Insulation Calculator

How to use the insulation R-value calculator

What this calculator does

This calculator estimates the nominal thermal resistance of a two-layer building assembly. It multiplies each material's listed R-value per inch by that layer's thickness, then adds both layer results. It also reports the SI equivalent, called RSI, and the reciprocal U-factor. The tool is best used for comparing early design options, checking a simple retrofit concept, or understanding how much each layer contributes. It does not replace a whole-wall analysis, a product datasheet, an energy model, or a local code review because framing, gaps, compression, moisture, temperature, fasteners, and thermal bridges can reduce actual performance.

When to use it

Use it when comparing two insulation products for a wall or roof, estimating the effect of adding rigid foam over an existing cavity layer, checking a classroom heat-transfer example, or preparing a simple material schedule. For climate-specific targets, compare the result with ENERGY STAR's recommended home insulation R-values and the requirements adopted by your local authority.

How to calculate

1. The calculator opens with a ready-to-use demonstration: 3 inches of standard fiberglass batt plus 1 inch of EPS foam. The result and a validated Excel workbook are available immediately.
2. Choose Preferred measurement system. Imperial displays thickness in inches and R-value; Metric displays thickness in millimeters and the primary result as RSI. Switching units converts the current thickness values rather than merely changing their labels.
3. For each layer, choose a Layer material and enter a positive Thickness of layer. Plain decimal notation is accepted; commas, scientific notation, unit text, and negative values are rejected to prevent ambiguous interpretation.
4. Read Total R-value, Total thickness, RSI value, U-factor, and Best-performing layer. The layer table shows the exact contribution and percentage share from each material.
5. Select Download Excel to export the current typed inputs and canonical results. Reset clears the demonstration values, results, table, validation state, and workbook cache; export remains disabled until both layers are complete and valid again.

Input guide

Preferred measurement system is required and accepts Imperial or Metric. Imperial thickness is entered in inches; Metric thickness is entered in millimeters. A value of 25.4 mm equals 1 inch. Changing the system preserves physical thickness through conversion. A common mistake is entering millimeters while Imperial is selected.

Each Layer material is required. The selection supplies a nominal thermal resistance per inch, such as 3.40 for standard fiberglass batt or 4.00 for EPS foam. Higher listed values generally increase the total more quickly for the same thickness. Material values are planning assumptions, not a guarantee for every branded product or installed condition.

Each Thickness of layer is required, must be greater than zero, and may be up to 1,000 inches or 25,400 millimeters. A realistic example is 3 inches of fiberglass and 1 inch of rigid foam. Increasing thickness increases that layer's contribution linearly. Do not include quote marks, “mm,” fractions such as 3 1/2, commas, or scientific notation; enter 3.5 instead.

Output guide and worked example

Total R-value is the sum of both layer R-values in °F·ft²·hr/BTU. Total thickness is the simple physical thickness sum in the active unit. RSI value converts the total R-value to m²·K/W by dividing by 5.6785917. U-factor is the reciprocal of total R-value in BTU/(h·ft²·°F), so lower values indicate less heat transfer under this simplified one-dimensional model. Best-performing layer identifies the layer with the larger R contribution; a tie is shown when both are equal. The table's R per inch, Layer R-value, and Share of total R columns explain why one layer dominates.

For the startup example, Layer 1 is standard fiberglass batt at 3.00 inches and R-3.40 per inch, giving 3 × 3.40 = 10.20. Layer 2 is EPS foam at 1.00 inch and R-4.00 per inch, giving 1 × 4.00 = 4.00. The total is R-14.20, the thickness is 4.00 inches, RSI is 14.20 ÷ 5.6785917 = 2.501 m²·K/W, and the U-factor is 1 ÷ 14.20 = 0.0704. Layer 1 contributes about 71.83% of the nominal total.