Hi. What type of underlayment should I put beneath my snap lock type standing seam metal panels? Should it be a high temp variety? Do I need an air gap just beneath the metal panels? Some building science people say yes some say no concerning an air gap. The reasoning they give for the no air gap is that if water or condensation gets under there between the underlayment and the metal it will get fried away anyway due to the heat from the sun on the metal. Plus asphalt shingles don’t have an air gap either so…. 

My plan has been to fasten the metal panels with the clips system that came with it straight to my 5/8″ ply sheathing with an underlayment of course. This is for a small timber framed shed roof that is a simple gable to gable type roof 6:12 pitch.

Also since the area of the roof is approx only 140 sq. ft. where can I get underlayment for that small an area? 

I am considering ZIP Systems Underlayment HT (hight temp rated to 240 deg F). Is there anything better?

Adding an air gap would complicate things a bit because then I’d have top consider that extra height in relation to all my trims and such as well as adding battens yadda yadda yadda. Something which I didn’t figure on. 

Am I complicating all of this? I always do that in my quest to “do it right”.
BTW, this timber framed shed has been a labor of love, hence my concerns (see attached photo).

Thanks all for any advice in advance.