Green rater Andrea Foss was on a worksite with a developer who had just failed several items on her LEED for Homes inspection when she came up with the idea for a list of 10 common failures in LEED projects.

Presenting at an educational session at the recent Greenbuild Conference, Foss, managing partner of Washington, D.C.-based green building consulting company Everyday Green, recounted the inspiration for her presentation.

“The developer was upset because she had failed some of the items,” Foss said. “She said, ‘I wish I had known to look for these things beforehand.’ ”

To help other green building professionals avoid making that developer’s missteps—as well as the mistakes she made while rehabbing her own home to LEED standards—Foss listed the 10 most common issues she’s found during her LEED inspections:

1. Manual J Equipment Sizing Calculations. Builders use Manual J calculations to determine the proper size of heating and cooling equipment based on a home’s size and tightness. Using the calculation properly is important in ensuring the equipment works efficiently in high-performance homes. Foss sees several common errors, however. One issue is when the calculation doesn’t reflect the home actually being built—the contractor just uses the defaults, or forgets to update the calculation even though the building has changed. Sometimes contractors will do the calculation retroactively, after the equipment is installed, or make one calculation that they use and another one “for the LEED people.”

“You lose the benefit if you’re just doing it for the paperwork,” Foss said. If you have questions, simply ask your rater—they’ve been through the LEED process before. “It’s better to ask questions before ordering the HVAC equipment,” she said.

Some common calculation errors to avoid: Make sure you have the right city listed, that the home’s infiltration is rated as “tight,” and that the insulation values of the walls and windows are accurate. And don’t try to mislead your green rater—it’ll just add time and frustration to the certification process.

2. Duct Leakage. LEED for Homes requires certified projects to reduce duct leakage to 6%, so it’s important to pay attention to detail. Look for duct boots that aren’t sealed to the drywall and places where the top of the duct is not sealed. Make sure installers seal connections with tape or mastic adhesive in addition to using a mechanical fastener—a zip tie won’t seal.

To avoid running into any problems, Foss suggested builders work with HVAC installers to make sure they know both how to properly install the equipment and how the testing will be performed. Seal the mechanical unit before it’s set in place, and test the system before close-in, especially if it’s the installer’s first time working on a LEED project.