When installing insulation, its R-value is one of the most important attributes to consider. You won’t see your insulation once it’s installed, but it’ll have a profound impact on your comfort and your home’s energy efficiency.
What Is R-Value?
To put it simply, R-value is a measurement of an insulating material’s thermal resistance. It measures the ability of heat to transfer from one side of something to another. The higher the R-value, the more insulating a material is.
Here is an example that demonstrates how important insulation is. Solid wood only has an R-value of 1. With an inch of standard fiberglass insulation, you get an R-value of 3.1 to 3.4. An inch of blown cellulose has an R-value of up to 3.7.
However, the total R-value of a system isn’t limited to the insulation. When considering drywall (which has an R-value of .45), siding, and sheathing, you may get more insulating value. Insulating properties increase as layers are added. Finding the total R-value can be a challenge so you should consult an insulation professional for help.
What Materials Have the Best R-Value?
A material’s R-value is generally measured per inch and increases with thickness. Here is a comparison of some common insulating materials and their R-value per inch:
- Blown fiberglass (wall): 3.7 to 4.3
- Blown mineral wool (wall): 3.1 to 4.0
- Polystyrene board: 3.8 to 5.0
- Polyurethane board: 5.5 to 6.5
- Foil-faced polyisocyanurate: 5.6 to 8.0
- Open cell spray foam: 3.5 to 3.6
- Closed cell spray foam: 6.0 to 6.5
What R-Value Do I Need?
The R-value of your insulation, plus that of other materials, determines the total thermal resistance of a system. For example, a layer of insulation with R30 and another layer that is R19 will total R49. If your wall insulation has an R-value of 15 and there’s a layer of wall material (like insulating sheathing) that is R3, the total will be R18.
The U.S. Department of Energy has divided the United States up into 8 climate zones based on overall insulating needs. Zone 1 is in South Florida, while Zone 2 covers regions near the Gulf Coast. For the most part, higher numbered zones are further north, although there are differences accounting for coastal areas and mountainous regions. Zones 7 and 8 are in Alaska.
ENERGY STAR recommends specific R-value ranges for attics, walls, floors, and crawlspaces by the zone one is in. For an uninsulated attic in Zone 1, it suggests an R-value of R30 to R49 and up to R60 for zones 2 through 8. If there are 3 to 4 inches of existing insulation, anywhere from R25 to R30 is suggested for Zone 1 while R25 to R38 is recommended for Zones 2 and 3. Energy.gov recommends R38 for Zone 4 and R38 to R49 for Zones 5 to 8.
For floors, R13 insulation is suggested for Zone 1, while Zone 2 structures should have insulating materials from R13 to R19 (up to R25 for Zone 3). In Zones 4 through 8, R25 to R30 insulation is suggested.
Walls are assessed differently. Assuming exterior siding is removed, an uninsulated wood-frame should have R5 insulative sheathing beneath new siding in Zones 3 and 4, and R5 to R6 insulative sheathing in Zones 5 through 8. Insulated wood-frame walls for homes in Zones 4 to 8 should have R5 insulative sheathing installed before new siding is placed.