There is very little insulation on top, and to what you may consider as insulation...
the small amount of insulation is used between the header, combustion chamber/box and any wires: thermostat and limit.
if you are referring to wires at top in the front of heater, behind removable front door... Then that is due to (can be considered as insulation as well - as that is what it's supposed to do: Keep the heat in the combustion chamber/box) broken fire tile/brick!
this is common and needs to be replaced.
Call a local service provider , or remove the top assembly to access the Heat Exchanger, disconnect plumbing, then remove the H.E.
You can now see the fire brick, ANY cracks or sacrificed integrity of that brick, need replacement. The heat travels Right THRU that crack!
An easier way to visually inspect brick, would be to disconnect gas line from heater, then remove burner tray. Many wrong installations, Actually NOT To Code, will have a union for the line INSIDE the heater, close to the gas valve. Although this is not to code, it makes for a very simple burner tray removal. Anyway, once burner tray removal is complete, use a mirror and flashlight, and you can inspect brick from bottom.
Link below what that brick kit looks like...
Good Luck
Parts4heating com Teledyne Laars R0325305 Tile Kit 400