Insulating/Ventilating Shed Roof

Hi peeps,
My first post here so here goes.
Im currently insulating my recently purchased shed,for general woodwork and the like.Shed is 12x8,3/4" t & g with a mineral felt roof on top of OSB.

Now I've spoken to a couple of people about this and some are telling me to insulate the roof and leave an air gap others are telling me because my shed roof has no ventilation(fascia/soffit vents) there is no point in leaving an air gap as all I'll do is create a vacumn of hot air.(suppose if there is no gap ,theres no room for a build up of hot air)

I've also been told to put insulation in and leave NO gap but just put an air inlet low on oneside of my walls and another outlet high on an opposite wall to create air flow(sounds reasonable enough)

I do plan on using a heater in the winter and would prefer it not to be like a sauna in the summer so which method would you guys think would be the best.

any insulation will be a bonus.

fit it tight to the inner surface of the shed.

insulation will help maintain the temperature inside the building whether it is warm or cool outdoors.

either create an airflow as suggested regards cut-outs, or leave a window open.