Enter the nominal r-value and the percent decrease in performance (%) into the Calculator. The calculator will evaluate the Effective R-Value. 

Effective R-Value Formula

ER = NV * PD/100


  • ER is the Effective R-Value ()
  • NV is the nominal r-value
  • PD is the percent decrease in performance (%)

To calculate the Effective R-Value, multiply the nominal r-value by the percent decrease in performance over 100.

How to Calculate Effective R-Value?

The following steps outline how to calculate the Effective R-Value.

  1. First, determine the nominal r-value. 
  2. Next, determine the percent decrease in performance (%). 
  3. Next, gather the formula from above = ER = NV * PD/100.
  4. Finally, calculate the Effective R-Value.
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

nominal r-value = 2.25

percent decrease in performance (%) = 3


What is the significance of the R-Value in insulation materials?

R-Value measures the thermal resistance of insulation materials. A higher R-Value indicates better insulation performance, meaning the material is more effective at reducing heat flow.

How does the percent decrease in performance affect the Effective R-Value?

The percent decrease in performance reflects the reduction in insulation effectiveness due to various factors like aging, moisture, or installation issues. It directly lowers the Effective R-Value, indicating decreased insulation performance.

Can the Effective R-Value increase over time?

No, the Effective R-Value typically decreases over time due to material degradation, wear, or external factors affecting the insulation’s performance.

Why is it important to calculate the Effective R-Value rather than just using the nominal R-Value?

Calculating the Effective R-Value provides a more accurate representation of an insulation material’s current performance. It factors in real-world conditions that may reduce the material’s insulating properties, offering a realistic view of its thermal resistance.