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 6×1.41 = 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|
|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|
|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|
|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|
|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|
|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|
|Roof Insulation||2.78 R-Value||13.95||27.8|
|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.