Does doubling insulation thickness yield double the R-value?

Insulation R-Value Relationship

The R-value of insulation is a measure of its ability to resist heat flow. It’s calculated as the ratio of temperature difference to the heat flow per unit area, typically expressed in units of thermal resistance per unit thickness (e.g., ft^2°F·h/BTU). The R-value of a given insulation type is generally a fixed value per inch of thickness. For example, fiberglass batts have an R-value of around 3.5 per inch, while spray foam insulation has an R-value of around 6.0 per inch.

R-Value Multiplier Effect

When you add multiple layers of insulation, the total R-value is calculated by adding the individual R-values of each layer. However, the incremental R-value of each additional layer decreases as the total R-value increases. For example, if you have a 4-inch thick wall with an R-value of 14 (3.5 per inch x 4 inches), adding another 4 inches of insulation would increase the total R-value to around 28 (3.5 per inch x 8 inches). However, the additional R-value from the second 4-inch layer would only be around 14 (3.5 per inch x 4 inches) since the incremental R-value of each additional layer decreases as the total R-value increases.

R-Value Calculation Example

To illustrate the relationship between insulation thickness and R-value, consider a wall with a total R-value of 28 achieved with 8 inches of fiberglass batts (3.5 per inch x 8 inches). If you add another 4 inches of fiberglass batts, the total R-value would increase to around 42 (3.5 per inch x 12 inches), but the additional R-value from the second 4-inch layer would only be around 14 (3.5 per inch x 4 inches). This example demonstrates how the R-value of a given insulation type is generally a fixed value per inch of thickness and how the incremental R-value of each additional layer decreases as the total R-value increases.