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.

What is the r value difference between 1.5″ polyiso and calsil on a 1″ pipe?

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.