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ACH (air changes per hour) or R-Value: What is important? …and Why

July 11, 2011 by Mark 

For years insulators have focused on R-value. Although R-value is an important figure, it’s not the only or most important factor when considering a building’s energy efficiency. Let’s examine the effects of R-value versus ACH.

R-value is a measure of a materials ability to resist changes in temperature. R-value is tested in a wind free environment at 75 degrees F. R-value is a test of material in a laboratory environment.

ACH – Involuntary air changes per hour is a measurement of the “tightness” of a building envelope. ACH is determined with a blower door test. ACH is a test of a specific building’s performance in the real world.

A little bit about Thermodynamics – Heat moves in one of three ways – conduction, convection, radiation.

Conduction – Heat transferred through material. Example: Cooking. When you put a pan on the stove and heat it, it becomes hot by conducting the heat from the burner, through the pan to the food. R-value measures conduction. The higher the value, the better the insulator.

Convection – Heat transferred through air. Example: Heating & cooling your home. Most homes have equipment that heats or cools air which is then distributed throughout the house. ACH measures the convection heat/cooling loss in your home by determining how much air leaks from the building envelope. The lower the number, the lower the heat loss through convection.

Radiation – Radiation is the thermal energy emmitted by an object that is warmer than surrounding objects. Radiation does not require air. The sun heats the earth by radiant heat.

When we examine the overall thermal performance of a building, because buildings are conditioned by convection, ACH is a more significant factor than R-value.

Example: At a blistering hot fourth of July party, two coolers are used to store cold drinks. One is an expensive brand name cooler with excellent insulation and the other is a cheap styro cooler. Each are loaded with the same amount of ice and drinks. Throughout the day, people help themselves to the drinks in the expensive cooler while the cheap cooler gets set to the side and never opened. Toward the end of the day, the ice has melted and the remaining drinks are getting warm in the cooler that has been opened dozens of times.

Each time the (high R-value) cooler is opened, convection introduces warm air which reduces thermal performance. On the other hand, in the cheap cooler (low R-value) that remained closed all day, the drinks are cold and the ice is solid.

In this example, as in your home, convection plays a more significant role in determining overall thermal performance than does R-value.


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