Rennie told media after the game, "If you had given me that score before the game, I would have taken that.
"I though in the first half we were really good, maybe even better than the 13-0 scoreline. We applied a lot of pressure, but the second half was disappointing.
"I just through we overplayed down our end of the field. But having said that, our lineout was excellent, our defence lineout was excellent. Likewise with the scrum.
"The big men put in a shift and that got us home in the end.
"I know the boys were setting up for a dropped goal under the posts at one point.
"With 90 seconds left they decided to hang on to it. We did that for about 80 minutes and then got penalised. It would have been good to deny them a bonus point but the key thing was us winning."
Glasgow next face Exeter and Rennie believed his side could compete with the English club.
"I think we've got a set-piece that'll be competitive down there at Exeter. I also thought we defended well for big chunks of this game.
"I just think we'll maybe need to play a little bit smarter next week," he said.
Exeter moved to the top of the table with a 31–12 bonus point away win over La Rochelle.
Munster's first five-eighths Tyler Bleyendaal kicked 12 points in his side's 32-13 over Ospreys in Swansea.
Isaia Toeava and George Moala were among the tryscorers for Clermont Auvergne's 53-21 win over Harlequins.
Leinster started their campaign with a 33-19 win over Benetton while Ulster broke Bath hearts when taking a 17-16 away win.
Still to play on Sunday are: Connacht v Montpellier, Northampton v Lyon and Racing v Saracens.
Meanwhile in the Challenge Cup section, Brad Mooar's Llanelli Scarlets had a 20-16 win over London Irish. While on Friday Toulouse turned around a 9-20 halftime deficit to beat Gloucester 25-20
And former Blues coach Pat Lam's Bristol side beat Zebre 59-21.
Worcester Warriors had a 57-14 win over Enisei-STM, Edinburgh beat Agen 31-10, Brive beat Stade Francais 27-11 while Toulon beat Bayonne 20-13 with wing Julian Savea scoring a try to give Toulon the lead 15-13 late in the game.
Cardiff beat Calvisano 38-16, Leicester, playing in the Challenge Cup for the first time, beat Pau 41-20 for whom Tom Taylor kicked 10 points.
Newport beat Castres 31-17 while Bordeaux beat Wasps 40-30 with Seta Tamanivalu among the try scorers for the French side.