When checking insulation specs sheets, R-value comes up quite often. *What is R-value?* We will look into what does R-value means, how it is calculated, what are the minimum recommended values of R for attics, 2×4 and 2×6 walls, and so on (you will find a **complete insulation R-value chart** further on), and what it’s so important to pick building materials with high R-values.

*What is insulation R-value?*

The R-value of building material is its **resistance** to the flow of heat. R-value is known as *thermal resistance*; **R stands for Resistance**. It is a numerical expression of how good an insulator of certain building material is. You can check types of insulations with typical R-values here for reference.

The higher the R-value of a material, the better insulator it is.

*Example:* R-30 material is 3 times better insulator than R-10 material. That’s why it’s recommended to use superb insulation material for attics (minimal recommended R-value is R-30), ductwork (R-42). Walls, for example, are not the best insulator; they typically should have an R-value greater than R-11.

Here are some estimates to illustrate how important good insulation (high R-value materials) really is:

**Poorly insulated buildings**will experience heat loss (winter) and heat gain (summer) as high as**50%**.*Example:*Monthly heating bill could be $100 but due to poor insulation you’re paying a $200 heating bill.- About
**30%**of heat loss/gain comes from the poorly insulated**ceiling**(importance of high R-value ceiling) and about**70%**of heat loss/gain is experienced from**walls, glass windows**, and so on (importance of good R-value for walls, for example). - As much as
**90% of ceiling heat loss/gain can be prevented with high R-value ceiling materials**. We also see much as 60% less heat loss/gain through high R-value walls.

With all that in mind, let’s first look at how R-value is defined. After that, we will look at the R-value chart for different materials. Based on that, you can choose the highest R-value materials to insulate your home with.

### What Does R-Value Mean In Insulation?

R-value – thermal resistance – is basically how you put *‘how good insulator’* any material is in numbers. What ASHRAE actually measures *(considerable research has been put towards this in the 50s and 60s)* is thermal conductivity or **k-value**.

k-value is a measurement of heat that flows through **1 sq ft of material** with **1-inch thickness** in **1 hour** for **each degree of the temperature difference** between indoor/outdoor temperature.

We can calculate R-value from k-value using this simple equation:

**R-value = 1 / k-value**

Basically, R-value is an inverse of the k-value. k-value (thermal conductivity) is measured, and then R-value (thermal resistance) is calculated from the k-value.

Let’s look at an example for R-value of wood to illustrate how R-values are calculated:

#### Example: What Is The R-Value Of Wood?

ASHRAE measured the k-value of both softwood and hardwood. They have determined that a total of 0.71 BTU is lost through 1-inch of 1 sq ft softwood in 1 hour. That means that the k-value for softwood is 0.71. For hardwood, the k-value is 1.41.

Based on this k-value, R-value for wood is calculated. According to the US Department of Energy (source here), *“R-value for wood ranges between 1.41 per inch for most softwoods and 0.71 for most hardwoods”*.

That means that a 6-inch softwood has an R-rating of 6Ã—1.41 = 8.46. We can basically say that ** 6-inch softwood has an R-value of R-8**, and 6-inches of hardwood has an R-value of R-4.

You can read more about how ASHRAE *(American Society of Heating, Refrigeration, and Air-Conditioning Engineers)* measures the k-value here.

ASHRAE also calculated the R-values from measured k-values, and publishes the results in what is known as *‘R-Value Charts’*.

Here is a comprehensive chart that includes various R-ratings for building materials:

## Insulation R-Value Chart

Building Material: |
R-Value (1 Inch Thickness) |
R-Value (5 Inches Thickness) |
R-Value (10 Inches Thickness) |

Closed Cell Spray Foam | 7.00 R-Value | 35.0 | 70.0 |

Open Cell Spray Foam | 3.80 R-Value | 19.0 | 38.0 |

Foam Board | 4.00 R-Value | 20.0 | 40.0 |

Gypsum Or Plaster Board | 0.9 R-Value | 4.5 | 9.0 |

Plywood | 1.25 R-Value | 6.25 | 12.5 |

Wood Panels | 1.25 R-Value | 6.25 | 12.5 |

Wood-Fiber Board | 2.38 R-Value | 11.9 | 23.8 |

Wood-Fiber Hardboard | 1.39 R-Value | 6.95 | 13.9 |

Softwood | 1.41 R-Value | 7.05 | 14.1 |

Hardwood | 0.71 R-Value | 3.55 | 7.1 |

Pine Wood | 1.25 R-Value | 6.25 | 12.5 |

Asphalt Tile | 0.32 R-Value | 1.6 | 3.2 |

Ceramic Tile | 0.08 R-Value | 0.4 | 0.8 |

Cork Tile | 2.22 R-Value | 11.1 | 22.2 |

Linoleum | 0.56 R-Value | 2.8 | 5.6 |

Plywood Subfloor | 1.25 R-Value | 6.25 | 12.5 |

Rubber Tile | 0.20 R-Value | 1.0 | 2.0 |

Plastic Tile | 0.20 R-Value | 1.0 | 2.0 |

Terrazzo | 0.98 R-Value | 4.9 | 9.8 |

Wood Subfloor | 1.25 R-Value | 6.25 | 12.5 |

Cotton Fiber | 3.85 R-Value | 19.25 | 38.5 |

Mineral Wool | 3.70 R-Value | 18.5 | 37.0 |

Wood Fiber | 4.00 R-Value | 20.0 | 40.0 |

Glass Fiber | 4.00 R-Value | 20.0 | 40.0 |

Roof Deck Slab | 4.17 R-Value | 20.85 | 41.7 |

Cellular Glass | 2.50 R-Value | 12.5 | 25.0 |

Corkboard | 3.7 R-Value | 18.5 | 37.0 |

Hog Hair | 3.00 R-Value | 15.0 | 30.0 |

Plastic (Foamed) | 3.45 R-Value | 17.25 | 34.5 |

Shredded Wood | 1.82 R-Value | 9.1 | 18.2 |

Macerated Paper | 3.57 R-Value | 17.85 | 35.7 |

Sawdust or Shavings | 2.22 R-Value | 11.1 | 22.2 |

Vermiculite | 2.08 R-Value | 10.4 | 20.8 |

Roof Insulation | 2.78 R-Value | 13.95 | 27.8 |

Concrete | 0.19-1.42 R-Value | 0.95-7.1 | 1.9-14.2 |

Brick (Common) | 0.2 R-Value | 1.0 | 2.0 |

Source: Courtesy of ASHRAE 1960 Guide

You can use this table to get an idea of what the R-value means in insulation for different building materials. If you need a more detailed chart, here is the insulation R-value per inch chart for 51 insulation materials you can check out.

*Example:* What is the R-value of spray foam insulation? Well, we differentiate between closed cell and open cell spray foam insulation. Closed-cell foam has a higher R-7 insulation value and open-cell foam has a substantial R-3.8 insulation value.

### What R-Value Insulation Do I Need? (Attic, Walls, Etc.)

Insulation is almost always a smart investment. You want as high R-value materials as possible.

Based on where in the US you live, there are different minimum recommendations for rooms/spaces that should be well insulated. They give you an idea of what R-value do you need.

Energy Star did a good analysis of exactly what R-value insulation you need for attic, 2×4 walls, 2×6 walls, floors and crawl spaces.

First of all, you need to check in which Insulation Climate Zone you live (there are 8 of them; South Florida is 1, North US is 7, etc.). You should check your zone at Energy Star here and come back for R-value recommendations.

**What R-Value For Attic Insulation?**

For attic insulation, you will need a minimum of R-30 value insulation.

- Zone 1 and 2: R-30 to R-49 is recommended.
- Zone 3: R-30 to R-60 is recommended.
- Zone 4 and 5: R-38 to R-60 is recommended.
- Zone 6 and 7: R-49 to R-60 is recommended.

Sometimes you see questions like ‘Is R-19 insulation good for attic?’ or ‘Is R-30 insulation good for attic?’. R-30 is the recommended minimum, yes, but R-19 is insufficient for the attic.

**What Size Insulation For 2×4 Walls? **

For 2×4 walls, the minimum recommended R-value insulation is R-13. For all zones across the US, it’s recommended to have R-13 to R-15 insulation for 2×4 walls.

**What Size Insulation For 2×6 Walls? **

For 2×6 walls, the minimum recommended R-value insulation is R-19. For all zones across the US, it’s recommended to have R-19 to R-21 insulation for 2×6 walls.

**What Size Insulation For Floors? **

Floors require a minimum of R-13 insulation. The further north you go, the higher is the minimum recommendation for floor insulation:

- Zone 1 and 2: R-13 floor insulation is recommended.
- Zone 3: Minimum floor insulation of R-25 is recommended.
- Zone 4, 5, 6, and 7: R-25 to R-30 floor insulation is recommended.

**What Size Insulation For Crawl Spaces? **

A minimum of R-13 insulation is recommended for crawl spaces. Same story here; the further north you go, the more insulation you need:

- Zone 1: R-13 insulation for crawl spaces is sufficient.
- Zone 2: R-13 to R-19 crawl space insulation is recommended.
- Zone 3: R-19 to R-25 insulation for crawl spaces is recommended
- Zones 4,5,6 and 7: R-25 to R-30 minimum R-value insulation is recommended for crawl spaces.

Hopefully, you now have a better understanding of what R-value means. We covered different R-values for different materials and you can check the minimum recommended values of R insulation for spaces that need to be insulated the most.

who is the author of this article? I need it for a citation ASAP! (posted 1/22/23)

Hi Neil, this article was written by one of our authors: Neil Fisher, Physics MSci. You can quite him or LearnMetrics. Thank you.

Thank You!