Pella Corp., a national manufacturer of windows, has purchased a nearly 50-year-old rival in Vassalboro.

The Iowa-based company announced Wednesday that it has acquired Duratherm Windows Corp., which has made custom hardwood windows, stairways and doors since 1967. Terms of the deal were not disclosed.

“It was a pleasant surprise to me when I received a call from Pella, especially since I wasn’t actively seeking to sell the business,” Tim Downing, president of Duratherm, said in a news release announcing the sale. “This partnership creates exciting possibilities for Duratherm. With Pella’s involvement, we will continue to be trailblazers within the architectural community.”

Downing and his 70 employees will maintain the upscale Duratherm brand for the time being. Then, over time, it will be integrated into the Pella Crafted Luxury collection.

“We’ve acquired Duratherm for Duratherm,” Tim Yaggi, president and CEO of Pella Corp., said in the release. “Our goal is to support their endeavors and help the team do more of what they already do exceptionally well. Duratherm rounds out Pella’s collection of elite brands and expands our geographic footprint on both the East and West coasts.”

Duratherm has been recognized for its adherence to green initiatives, such as replacing its oil-based heat with wood pellets and operating an organic garden for employee use. It also has embraced workplace wellness programs, providing a model for other manufacturers.

This is the third recent acquisition for Pella. It previously acquired Michigan-based Grabill Windows & Doors in 2015, and New York’s Reilly Windows & Doors in July. With the purchase of Duratherm’s facility in Vassalboro, Pella will operate 13 manufacturing locations in the United States.