Pompey started promisingly when Jed Wallace's 25-yard effort from wide on the right surprised Northampton keeper Matt Duke and bounced on the top of the bar.
But Pompey faded badly after that, not that Northampton offered much of an attacking threat, and their sole first-half threat bizarrely came from Ben Tozer's long throw which bounced in the Pompey six-yard box and then on to the bar with nobody getting a touch.
It was an hour before either keeper was extended, Pompey's Trevor Carson forced to claw away Moyo's looping header.
Matty Harriott also warmed Carson's hands before Therry Racon forced Duke into his first save on 62 minutes with a low raking effort.
Barker said: "We've kept a clean sheet for two successive games, which this team hasn't done this season.
"That's two games unbeaten and four points in three days, and people have to realise we are not in a position where we can turn up and assume we will win football matches."