First I had to look up what "purlins" were as I was unfamiliar with the term. They appear to be secondary support of the roof structure.
What realy governs the use of plenum rated cable is the building & electrical codes of your area. It all depends if the city, town, state, etc. have adopted the NEC or have it supplemented with local codes. The final say is the AHJ (Authority Having Jurisdiction). That being said, if you area is covered by standard codes, like the NEC, then plenum rated cable is only required in plenum spaces (used for air handling). In general, it does not sound like using non-plenum in this case would be in violation but please check with your AHJ.
Joseph Golan, RCDD