ISO R-Value Chart: Thickness Of Polyiso Insulation (1-20 Inches)

Polyiso is a very good insulation material. We commonly referred to it as ISO; the molecule that makes polyiso is polyisocyanurate. We are going to look into insulation R-value of polyiso. Below you will also find a ‘Polyiso R-Value Thickness Calculator’ (insert inches, get ISO R-value) and ‘ISO R-Value Thickness Chart’.

Here is the key metric we have to know in order to calculate the thermal resistance capability of ISO:

Insulation R-value of polyiso is R-5.8 per inch of thickness (typically). You can also see polyiso boards with R-value as low as R-4.8 and as high as R-6.5

If you compare the R-value of polyiso to other materials in this insulation R-value chart, you will see that polyisocyanurate has above-average insulation capabilities. That’s why we use polyiso in several insulation types (you can also check all types of insulation here), including:

  • Polyiso rigid foam or polyiso foam board. In the industry, we refer to these boards as ISO boards. ISO boards have a thickness from 0.5 inches to 2.5 inches, with a corresponding R-value of R-2.9 to R-14.5.
  • Polyiso insulating concrete forms. These concrete forms can increase the R-value of walls up to R-20.
  • Polyiso structural insulated panels (SIPs). Here polyiso foam boards are placed between two oriented strand boards. We use 4-inch polyiso SIPs with an R-value of R-12 and 8-inch polyiso SIPs with an R-24 insulation value.
polyiso r value per inch and insulation types
Polyisocyanurate is such a good insulation material (due to high R-value) that is used in several types of insulation.

Using polyisocyanurate R-value per inch, we can calculate what is the R-value of any polyiso thickness. To help you out with this calculation, we have designed ‘Polyiso R-Value Thickness Calculator’. Basically, you just insert how many inches of ISO you are using, and the calculator will determine the R-value of that thickness of polyiso.

Below the calculator, you will also find the ‘ISO R-Value Thickness Chart’ that specifies what R-values different thicknesses of polyisocyanurate have.

Let’s start with the calculator and then proceed to the all-important chart:

Polyiso R-Value Thickness Calculator


R-value ISO calculator is quite easy to use. Let’s solve two example to see how it works:

Example 1: What is the R-value of 2 inches of polyiso? You just slide the slider in the calculator to ‘2’ inches and you get the result: 2-inch polyiso board has an R-value of R-11.6.

Example 2: Let’s say that we have a very thick 8-inch ISO board. What is the R-value of 8 inches of polyiso? You just slide the slider in the calculator to ‘8’ inches and you get the result: 8-inch polyiso board has an R-value of R-46.4.

Here are calculations for 1-inch to 20-inch thick ISO boards, summarized in the polyisocyanurate R-value thickness chart:

ISO R-Value Thickness Chart

Polyiso Thickness: R-Value:
1 Inch Polyiso R-5.8
1.5 Inch Polyiso R-8.7
2 Inch Polyiso R-11.6
2.5 Inch Polyiso R-14.5
3 Inch Polyiso R-17.4
3.5 Inch Polyiso R-20.3
4 Inch Polyiso R-23.2
4.5 Inch Polyiso R-26.1
5 Inch Polyiso R-29.0
5.5 Inch Polyiso R-31.9
6 Inch Polyiso R-34.8
6.5 Inch Polyiso R-37.7
7 Inch Polyiso R-40.6
7.5 Inch Polyiso R-43.5
8 Inch Polyiso R-46.4
8.5 Inch Polyiso R-49.3
9 Inch Polyiso R-52.2
9.5 Inch Polyiso R-54.5
10 Inch Polyiso R-58.0
11 Inch Polyiso R-63.8
12 Inch Polyiso R-69.6
13 Inch Polyiso R-75.4
14 Inch Polyiso R-81.2
15 Inch Polyiso R-87.0
16 Inch Polyiso R-92.8
17 Inch Polyiso R-98.6
18 Inch Polyiso R-104.4
19 Inch Polyiso R-110.2

As you can see, if you stack ISO boards one on top of the other 18-inches thick, they will have a combined R-value over R-100. That’s quite an amazing R-value.

Hopefully, now you know a little more about ISO R-values and how thick polyiso you need to use in your situation. If you have any questions regarding ISO R-values, you can use the comment section below and we will try to help you out.

6 thoughts on “ISO R-Value Chart: Thickness Of Polyiso Insulation (1-20 Inches)”

  1. Will the R-value of poly iso increase by doing multiply layers? Is 5 layers of 1″ a higher R-value than one layer of 5″?

    • Hello Devin, in theory, 5 layers of 1-inch ISO have the same insulation R-value as one 5-inch ISO layer. Basically, a little bit of air can be captured in between several layers but that shouldn’t have a significant impact on the overall R-value. Hope this helps.

    • Hi Walter, 4-inch polyiso costs about $1.20 to $1.60 per square foot of flat room. If you have 1400 square feet of flat root, that’s anywhere between $1680 and $2240. So, around $2000. Hope this helps.

    • Hi Chris, the typical polyiso R-value is 5.8 per inch (1.5 inch would have 8.7 R-value) and typical calcium silicate (calsil) has an R-value of 2.6 per inch (1.5 inch would have 3.9 R-value). From these numbers, you can see that, at 1.5 inch thickness, the difference between polyiso and calsil is 4.8 R-value. In short, polyiso has more than double the insulation capability compared to calsil. Hope this helps.


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