A second-half try by England wing Tom Voyce and 11 points from the boot of fly-half Alex King saw Wasps through, despite a late revival from Castres.
The French closed the gap to just three points when lock Pascal Pape was shunted over the line, but Castres failed to find the extra all-important score.
"It's a fantastic result for the club," said a relieved Ibanez.
"We were fighting for a home quarter-final berth and it is great to have achieved that, it'll be a brilliant day and I know our fans are already looking forward to it.
"We were expecting a tough battle and we got it all right.
"It meant all the preparation has paid off, we focused hard on this game and we showed a lot of patience, as well as composure to secure the win.
"It shows the strong team spirit.
"We have not had the best record in Europe, so it is great to still be in the hunt for the Guinness Premiership title and a Heineken Cup semi-final place.
"We will have a good celebration after the win in France, I am looking forward to seeing a few friends."
Wasps enjoyed a dream start by dominating the opening proceedings.
Former England fly-half King dropped a goal and landed two penalties in the opening exchanges as the guests raced into a 9-0 lead after 15 minutes, but Castres clawed their way back into the game with fly-half Cameron McIntyre landing two penalties to close the gap to 9-6 up at half-time.
Voyce sprinted under the posts just after the break from a counter-attack, launched by Ibanez, and King's conversion put Wasps 16-6 clear with half an hour to play.
Castres lock Pascal Pape was shunted over the line by his pack for a converted try to put his side in contention at 16-13.
The last 10 minutes were all Castres as they mounted a furious assault on Wasps but the yellow-and-black line held firm.