What Is R-Value? A Look At Insulation

What Is R-Value? A Look At Insulation

A lot of people I talk to have this kind of understanding about insulation and R-Value: Insulation is good, and the bigger the R-number, the better.

Well, that may be true as a generalization, but there is more to keeping a house comfortable than insulation... but I digress. Back to the topic... So let's first ask: what is R-Value, anyway?

R-value is the measurement used to describe a material’s level of thermal resistance. It may be more than you want to know, but R-Value is the inverse of the U-Value, a measurement of thermal conductivity. 

And, of course, R-Value is often the standard consideration when discussing the effectiveness of insulation. 

How Does R-Value Relate to Insulation?

Heating and cooling accounts for between 50% to 70% of energy used in a typical American home. Inadequate insulation, or insulation improperly installed, can waste a lot of this energy -- energy that you have already paid the utlity company to heat or cool.

Insulation provides resistance to the flow of heat.  A higher R-value equates to higher resistance to heat flow (that is, greater effectiveness in insulating).  An insulation material’s R-value -- along with how and where it is installed -- will determine its thermal resistance and effectiveness.

Adding the R-values of each layer of material contained in one building component, such as a wall or ceiling with multiple layers of insulation, will help determine the thermal resistance of the whole component.

R-Value Doesn't Matter If It Is't Installed Right

There are other factors that contribute to thermal resistance. First and foremost, they R-Value applies to propery installed insulation. Stuffing batt insulation sized for 5 inches into a 4-inch wall cavity will actually lower its R-value. Compressing a single layer of insulation reduces its effectiveness.

Insulation isn't the only thing that affects heat transfer. Glass windows, studs, structural gaps in any building penetrations, poor (or no) weatherstripping can all contribute to wasted energy.

Studs and windows provide a parallel heat conduction path, and insulation between studs in a wall does not restrict heat flow through the studs.  This heat flow is called thermal bridging, and the overall R-value of the wall will be different from the R-value of the insulation itself. 

Calculating and Converting R-Value

For those interested -- this is the equation for determining R-value:

R-value = temperature difference x area x time ÷ heat loss

The temperature difference is expressed in degrees Fahrenheit, the area in square feet, the time in hours, and heat loss in BTUs. 

Side note: Canadian R-values use different units of measure (Celsius, meters, etc.).

The FTC and DOE on R-Value

In the 1970s, the Federal Trade Commission (FTC) mandated that insulation manufacturers disclose R-values at the point of sale to protect purchasers from false claims. This action also created a standard of comparison for products. 

The U.S. Department of Energy issued recommendations for insulation R-values in new and existing homes. Their recommendations for attics, cathedral ceilings, walls and floors are generally greater than what is actually required by most current building codes.