When converting a shed for livestock of any kind, temperatures and air-supply must be paramount if you are to avoid causing suffering to the animals/birds/fish/elephants/whatever that will live there....
Anyone stepping into a shed in the summer will immediately notice the heat being radiated from the ceiling. The (usually) black roof felt absorbs huge amounts of heat from the sun, and emits it as radiated heat inside the shed. It can affect the temperatures inside massively, much like leaving a car parked in the sun, and we all know what can happen to animals left inside those without there being enough ventilation to allow the air to carry away excess heat.
To avoid prematurely cooking our chickens, here's what I've done in the past to my own sheds, and it has helped tremendously in saving the trouble and effort involved in trying to solve heat/air problems further down the road.
Firstly, a coat of aluminium pain on the roof wouldn't go amiss, if practical. After that, I cover the inside of the ceiling in cooking foil, shiny side to the roof and using spray glue, to reflect as much heat back as possible. Then, using a sheet of 2" thick polystyrene sheet from the local builders merchants, I insulate between the rafters on the ceiling, again using spray glue to stick them there, and finally covering the lot with thin plywood sheet tacked to the rafters. (If you can afford to do this to the whole shed, you'll create a veritable palace for them.)
Doing the above will reduce the heating effect of the ceiling almost to zero noticeable effect, with the added bonus of it keeping the shed warmer in winter as heat doesn't escape through the roof either.
Air is the next priority for me, and the rule is plenty of it, but no draughts. This is usually easily solved by siting the air vents at different levels, one high up, near the roof, will naturally act as an exit point for air as heat rises, so site your intake vents low down so the incoming air doesn't simply go straight through the shed and out of the other side causing a draught.
Blimey, I did ramble didn't I?