What is
insulation
r-value?
The R-value of insulation is a measure of the effectiveness of insulation. The higher the R-value of a material, the better it insulates. This number provides an easy point of reference to compare different insulating materials of different types, sizes, and properties.
The R-value is regulated by the Federal Trade Commission to prevent ‘unfair or deceptive acts or practices.’ In effect, it is a federally-issued standardized means of understanding the insulating properties of a material.
The purpose of this guide is to show in more detail what is meant by an R-value – to help you understand the science behind it as well as the implications of different R-values. This will help you to make an informed decision when it comes to determining the best insulation for your home.
WHAT IS INSULATION AND HOW IS IT
RELATED TO
THE R-VALUE?
Fundamental to understanding what is meant by R-value is understanding what insulation is and what it does. At its heart, this is a question of why humans build shelters in the first place.
WHAT DOES INSULATION DO?
Insulation is a material that provides resistance to heat flow. The material prevents (primarily) heat from passing through it, thereby retaining warmth inside a home when the weather is cold or preventing heat from entering the home during the hotter summer months.
Almost all materials used in building a home have some form of insulating properties, from wood to brick to stone. However, dedicated insulating materials are designed specifically with that purpose in mind.
The most commonly used insulating material in modern homes is fiberglass, usually in the form of batts or rolls, and usually placed in the attic space of a home. This prevents heat from exiting through the roof of a house. Our guide on fiberglass rolled and batt insulation goes more in-depth on this specific type of insulation if you would like to learn more.
The benefits of insulation are that it helps to keep the house warm in the winter and cool in the summer. Because you’re not losing heat in the cooler months, nor receiving it in the hotter months, you spend less on utilities and your home is easier to maintain at a comfortable temperature.
For insulation cost information, go to our page on the average cost of insulation.
WHAT DOES R-VALUE MEASURE?
Since insulation is defined by preventing the flow of heat across it, another way to think about insulation is as resistance.
The resistance value, or R-value, is a number given to insulation to indicate how well it prevents heat from passing through. The higher the R-value, the higher the resistance to heat, and therefore the better insulating a material is.
FOR EXAMPLE:
An inch-thick piece of solid wood has an R-value of 1.
An inch of blown fiberglass insulation has an R-value of 3.1-3.4.
An inch of closed-cell spray foam insulation has an R-value of 6.
To help understand what is meant by that, imagine a container of boiling water surrounded by a one-inch thick block of different materials. The one insulated by closed-cell spray foam would take around twice as long to cool down as the fiberglass one, and six times as long as the wooden one.
R-value, therefore, provides an easy point of reference for understanding insulating properties. R-value is the industry standard for insulation, meaning contractors, commercial manufacturers, and consumers all share the same language and reference points when it comes to insulation.
HOW DOES INSULATION
WORK?
THE SCIENCE OF HEAT TRANSFER
You may remember from your high-school science classes that heat transfer is a form of energy transfer. Heat transfers at an atomic level, connected with the vibration of electrons. When heat transfers through a solid it is called conduction, whereas through liquids and gases is called convection.
Radiant heat travels in a straight line, heating anything solid in its path that is an absorber of energy.
THE SCIENCE OF INSULATION
The process of insulation is effectively the process of slowing energy conducting through a material. Insulating materials work in two main ways:
- Reflective insulation
- Radiant barriers
Reflective Insulation
Reflective insulation – as the name suggests – works by sending the heat back in the direction it came from, in the same way as a mirror reflects light. On insulation batts, for example, the part that faces the air space is designed to reflect heat back.
Radiant Barriers
Radiant barriers are designed to stop heat from moving through a surface. Heat moves along a gradient from an area of high temperature to an area of low temperature. The greater the difference between these two, the harder it is to prevent heat transfer. Radiant barriers help to prevent this heat transfer, often working in conjunction with reflective insulation.
Insulation such as fiberglass works by utilizing materials that don’t conduct heat. For example, fiberglass is made of blown glass fibers; glass is a poor conductor of heat, particularly when it is in tiny fibers.
Within fiberglass, the combination of very small air pockets and very small fibers of glass means that heat is unable to travel through.
WHAT AFFECTS
R-VALUES?
The R-value of a material depends on a combination of three factors:
1
The chemical makeup of the material
2
The density of the material
3
The thickness of the material
In some materials, the R-value is also affected by the following:
Temperature
Moisture levels
Age
This again illustrates the purpose of the R-value, since it allows for all metrics to combine into a single number.
The table below shows some of the R-values of various common insulating materials at different thicknesses.
Material | R-value per inch | 3.5 in. | 5.25 in. | 10 in. | 12 in. | 15 in. |
|---|---|---|---|---|---|---|
Fiberglass (batt) | 3.1 - 3.4 | 10.8 - 11.9 | 16.3 - 17.8 | 31.0 - 34.0 | 37.2 - 40.8 | 46.5 - 51.0 |
Fiberglass blown (attic) | 2.2 - 4.3 | 7.7 - 15.0 | 11.5 - 22.6 | 22.0 - 43.0 | 26.4 - 51.6 | 33.0 - 64.5 |
Fiberglass blown (wall) | 3.7 - 4.3 | 12.9 - 15.0 | 19.4 - 22.6 | 37.0 - 43.0 | 44.4 - 51.6 | 55.5 - 64.5 |
Mineral Wool (batt) | 3.1 - 3.4 | 10.8 - 11.9 | 16.3 - 17.8 | 31.0 - 34.0 | 37.2 - 40.8 | 46.5 - 51.0 |
Mineral Wool blown (attic) | 3.1 - 4.0 | 10.8 - 14.0 | 16.3 - 21.0 | 31.0 - 40.0 | 37.2 - 48.0 | 46.5 - 60.0 |
Mineral Wool blown (wall) | 3.1 - 4.0 | 10.8 - 14.0 | 16.3 - 21.0 | 31.0 - 40.0 | 37.2 - 48.0 | 46.5 - 60.0 |
Cellulose blown (attic) | 3.2 - 3.7 | 11.2 - 12.9 | 16.8 - 15.0 | 32.0 - 37.0 | 38.4 - 44.4 | 48.0 - 55.5 |
Cellulose blown (wall) | 3.8 - 3.9 | 13.3 - 13.6 | 19.9 - 20.8 | 38.0 - 39.0 | 45.6 - 46.8 | 57.0 - 58.5 |
Polystyrene Board | 3.8 - 5.0 | 13.3 - 17.5 | 19.9 - 26.2 | 38.0 - 50.0 | 45.6 - 60.0 | 57.0 - 75.0 |
Polyurethane Board | 5.5 - 6.5 | 19.2 - 22.7 | 28.9 - 34.1 | 55.0 - 65.0 | 66.0 - 78.0 | 82.5 - 97.5 |
Polyisocyanurate (foil-faced) | 5.6 - 8.0 | 18.2 - 28.0 | 29.4 - 42.0 | 56.0 - 80.0 | 67.2 - 96.0 | 84.0 - 120.0 |
Open Cell Spray Foam | 3.5 - 3.6 | 12.2 - 12.6 | 18.4 - 18.9 | 35.0 - 36.0 | 42.0 - 43.2 | 52.5 - 54.0 |
Closed Cell Spray Foam | 6.0 - 6.5 | 21.0 - 22.7 | 31.5 - 34.1 | 60.0 - 65.0 | 72.0 - 78.0 | 90.0 - 97.5 |
The required R-value in your home will depend on the climate you live in (i.e. the temperature differential between the inside and outside of your home), your budget, and the part of your home you are looking to insulate.
WHAT R-VALUE
DO I NEED?
In order to calculate the best R-value for you, you’ll need to calculate a number of factors. However, you can use the following as indicators:
ATTIC INSULATION
If you live in a cold climate you should aim for a minimum of R-49 in your attic. Using the table above, you can see that this corresponds to roughly 16 inches of fiberglass batt insulation.
If you live in a warmer climate, you will require an R-value of 38 or above, which is about a 12-inch thick batt.
WALL INSULATION
The limiting factor when it comes to wall insulation is often the thickness of the wall. However, in this case, you can vary the density of the insulation, which impacts the overall R-value.
For in-wall batts, you can achieve an R-value of between 11 and 15. If you use spray foam insulation, you can reach an R-value of anywhere between 14 and 28.
FLOOR INSULATION
Floor insulation requires additional care because of issues with venting and moisture. However, the Department of Energy recommends an R-value of 25 in colder climates and 11 in warmer parts of the country.
Ultimately, you don’t need to understand the science or the rationale behind R-value in order to purchase insulation. However, being an informed consumer means having as much information at your disposal as you can.
Although R-value is designed to be as simple a metric as possible, it’s vital to understand why an R-value is higher on a particular material, and what the implications are for your home.
You can then factor that in to the financial calculations – in consultation with your contractor – and therefore make the best decision for your family and your home.