 divided by total R-value for a given assembly.

Can I mix inches and millimeters?

Yes. Each layer can use its own thickness unit. The calculator converts all thicknesses to inches before applying material R-values per inch.

Does air space count as insulation?

Yes. A trapped air space adds thermal resistance. This calculator includes air space as a selectable layer with an R-value of about 1.43 per inch.

Are these R-values suitable for code compliance?

Values are typical reference numbers for planning. Local building codes may require specific tested values. Confirm requirements with your jurisdiction before construction.

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What Is an Insulation Calculator?

How Total R-Value Is Calculated

Frequently Asked Questions

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