Rigid Insulation R-Value Per Inch (+ EPS, XPS, ISO Thickness Charts)

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 Expanded PolyStyrene. We differentiate between higher R-value high-density EPS and lower R-value low-density EPS.
  • R-values of XPS (also known as Styrofoam). XPS stands for eXtruded PolyStyrene. The same as with EPS, we differentiate between higher R-value high-density XPS and lower R-value low-density XPS.
  • R-values of ISO (also known as polyiso). ISO is an abbreviation of polyISOcyanurate. This is foil-faced polyisocyanurate; the R-value of ISO decreases over the years.

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:

Rigid Insulation R-Value Per Inch
Small air pockets visible in Styrofoam.


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.

2 thoughts on “Rigid Insulation R-Value Per Inch (+ EPS, XPS, ISO Thickness Charts)”

    • 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.


Leave a Comment