Deanna White told a contractor she couldn’t afford the $42,200 loan he recommended for improvements to her house in Inglewood, Calif. The contractor, she recalled, said she wouldn’t be on the hook because the loan was part of a “government program.” She applied and was approved.

Two years later, Ms. White is struggling to make payments on the loan, which was packaged with more than 10,000 similar loans into bonds and sold to investors. Under its terms, Ms. White’s five-bedroom house could be foreclosed on if she defaults.

Her loan is part of a booming corner of the lending industry called Property Assessed Clean Energy, or PACE. Such loans, set up by local governments across the U.S., are designed to encourage homeowners to buy energy-efficient solar panels, window insulation and air-conditioning units.

About $3.4 billion has been lent so far for residential projects, and industry executives predict the total will double within the next year. That would likely rank PACE loans as the fastest-growing type of financing in the U.S.

As the loans spread, so do problems that echo the subprime mortgage crisis. Plumbers and repairmen essentially function as loan brokers but have scant training and oversight. They often pitch PACE loans to help land contracting jobs and earn referral fees from lenders, according to loan documents and more than two dozen borrowers, industry executives and employees.