Hot tin roof. HELP!

My grow room is almost setup, but I just ran into a problem today. I've got it setup in a shed out back and I went in to check what I still needed and it was kinda hot in there already with nothing running yet. It's only 78 degrees outside right now, but the roof is made out of metal and it's hot to the touch. It's definitely heating up the room and it's only going to get worse once the 600W light is on. Does anyone have any ideas on how to insulate the roof from the rest of the room? I'm already going to paint the top of the roof flat white to try and reflect as much of the sun as I can, but I'll probably need to do something on the inside too. Maybe something as simple as putting some plywood like 6 inches below the ceiling to make some air space between the roof and the rest of the room? I don't know... Any ideas?

Oh, and I don't have a thermometer yet so I'm not sure how much hotter the room is than outside, but it feels like at least 10 degrees hotter than outside. I'm going to get a thermometer today so I can have more info.

bump for this, i had the same problem, i installed two vents near the roof of mine yesterday, one facing north and the other south, thermometer is in there now so i will know temps inside shortly, definatley feels like it helped a lot though, would like to hear some other suggestions though.

Look for a product called Radiant Barrier. It's a silver metallic roll on or spray on film, that reflects 99.99% of all types of radiant wave lengths. My father started to use it on the homes he designed. Our attic's would get near 120F in the summer times. After applied temps dropped to high 80's.

With the addition of a vent or two, that will definitely drop those temps.

I used brilliant white acrylic gloss on galv steel that was heating up to 160F in direct sun, after two coats its now cool to the touch. You could also add a shade cloth a few inches above to absorb the bulk of heat instead of the roof. The airspace will allow the heat to disperse if there`s a breeze.

cool thanks for the help and quick response.

TempShield™ Single Bubble Foil/Foil Insulation might work along with outlet fan

http://www.radiantbarrier.com/bubble-foil-insulation.htm

I have an attic grow and what I did was just insulated as well as possible. If its light proof you can also run your lights at night instead of during the day. And another thing is if it is raised off the ground you could make your intake air come from underneath your shed where the air is cooler.

run a sprinkler on the roof when the temps get to hot.

i put a thermometer in there yesterday and it got to just about 100 degrees, think im gonna rethink my plan and try something else, thanks for all the help.