Serious Materials, one of the better-funded green building startups with its recent round of $50 million in the bank, has bought up an energy-efficient window manufacturer called Alpen Windows.

Started as Quiet Solution, a maker of highly soundproofed building materials, Serious moved more aggressively into green building last year. It has several lines of insulating or environmentally friendly wall material.

The acquisition, for an undisclosed price, should help the Serious round out its offerings. Alpen windows are advertised as being better than Energy Star-labeled products, with extremely good thermal insulation properties.