R-values for insulation often appear on specification sheets and measure how well the material resists heat flow. As the R-value increases, so does the insulation per inch of thickness.
Many homeowners ask, “What R-value do I need?” This article will discuss what R-Value means, how to calculate the R-value, and how R-value affects your home’s insulation.
What Is R-value?
R-value refers to thermal resistance, with the “R” standing for “resistance.” R-values for insulation represent the insulating capacity of a certain material. Insulation with a higher R-value will provide a more comfortable living space and save energy costs.
What Does R-value Mean in Insulation?
The R-value measures how well a material insulates in numbers. The American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) has created a means to measure thermal conductivity, or k-value, using research dating back to the 1950s.
K-value measures the amount of heat that flows through one square foot of material with one-inch thickness in one hour. The k-value can help calculate the required insulation R-value, R-value for roof insulation, and ceiling insulation R-value.
To calculate the R-value from the k-value, we can use the equation: R-value = 1/k-value.
By measuring a material’s k-value (thermal conductivity), we can calculate the R-value (thermal resistance). R-value acts as the inverse of the k-value, so the lower the k-value, the better the insulation. Let’s examine the calculation of R-values by using wood as an example.
As part of an ASHRAE study, experts measured the k-values of softwood and hardwood. Based on those k-values, the U.S. Department of Energy reported that softwoods have an R-value of 1.41 per inch while hardwoods have an R-value of 0.71.
Using this data, we can answer the question, “What R-value do I need?” The R-value for five inches of softwood equals 5 x 1.41 = 7.05. In other words, five inches of softwood requires an R-value of 7 and five inches of hardwood an R-value of 3.5.
R-Value Insulation Chart
The chart below contains the R-values for different building materials. For example, closed-cell spray foam has a higher R-7 insulation value, while open-cell foam has an R-3.8 value.
| Building Material | R-Value (One Inch Thickness) | R-Value (Five Inches Thickness) | R-Value (Ten 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 |
What R-value Insulation Do I Need?
Investing in insulation will eventually pay off, so you want to select materials with a high R-value. The recommended insulation level for rooms and spaces depends on where you live. Use the insulation chart or consult a professional to determine what R-value you need.
Before starting, you must find out which insulation climate zone you live in. Energy Star has created recommended R-values based on seven zones. Additionally, Energy Star performed an analysis to determine what R-value insulation you need for your walls, attic, floor, and crawlspace.
R-value for Attics
Match your climate zone to the recommended R-value below. For example, if you live in zone four, consider getting insulation with an R-value between 38 and 60. The R-value for roof insulation and attic insulation should have a minimum value of 30.
- Zone One and Two: R-30 to R-49
- Zone Three: R-30 to R-60
- Zone Four and Five: R-38 to R-60
- Zone Six and Seven: R-49 to R-60
R-value for Walls
2×4 walls require a minimum wall insulation R-value of 13, so select insulation with an R-value of 13 to 15. For 2×6 walls, you need an R-value of 19, and the recommended wall insulation R-value for all zones falls between 19 to 21. If you decide to insulate the ceilings, they should have a minimum ceiling insulation R-value of 30.
Frequently Asked Questions
Is a Higher R-value Better for Insulation?
The R-value denotes the effectiveness of the insulation, so choose insulation materials with the highest R-value possible. A higher R-value means better insulation performance, especially if you live in colder areas.
Which Insulation Has the Best R-value?
Closed-cell spray foam has the best R-value of 7 per inch. Spray foam also has exceptionally high density, making it one of the most effective types of insulation.
Does Doubling Insulation Double R-value?
Doubling insulation does double R-value. Due to the law of diminishing returns, doubling insulation from R-10 to R-20 will reduce energy consumption more than doubling R-20 to R-40. Attics usually have plenty of space for doubling insulation.
Conclusion
Attic Projects provides one-stop insulation services. Our team uses high-quality materials that will insulate and protect your home for years to come. We can handle spray foam insulation, attic insulation upgrades and removals, loose-fill or blown-in systems, batt insulation, and vapor barriers.
If you need more information on “What R-value do I need?” contact Attic Projects today to get a free quote and insulation consultation. Call us at (858) 282-0516.
