WHAT IN ADDITION TO R-VALUE IS IMPORTANT?

R-Value is very important when considering the energy efficiency of a building material, but it is not the only thing. To understand how all these factors interact, one must first understand the many ways that heat can effect a building.

Heat is the substance! Cold is the absence of any heat. Heat is energy, when energy is present, heat is generated. Cold occurs when there is no heat. Heat is measured in BTU’s (British Thermal Units). The higher the BTU’s the more heat.

BTU = British Thermal Unit: One BTU is the amount of heat required to raise the temperature of one pound of water one degree Fahrenheit

Temperature: A measure we use to tell how hot an item is, i.e. the air is 105 deg. BTU’s and volume determine the Temperature of an item. An example would be that if 10 BTU’s were added to one pound of water, the temperature would raise 10 deg. But if 10 BTU’s were added to two pounds of water, the temperature would raise only 5 deg.

Heat is transferred from one object to another object in several ways:

Conductance: When one object is warm and touches another object, heat is transferred, this is called conductive heat. The first object transfers heat by conductance (direct touch) resulting in heating the second object. If your hand is cold and you touch a warm pipe, your hand will warm up by conductance. Heat from the outside of a building will conduct through the wall material, then conduct to the air in a room, the air will then conduct to you and you will feel warmer. The resistance to conductance is called R-Value. See related sheet on "Understanding R-Value".

Radiated Energy: Radiated heat transfer acts in addition to conducted heat. When you stand in the direct sun, you feel hot where the sun strikes your skin. By holding your hand between your skin and the sun, you block the direct sun. The air temperature surrounding your skin has not changed but you have blocked the radiated heat, and your skin feels cooler. Radiated heat passes from one object through a gap or space to another object without warming the space between. R-Value does not directly stop or effect the radiated transfer. Radiated heat can pass through solid items as if they were not there, much like an X-ray does. Window film does not change the R-Value of the window, but does reflect some of the radiated heat, the effect is a lower Air Conditioning bill. Sometimes this is called "Effective R-Value" changes.

Air Infiltration: One volume of air can contain a larger amount of BTU’s than another volume of air. When the high heat volume of air is moved from the outside of the building to the inside of the building it brings with it this increased heat causing the inside temperature to rise. Air infiltration can enter the building through open doors, windows and cracks. A "Blower Door Test" is used to measure a buildings air infiltration with results measured in ACH (Air Changes per Hour). R-Value does not measure the amount of air infiltration or heat gain and by itself, does not effect the amount of air infiltration. This type of heat gain is in addition to R-value

Internal Heat Gains: When living bodies are present, energy is burned and gives off heat. This heat will increase the temperature in a room. The more people the more heat. Other things also add to this internal heat load, lights, appliances, televisions, computer, etc. R-Value does not measure or effect this type of heat gain.

The relationship of building materials to the above items needs to be looked at very carefully. A material with an R-Value rating does not give you any indication on how the material will perform against radiated energy, air infiltration or internal heat gains. But the same material that provides a high R-value may be able to provide protection against radiated energy and air infiltration also. Internal heat gains are not directly effected by building materials.