Homes are supposed to breathe. But some inhale excessively from the outdoors and exhale too much from inside. The result: Drafty rooms, high utility bills, dirty and leaky ducts, and a bigger-than-necessary carbon footprint. If you think your home could be more energy efficient but aren’t sure where its leaks live, an energy audit can diagnose your energy issues and help you decide which to tackle.

Audits identify a mixture of major and minor air leaks. So if you’re budget-minded, you might opt for inexpensive fixes like adding caulk or insulation at leak points and installing weather-stripping. If you’re embarking on a remodel, you can make bigger investments, such as adding insulation.

The question is whether to hire a pro or conduct a free do-it-yourself audit guided by online tips. There are pros and cons to either approach.

Paying for a pro

Professional audits aren’t cheap: They run from $150 (visual) to $400-$600 or more (diagnostic). But the information they reveal can help you make targeted repairs that lower energy bills by 5% to 30% annually, according to the U.S. Department of Energy’s office of Energy Efficiency and Renewable Energy. With energy bills averaging about $2,200 annually, according to Energy Star, following an auditor’s recommendations could save you up to $660 within a year.

Paying for an audit may not make sense if you have a newly-constructed home, which likely follows the most up-to-date building codes. Energy audits should also take a back seat to urgent home issues that compromise safety, such as old or faulty electrical or structural issues, like roof or foundation problems. So if you own a fixer-upper, it’s worth addressing safety issues before optimizing energy issues.

DIY audits

A do-it-yourself audit may help you make an educated guess about how airtight your home is—or isn’t—and point you toward fixes. A typical DIY test: Hold up a lit candle to windows, doors, and electrical outlets to see if a draft blows the flame.

But be aware that when you fix a problem you uncover yourself, you could err. For instance, you might pay for new windows when you need to insulate existing window frames instead. You could also over-seal your home, creating indoor air quality issues (dirty air, mold) that compromise your health.

Services of a professional audit

Pro audits give you access to high-tech tools that pinpoint the exact location of duct leaks; exactly how airtight your home is (and should be according to local code); gas leaks; and which direction drafts are blowing. Draft direction can alert an auditor that your attic is greedily sucking up your warm air, for instance. They also ferret out drafts between insulated and less-insulated (garage, basement/crawlspace, attic) portions of a home and assess the performance of heating and cooling systems.

Two types of professional audit

A visual inspection (like a home inspection, but focused narrowly on energy issues) might be sufficient if you have semi-finished or exposed spaces (unfinished basements, exposed ducts, crawlspaces, and attics). A diagnostic inspection includes visual work, but also employs tools and devices to pinpoint air leaks.

• Blower door tests use high-powered fans to depressurize a home so that a technician can measure draft levels.

• Thermal or infrared scanning measures surface temperature variations along walls to spot exact locations of air leaks or insulation lapses.

• Smoke puffers release a form of “dust” during a blower door test to reveal the direction drafts are blowing.

• Duct blasters inject and measure air pressure, air flow, and leakage in ducts.

• Gas leak detection devices help assess indoor air quality.

These technologies provide far more specific information about a home’s issues than a typical DIY audit.

Common energy issues

A technician should be able to tell you how much total air leakage exists in your home (10 sq. ft. is like having a door open all the time), where it comes from, and how best to address it, says Robert Stockmann, of Pinnacle Home Inspections in Bellingham, Wash. The most common issues he finds are:

• Ducts in uninsulated areas (crawlspaces, attics, unfinished basements), which need cleaning, insulation, re-sealing

• Moisture in crawl spaces and basements

• Air that’s entering or exiting the home via range hoods, attic trap doors, and poorly sealed doors

Hire an auditor, smartly

Energy audit is a loose term these days, so when hiring an auditor, ask questions. Make sure the auditor doesn’t work for a window company; has a professional affiliation with or training from an auditing organization such as RESNET or the Building Performance Institute; and can provide a written report. If you need diagnostic advice, ask if the auditor can use tools that assess what’s going on behind walls and inside ducts. Your local utility company may offer audits or be able to recommend auditors.

Because an audit is a precursor to further spending for repairs, if your DIY audit indicates you need extensive, expensive, or hard-to-do repairs, consider a paid audit as a kind of second opinion. Likewise, any paid audit that indicates you need only minor fixes may seem unnecessary—but if you consider that small fixes may keep you from overspending on major ones, the money may be worth it.

Jane Hodges has written about real estate for publications including MSNBC.com, Seattle Magazine, and The Seattle Times. In 2007, she won a Bivins Fellowship from the National Association of Real Estate Editors to pursue a book on women and real estate. Her work has also appeared in The New York Times, CBS’s BNET, and Fortune. She lives in Seattle in a 1966 raised rancher with an excellent retro granite fireplace. Latest home project: Remodeling a basement bathroom.