Volstad (1-0) got his first win since 2012 with the Chicago Cubs. He allowed a run over 4 1/3 innings, and six relievers kept Houston scoreless from there. Juan Minaya worked around two walks in the ninth for his fifth save.
Tim Anderson homered to extend his hitting streak to a career-best 12 games, and Kevan Smith added two hits for Chicago.
The victory snaps a six-game winning streak for the Astros, who clinched the AL West on Sunday but are still making a push for the league's best record.
Chicago built a 2-0 lead off Dallas Keuchel (13-5) by the third inning. Brian McCann got Houston within 1 on a solo homer in the bottom of the inning.
Anderson launched a towering shot to center to begin the eighth inning. He has two homers, three doubles and six RBIs during this streak.
