Emil Hallfredsson fired Barnsley to a second straight win to spark talk of a Coca-Cola Championship play-off push at Oakwell.
The Icelandic midfielder capped a slick move from the hosts by smashing his third goal of the season to sink Watford 1-0.
Barnsley played the last 15 minutes with 10 men after Ryan Shotton was sent off for a foul on Lloyd Doyley.
The Tykes came close to opening the scoring in the 54th minute when Nathan Doyle let fly from 25 yards and rattled a post.
And 60 seconds later the deadlock was finally broken. Teixeira did well to keep the ball in play and found Andy Gray, whose pass sent Hallfredsson galloping through.
They came close to doubling their lead in the closing stages when Gray raced through only to be denied by Loach, while Tykes keeper Luke Steele secured three points for his side with a fine stoppage-time save from Lansbury.