styrofoam is already insanely light. It's the lightest possible insulation there is for a garage door. It sounds like your springs are incorrect size range for the weight of the garage door, and springs can't be overtightened, they need to be replaced with a proper spring. Do you own a bathroom scale? Disconnect your garage door opener and set the scale under the door and see how much weight is on the door, it should be 8-10 lbs.