R-value is short for resistance value and describes how well a product resists heat flow. In construction terms, most products have an R-value, including studs, wall systems, windows, and insulation. The equivalent metric unit is RSI.
All insulation has a unique R-value. R-value increases with the thickness of the product. Resistance values are often expressed “per inch” to enable realistic comparisons. Here are some of the more common insulation products and their R-values.
See ColoradoENERGY.org-R-ValueTable for an excellent detailed list of product R-values. The value of proper insulation can be illustrated by comparing a fiberglass batt’s R-value to poured concrete. A 3.5-inch thick fiberglass batt provides R-11. To achieve the same R-value, poured concrete must be 11.5 feet thick.
How Much R-Value is Necessary?
Many people equate insulation and R-values with keeping out the cold, but insulation actually prevents heat from flowing to cold areas. Insulation keeps more heat inside the house or–in warmer climates–keeps excess heat out of the house.
Even in warm climate zones, the Department of Energy recommends homeowners add significant amounts of insulation. Insulation with higher R-values helps reduce heating and cooling costs and makes for a more comfortable home.
The Department of Energy states that approximately 9 out of 10 US homes are underinsulated. Bringing insulation up to the recommended R-value and sealing air leaks can reduce energy costs by 10% annually. If all of the air leaks in the average home were combined, they would create a hole of 25 square inches.
This map from the United States Department of Energy shows recommended Insulation R-values for various climate zones.
Recommended Insulation Levels for Retrofitting Existing Wood-Framed Buildings
Wall Insulation – Whenever exterior siding is removed on an:
Uninsulated wood-frame wall:
- Drill holes in the sheathing and blow insulation into the empty wall cavity before installing the new siding, and
- Zones 3–4: Add R5 insulative wall sheathing beneath the new siding
- Zones 5–8: Add R5 to R6 insulative wall sheathing beneath the new siding.
Insulated wood-frame wall:
- For Zones 4 to 8: Add R5 insulative sheathing before installing the new siding.
Courtesy: US Department of Energy
Types of Insulation
There are many types of insulation with varying R-values to use in specific locations of your home.
- Batt Insulation. The most popular type of insulation is batt insulation which friction fits between wall studs. The most popular batts are fiberglass and mineral wool. They are also available in rolls–usually 50’ long.
- Loose Fill Insulation. Typically cellulose or fiberglass, this insulation is most effective in attics but is becoming more popular in walls. Installers can apply cellulose wet or dry. Wet application sticks to walls and ceilings even after it dries.
- Rigid Board Insulation. Very effective on concrete walls because it provides a vapor barrier. Also an excellent exterior product under new siding or stucco.
- Radiant Barrier Insulation. Often attached to certain types of rigid foam to provide increased R-values up to R-6.2/inch.
- Spray Foam Insulation. Spray foam goes on the insides of walls before drywall application, the undersides of roof decks, and basement walls. It can seal any wall penetrations to prevent air leaks.
How Insulation Works
Insulation provides a thermal-resistant barrier. The combination of dead air spaces and the product itself prevents the movement of heat and air through walls, roofs, and floors.
Heat conduction is somewhat like sound. Sound does not travel directly to your ear. Instead, it agitates molecules at the source, which in turn continues the process until the noise arrives and can be heard. The speed of sound is relatively slow, which causes the delay between seeing the distant hammer hit the wall and hearing the sound.
Dead air is simply air that cannot move to conduct heat onward. All insulations use some variation of dead or trapped air to prevent heat conduction.
Changes In R-Value
The R-value of insulation is only one part of an efficient building envelope. Heat moves by conduction, convection, radiation, and air infiltration. Consider all of these variables when determining how well an insulation product will work. R-value is an effective method of determining the heat flow resistance of types of insulation, but it only measures conduction.
R-value numbers by themselves cannot account for other factors that affect heat flow. These include wind, humidity, and rapid temperature changes. All of which can cause increased airflow and leaks in the building envelope. The combination of these factors can effectively change the R-value of insulation and determine how well it works.
It is very difficult to measure this new R-value. Wind, humidity, and temperature are constantly changing. Buildings have different sources and sizes of air leaks. Leaving R-value as the preferred choice for determining how warm, cold, or comfortable an insulation product will keep a building.