Tank Insulation

[Seed_Lord]

Hey peeps, just wondering if anybody with some knowledge of insulation could give me a rough idea on this, basically might be moving into a flat by myself, want to start breeding discus so going to have 5+ tanks, with plans to glue 2-3 inch celotex (best i can find) to the tanks, literally enclosing them as much as possible with a viewing window cut out (which will slot back in) and a proper hood made out of it, so will be as air tight as possible, with holes for filter heater etc, just wondering how well this would stand up to a cold room? like how long say a 200-300w heater would be on a day to keep a 30 gallon tank at 30 degrees? cos obviously keeping a warm tank in a cold room with no insulation will strain the heaters which means more electric which means higher bills, flat ive been looking at is all electric which means electric heaters so want to use them as little as possible i dont mind throwing on a jumper.

Cheers