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.
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.
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.
what is the cost for 4″ polyiso insulation for flat roof of 1400 sf
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.