Lets calculate the total area. We have a rectangle of sides 8+18 = 26 m and 8+25 = 33 m. So:

The "ghost area" is a rectangle of sides 25 m and 18 m. The area will be:

So the L-shaped area is:

To get the volume just multiply the result above by 1 m, which is the height of insulation:

Answer: it is required 408 cubic meters for the job.