Help With Temp & Humidity.

Ight , so i got a little problem , i have a grow box that stands 2ft high , 1 ft wide , and 1 ft deep , its lined with mylar sort of (inside of chip bags) , ive used it befor and it works great , but the weird thing is , when my lights turn off the temp and humidity are Perfect! , temps at 70 and humidity is at 60-65 but when the lights are on , temp stays where it is , but the humidity drops like crazy to 40-50 , its so weird , i got an intake fan , and an outtake fan , and i have no idea what the problem is , any help would be apreachiated! , Thank You.

what kinda lights ?, i would have to say whe he light are on it dry up the air a bit,

the only way you can keep the same humidity is to have a vented hood so that you can have a separate intake and exhaust for your light. this will allow you to keep the same internal humidity (give or take a few %) levels as your external levels.
A cheeper way to do it is to ad a bowl/jar of water to your grow room. your light will heat the water causing the humidity to rise, but this also depends on the light(s) you are running.

as campfire stated, the light will drop the humidity a bit if your using a HID light.

does your temp stay at 70 the whole time lights are on?

ur humidity is a little high - i think ideal temp is 78 degress and ideal humidity is 55 - just my opnion tho

If the exhaust fans are running more when the light is on then I would say that is why your humidity levels drop in the day. 40-50% is not too bad.