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|
|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|
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.
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.