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:
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:
|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:
|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:
|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.