ROCKLAND, Maine — It all started so small, Richard Cadwgan marveled this week as he looked around at the Window Dressers’ busy teaching workshop space in Rockland’s Lincoln Street Center — space that is packed with the makings for thousands of low-cost, high-efficiency thermal window inserts.

Back in 2009, Cadwgan, a retired geotechnical engineer who lives in Rockport, got involved with an effort to make the First Universalist Church in Rockland more energy efficient. The church had 19 old aluminum-frame windows in the sanctuary, letting in light but also Arctic blasts of frigid air.

“I looked around and found out that a single custom-built window was $1,300. We had 19 to do. Just forget it!” he said.

But then he learned that a man in Damariscotta was teaching people how to build pine and polyolefin window inserts that kept the heat in and the cold out. Cadwgan went to Topher Belknap’s workshop and made 26, enough for the sanctuary and for the common rooms downstairs. It didn’t take long for parishioners to notice how much warmer they were, and asked if he could build them for their houses, too.

“And that’s what happened, this downhill journey,” he joked, gesturing around him at the many neatly stacked piles of materials that volunteers with Window Dressers will assemble into inserts this fall. “It’s growing like a weed.”

That’s for sure. The nonprofit agency that Cadwgan and Frank Mundo of Rockport founded in 2010 began with about 100 inserts. They tripled that the next year, and this fall, they are shooting for 3,000. Last year, it was awarded the Governor’s Award for Environmental Excellence for outstanding environmental leadership and innovation.

The growth has happened for a few reasons — firstly, because the inserts work, Cadwgan said. A correctly made and correctly measured insert really makes a difference in Maine’s housing stock, where up to 30 percent of heat can be lost through old, drafty windows. The windows can help homeowners lower their fuel costs by as much as 20 percent.

“They definitely make a difference,” said volunteer Corliss Davis of Belfast, who installed several in her home two winters ago.

The program also has benefited from innovation. Cadwgan and others grew tired of using a noisy hairdryer to heat-shrink all the inserts, and so they designed what they call the “pizza oven”: a vertical infrared oven that heat-shrinks the inserts in only 10 seconds. The windows also are measured with lasers and the wooden insert frames are cut using precise, computerized technology.

“We’ve used assembly-line ideas and applied them to an inherently labor-intensive job,” Cadwgan said. “Nobody has a set-up like this.”

Another reason the nonprofit is growing is because its business model is elegantly simple. Window Dressers have two kinds of customers — paying customers, who are charged about $19 per window for the unpainted pine inserts — and low-income customers, who are asked to make a $10 donation to cover all their windows.

“For every three windows people buy, we give one window away. We give away 25 percent of our production. That’s how the system works,” Cadwgan said. “We use hundreds of volunteers in a season. It’s just unbelievable.”

This year, things are a bit different, he said. Mundo, who worked hard to expand the Window Dressers’ reach by developing the community build idea, ran into some health problems this fall and is not as involved.

“Up to this point, we’re self-funding. We don’t ask the state for money. We don’t ask the federal government. We don’t need it,” Cadwgan said. “But the board decided we needed to bite the bullet and hire a full-time operations manager. For the first time, we’re looking for support.”

So far, they have raised about $40,000, with a significant chunk of given by the midcoast’s St. Brendan the Navigator Parish to build inserts for low-income folks around the area. Window Dressers this year has expanded to Brunswick, Portland and Searsport, and organizers hope to add Waterville and Lewiston next year.

“We need to grow,” Cadwgan said.

Though it is too late to order window inserts for this year, there’s still plenty of time to volunteer at a community build this fall or to be added to the customer list for next year, he said. Those interested can call Window Dressers at 975-5967 for more information.