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Insulation R-Value Calculator

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Add up multiple layers of insulation - batts, foam board, existing material - to find the combined R-value of a wall, attic, or floor assembly.

Insulation R-Value Calculator

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Total combined R-value
R-26

The formula

Total R-value = R-value of layer 1 + R-value of layer 2 + ... (layers add together in series)
Example

A wall with R-13 fiberglass batt, R-8 rigid foam board, and R-5 existing insulation: 13 + 8 + 5 = R-26 total.

Step-by-step guide

  1. List each layer of insulation in your assembly, from any existing material to new material you're adding.
  2. Enter the R-value of each layer - check the product label or manufacturer specification.
  3. Read the combined total and compare it against your region's recommended R-value.

Typical recommended R-values by climate and location

Recommended R-values vary significantly by US climate zone and which part of the house you're insulating. As a general reference: attics in colder climates often target R-49 to R-60, while milder climates may target R-30 to R-38. Wall insulation commonly targets R-13 to R-21, and floors over unheated spaces often target R-25 to R-30. Always check your specific local energy code or a current regional recommendation table, since these figures are commonly updated and vary by exact location.

Common mistakes

Compressing batt insulation to fit a smaller cavity than it's rated for - compressed insulation loses R-value, so it should fill its cavity at its intended thickness, not be squeezed in.
Assuming higher R-value alone guarantees a well-insulated space - air sealing gaps and leaks often matters just as much as raw R-value, since air movement can bypass insulation entirely.

Frequently asked questions

Do R-values really just add together?

Yes, for layers stacked in series (one behind another) with no significant air gaps or thermal bridging between them, R-values are additive - this is a standard, well-established principle in building science.

What is R-value, exactly?

R-value measures a material's resistance to heat flow - a higher R-value means better insulating performance and slower heat transfer through that material.

Does compressing insulation reduce its R-value?

Yes - batt insulation is rated for a specific thickness, and squeezing it into a smaller space than intended reduces its effective R-value below the number printed on the package.

Where can I find the recommended R-value for my area?

Local building codes and regional energy authorities publish climate-zone-specific R-value recommendations, since requirements vary based on your area's typical winter and summer temperatures.

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