The term "audit" means different things to different people. Here is an overview of definitions to help you decide what type of service is best for you.
Online Energy Audits: These are online tools that allow you to enter your utility usage, cost and home description into an online tool. The Energy Star Home Energy Yardstick is an example of one of these. The tool allows you to compare your energy usage to others around the country and get a good idea of where you stand. It also provides general ideas of how you can lower your energy usage.
In-Home Energy Audit: These usually cost between $75 and $175 dollars. In this audit a representative of the company will come to your home, review your utility bills, do a visual inspection of your home and make suggestions that can improve your energy usage. You can expect to receive some general advice as well as more specific advice for your home, based upon the visual inspection. These professionals will usually safety test your furnace and leave you with a list of improvements.
Home Energy Performance Review: This is the most comprehensive of the services being referred to as "audits." These reviews run from $350 to $750 (or more) based upon the amount of testing involved. In addition to everything listed in the audit above, the Home Energy Performance Review may include the following diagnostic testing and reporting to evaluate your home. Some of these services are priced as part of an "audit" package and some are usually priced a la carte.
Combustion Appliance Zone (CAZ) Testing: Professionals will safety test your gas and propane appliances to verify that combustion gases are being exhausted to the outside. Pressures in your home can potentially pull carbon monoxide fumes back inside and create a dangerous situation for your family.
Blower Door Test: A blower door is a powerful fan that mounts into the frame of an exterior door. The fan pulls air out of the house, lowering the air pressure inside. The higher outside air pressure then flows in through all unsealed cracks and openings. The auditors may use a smoke pencil to detect air leaks. These tests determine the air infiltration rate of a building. There are two types of blower doors, calibrated and uncalibrated. It is important that auditors use a calibrated door. This type of blower door has several gauges that measure the amount of air pulled out of the house by the fan. Uncalibrated blower doors can only locate leaks in homes. They provide no method for determining the overall tightness of a building. The calibrated blower door's data allow the auditor to quantify the amount of air leakage and the effectiveness of any air-sealing job.
Duct Blaster: A duct blaster combines a small fan and a pressure gauge to pressurize a house’s duct system and accurately measure air leakage of the ductwork. Duct leakage can increase heating and cooling costs over 30% and contribute to comfort, health and safety problems. Leaky supply ducts can send expensive conditioned air into unconditioned spaces such as attics and crawlspaces and leaky return ducts draw unconditioned air into the duct system.
Infrared Imaging: These tools see light that is in the heat spectrum. Images on the video or film record the temperature variations of the building's skin, ranging from white for warm regions to black for cooler areas. The resulting images help the auditor determine whether insulation is needed. They also serve as a quality control tool, to ensure that insulation has been installed correctly. Infrared scanning allows energy auditors to check the effectiveness of insulation in a building's construction. The resulting thermograms help auditors determine whether a building needs insulation and where in the building it should go. Because wet insulation conducts heat faster than dry insulation, thermographic scans of roofs can often detect roof leaks. In addition to using thermography during an energy audit, you should have a scan done before purchasing a house; even new houses can have defects in their thermal envelopes.
Computer Modeling: Home Energy Modeling is a way to predict the impact of improvements on your home, before the work is done. By modeling your home and mapping out the proposed changes updates can be made in the most cost effective and energy efficient way.
HERS Index 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. A current and projected HERS Index is frequently required if you are applying for energy efficiency loan and financing programs.
Energy Star Rating with Plan Reviews: Includes 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.
Test Out: The auditor will return to your home after energy improvements are performed to verify that the expected savings will be achieved.
Source: Some information on this page provided by the US Department of Energy, http://www.energysavers.gov/your_home/energy_audits/index.cfm/mytopic=11190