Grow room temp. Way too high!!! Help!!!

Hi folks, i'm a newbie at all this and need some advice. Due to height restrictions (the attic not me..) i've had to build a 4x4x4 white box and i've fitted a 400W HPS box and a 100mm inline fan with a carbon filter. I've also put in 2 100mm vents at the bottom with valves on them to let air in but not out but the temps are reading over 90 with the inline fan running constantly! Here's a pic of the inside for a rough idea:



This is going to cost a fortune in electricity with the lights mini-fan etc. running 18 hours a day if I can't get the temps. down.

I hoped the exhaust fan would clear the room quickly, which i think it does but when the fan goes off (i had it on a timer set for 10mins every hour) the HPS lamp shoots the whole room way up to over 100 in a few minutes again!

Do i have to run this fan constantly or can anyone offer and advice on cooling things down a touch. Thanks in advance folks.

put some holes in the top, of add an expel air in the top"comp fan"
heat rises.

you could lower them a bit, till they take.
small plants dont really like hps, esp that close.
dont you have any flos to start them off?.

Thanks for your quick reply, no i don't have any Flo's to start them off which i thought was going to be an issue but the HPS light was nice and cheap which is why i went for it. After coughing up for the materials/seeds/fan/filter etc. the bank balance was staring to complain. Turning out to be a mistake cutting that corner i think...

I might try putting in a couple of those large white sliding vents into the top, does any one think that'll remove enough heat when they're opened up?. If i put a Ventaxia fan in as well, the elec. costs are going to go up again. Along with noise and risk of the room being 'heard'. These inline fans aren't as quiet as the makers say...

Again folks, any advice is grealty appreaciated.

The vents in the top should help. The fans are the cheapest part of the electric bill.

Im having the same problem. Is your light sealed with its own cooling fan? I made a diy cooltube which has helped, next a bigger fan.

No it's just an old style HPS light with the ballast and ignitor etc. all in one box. I saw the cooltubes on ebay and wondered if i could disassemble the light box and as you've done, and make my own (cost is a major factor now). Is there a DIY giude on here for this? or could you give me a few pointers. The idea of the heat from the lamp being sucked right out the grow room seems the way to go, i'll get bigger plants in my small space too. Thanks for your help.