Rigid insulation like rigid foam and foam boards have **high R-values**. That’s why materials like polyiso *(polyisocyanurate)*, EPS *(expanded polystyrene)*, and XPS *(**extruded polystyrene)* like Styrofoam make for such good insulation materials. We are going to look what is rigid insulation R-value ** per inch** and the rigid insulation thickness chart (for 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10-inch foam boards).

Namely, we will look into 3 main types of rigid insulation:

**R-values of EPS.**EPS stands for. We differentiate between higher R-value high-density EPS and lower R-value low-density EPS.**E**xpanded**P**oly**S**tyrene**R-values of XPS**(also known as Styrofoam). XPS stands for*e*. The same as with EPS, we differentiate between higher R-value high-density XPS and lower R-value low-density XPS.**X**truded**P**oly**S**tyrene**R-values of ISO**(also known as polyiso). ISO is an abbreviation of*poly*. This is foil-faced polyisocyanurate; the R-value of ISO decreases over the years.**ISO**cyanurate

R-values for rigid insulation **vary from R-3.85 to R-6.8 per inch** of thickness. Polyiso has the highest R-value, followed by XPS and EPS. The key to all rigid insulation high R-values are small air pockets that exhibit high thermal resistivity properties:

Altogether we are going to check out **5 relevant charts** for rigid insulation R-values:

- We will start with the list of the R-values of these rigid insulations
**per inch of thickness***(Chart 1)*. - After that, we are going to look at
**rigid insulation thickness charts for EPS, XPS, and ISO**at various thicknesses*(Chart 2, Chart 3, and Chart 4)*. - Additionally, we will look at how thick rigid insulation we need to achieve
**R10, R20, R30, and R49**insulation value*(Chart 5).*

*Note:* Rigid insulation boards are commonly manufactured in **2×4, 4×4, and 4×8 board sizes**. Nonlaminated boards have thicknesses of 0.5, 0.75, and 1 inches. Laminated boards have thicknesses of 0.5, 1, and 2 inches.

Let’s start with the specified per inch R-value of rigid insulation:

## Rigid Insulation R-Value Per Inch (Chart 1)

Rigid Insulation Material: |
Insulation R-Value Per Inch: |

High-Density EPS | R-4.2 Per Inch |

Low-Density EPS | R-3.85 Per Inch |

High-Density XPS | R-5.2 Per Inch |

Low-Density XPS | R-4.15 Per Inch |

ISO (Penthane Expanded, New Polyiso) | R-6.8 Per Inch |

ISO (Penthane Expanded, 5-10 Years Old Polyiso) | R-5.5 Per Inch |

As you can see, polyiso has the highest rigid insulation R-value. A new polyiso will have an R-value of R-6.8 per inch. This is a 31% higher R-value than high-density XPS and a 62% higher R-value than high-density EPS.

It is worth noting that the thermal resistance of ISO degrades in time. Within 5 to 10 years, the R-value of polyiso will likely fall from R-6.8 to R-5.5. This is a 24% drop in R-value.

High-density XPS has an R-value of 5.2 per inch; low-density XPS has an R-value of 4.15 per inch. Styrofoam is a trademark name for XPS; the R-value of Styrofoam is 5.2 per inch since it is a high-density XPS.

High-density EPS has an R-value of 4.2 per inch; low-density EPS has an R-value of 3.85 per inch.

If you compare the high-density R-value of XPS and EPS, you can see that **XPS has a 24% higher R-value per inch than EPS**.

Both XPS and EPS are made out of the same material: *polystyrene*. The difference in thermal resistivity is due to how this polystyrene is manufactured. XPS is made using the process known as extrusion; producing a closed-cell tightly packed structure. EPS is made using an expansion process where beads of foam are made to expand and fuse together.

Now, knowing the R-values per inch of thickness will help us determine what R-values 2-inch, 3-inch, 4-inch, and so on rigid insulation boards have. We will start with ISO boards (Chart 2), and proceed to XPS boards (Chart 3) and EPS boards (Chart 4):

## ISO Thickness R-Value (Rigid Insulation Chart 2)

Given that the R-value per inch of ISO is time-dependent, we use an R-5.8 per inch as a median insulation metric in this chart:

ISO Thickness: |
R-Value: |

1 Inch ISO Board | R-5.8 |

1.5 Inch ISO Board | R-8.7 |

2 Inch ISO Board | R-11.6 |

2.5 Inch ISO Board | R-14.5 |

3 Inch ISO Board | R-17.4 |

3.5 Inch ISO Board | R-20.3 |

4 Inch ISO Board | R-23.2 |

4.5 Inch ISO Board | R-26.1 |

5 Inch ISO Board | R-29.0 |

5.5 Inch ISO Board | R-31.9 |

6 Inch ISO Board | R-34.8 |

6.5 Inch ISO Board | R-37.7 |

7 Inch ISO Board | R-40.6 |

7.5 Inch ISO Board | R-43.5 |

8 Inch ISO Board | R-46.4 |

8.5 Inch ISO Board | R-49.3 |

9 Inch ISO Board | R-52.2 |

9.5 Inch ISO Board | R-54.5 |

10 Inch ISO Board | R-58.0 |

Let’s say we are looking for an R-value of a 2-inch ISO foam board. You can check this chart and see that a 2-inch ISO foam board has an R-value of R-11.6.

If you have a rigid ISO foam board that is more than 9 inches thick, you can expect that the R-value of such board will be over R-50. This is more than enough even for attics (the best attic insulation is usually R-49).

Let’s check XPS and EPS charts as well:

## XPS Thickness R-Value (Rigid Insulation Chart 3)

Here we use the R-value of high-density Styrofoam or XPS of R-5.2 per inch:

ISO Thickness: |
R-Value: |

1 Inch XPS Foam Board | R-5.2 |

1.5 Inch XPS Foam Board | R-7.8 |

2 Inch XPS Foam Board | R-10.4 |

2.5 Inch XPS Foam Board | R-13 |

3 Inch XPS Foam Board | R-15.6 |

3.5 Inch XPS Foam Board | R-18.5 |

4 Inch XPS Foam Board | R-20.8 |

4.5 Inch XPS Foam Board | R-23.4 |

5 Inch XPS Foam Board | R-26 |

5.5 Inch XPS Foam Board | R-28.6 |

6 Inch XPS Foam Board | R-31.2 |

6.5 Inch XPS Foam Board | R-33.8 |

7 Inch XPS Foam Board | R-36.4 |

7.5 Inch XPS Foam Board | R-39 |

8 Inch XPS Foam Board | R-41.6 |

8.5 Inch XPS Foam Board | R-44.2 |

9 Inch XPS Foam Board | R-46.8 |

9.5 Inch XPS Foam Board | R-49.4 |

10 Inch XPS Foam Board | R-52 |

Styrofoam, for example, is a trademark name for high-density XPS foam boards. You can get 2-inch, 3-inch, 4-inch Styrofoam, and so on.

2-inch Styrofoam has a typical insulation R-value of R-10.4. You can check XPS R-values for different thicknesses from the chart above.

## EPS Thickness R-Value (Rigid Insulation Chart 4)

Here are R-values for different thicknesses of high-density EPS:

ISO Thickness: |
R-Value: |

1 Inch EPS Foam Board | R-4.2 |

1.5 Inch EPS Foam Board | R-6.3 |

2 Inch EPS Foam Board | R-8.4 |

2.5 Inch EPS Foam Board | R-10.5 |

3 Inch EPS Foam Board | R-12.6 |

3.5 Inch EPS Foam Board | R-14.7 |

4 Inch EPS Foam Board | R-16.8 |

4.5 Inch EPS Foam Board | R-18.9 |

5 Inch EPS Foam Board | R-21 |

5.5 Inch EPS Foam Board | R-23.1 |

6 Inch EPS Foam Board | R-25.2 |

6.5 Inch EPS Foam Board | R-27.3 |

7 Inch EPS Foam Board | R-29.4 |

7.5 Inch EPS Foam Board | R-31.5 |

8 Inch EPS Foam Board | R-33.6 |

8.5 Inch EPS Foam Board | R-35.7 |

9 Inch EPS Foam Board | R-37.8 |

9.5 Inch EPS Foam Board | R-39.9 |

10 Inch EPS Foam Board | R-42 |

Example: What is the 2-inch EPS foam board R-value? Check the chart above and you can see that the R-value of a 2-inch EPS foam board is R-8.4.

In many cases, however, you want to know how thick rigid insulation you need to achieve R-10, R-20, R-30 insulation value, and so on. In this last chart we have calculated how many inches of rigid insulation you need for certain R-value with new ISO boards, high-density XPS, and high-density EPS:

## R-10, R-20, R-30, and R-49 Rigid Insulation Thickness Chart (Chart 5)

Let’s say you want your walls to have an R-30 value. How many inches of XPS, EPS, or ISO do you need? Here is a chart for R-10, R-20, R-30, and R-49 insulation values that will tell you how many inches of rigid insulation of the specified type you need:

Rigid Insulation Type: |
Thickness For R-10: |
Thickness For R-20: |
Thickness For R-30: |
Thickness For R-49: |

High-Density EPS | 2.38 Inches | 4.76 Inches | 7.14 Inches | 11.67 Inches |

Low-Density EPS | 2.60 Inches | 5.19 Inches | 7.79 Inches | 12.73 Inches |

High-Density XPS | 1.92 Inches | 3.85 Inches | 5.77 Inches | 9.42 Inches |

Low-Density XPS | 2.41 Inches | 4.82 Inches | 7.23 Inches | 11.81 Inches |

ISO (Penthane Expanded, New Polyiso) | 1.47 Inches | 2.94 Inches | 4.41 Inches | 7.21 Inches |

ISO (Penthane Expanded, 5-10 Years Old Polyiso) | 1.82 Inches | 3.64 Inches | 5.45 Inches | 8.91 Inches |

Here you can see that the R-value of 2-inch rigid insulation is higher than R-10 if you use ISO or high-density XPS.

The higher the R-value of rigid insulation, the less thick foam board you will need to achieve the same insulation R-value.

Hopefully, this illustrates how to think about the R-values of rigid insulation. If you have any questions, you can use the comments below and we will try to help you out.

You can also consult R-value charts for other insulation materials here. To adequately understand all types of insulation we use in building houses, you should check the explanation of 9 types of insulation here.

Is there a chart identifying the long term moisture impact on “R” values?

Hello Victor, no, the longitudinal R-value studies are available for only a few materials. You can check the complete insulation R-value table here; some of the materials have ‘After 5 Years’ or ‘After 10 Years’ R-value. These longitudinal studies include the effect of moisture to R-values. Hope it helps.