Glossary

AFUE- The most widely used measure of a furnace's heating efficiency. It measures the amount of heat actually delivered to your house compared to the amount of fuel that you must supply to the furnace. Thus, a furnace that has an 80% AFUE rating converts 80% of the fuel that you supply to heat -- the other 20% is lost out of the chimney. Note that the AFUE refers only to the unit's fuel efficiency, not its electricity usage.

ASHRAE -  American Society of Heating, Refrigerating and Air-Conditioning Engineers

HERS Rating
- The HERS Index is a scoring system established by the Residential Energy Services Network (RESNET) in which a home built to the specifications of the HERS Reference Home (based on the 2006 International Energy Conservation Code) scores a HERS Index of 100, while a net zero energy home scores a HERS Index of 0. The lower a home’s HERS Index, the more energy efficient it is in comparison to the HERS Reference Home.  Each 1-point decrease in the HERS Index corresponds to a 1% reduction in energy consumption compared to the HERS Reference Home. Thus a home with a HERS Index of 85 is 15% more energy efficient than the HERS Reference Home and a home with a HERS Index of 80 is 20% more energy efficient.

Home Energy Rating- A home energy rating involves an analysis of a home’s construction plans and onsite inspections. Based on the home’s plans, the Home Energy Rater uses an energy efficiency software package to perform an energy analysis of the home’s design. This analysis yields a projected, pre-construction HERS Index. Upon completion of the plan review, the rater will work with the builder to identify the energy efficiency improvements needed to ensure the house will meet ENERGY STAR performance guidelines. The rater then conducts onsite inspections, typically including a blower door test (to test the leakiness of the house) and a duct test (to test the leakiness of the ducts). Results of these tests, along with inputs derived from the plan review, are used to generate the HERS Index score for the home.

Kilowatt-hours (kWh)- is a unit of energy equal to 1000 watt hours (the multiplication of watts and hours (When you use 1000 watts for 1 hour=kilowatt-hour).  What can be confusing is that Watts is the rate of use at this instant where Watt-hours is the total energy used over time.  You can't always trust the wattage printed on a device because many devices don't use the full listed wattage all the time.  For example, the compressor in a refrigerator doesn't run constantly, only sometimes, so you can't go by the listed wattage for a fridge.

R-Value- Indicates an insulation's resistance to heat flow. The higher the R-value, the greater the insulating effectiveness.  The R-value depends on the type of insulation and includes its material, thickness, and density. The effectiveness of an insulation's resistance to heat flow also depends on how and where the insulation is installed. For example, insulation that is compressed will not provide its full rated R-value. The overall R-value of a wall or ceiling will be somewhat different from the R-value of the insulation itself because some heat flows around the insulation through the studs and joists. Therefore, it's important to properly install your insulation to achieve the maximum R-value.

Therm- unit of heat energy equal to 100,000 British thermal units (BTU). It is approximately the energy equivalent of burning 100 cubic feet of natural gas.  Since meters measure volume and not energy content, a therm factor is used by gas companies to convert the volume of gas used to its heat equivalent, and thus calculate the actual energy use.

U-Value- The measure of a window's ability to reduce heat loss during indirect radiation exposure; such as during the winter months in moderating climates. Lower U values translate into less indirect heat lost from the interior of the home resulting in lower heating costs.  U-value is the inverse of R-value. To find a correlating R-value from a given U-value, simply divide the number 1 by the U-value. Lower U-values correlate to higher R-Values.  Lower U-values are important because many municipalities are adopting the IRC 06 which mandates all energy efficient window and exterior door units to carry a minimum U-Value of .40.  Top energy efficient windows today carry U-values hovering in the .22 through .30 ranges.