On my bathroom floor I can feel a ton of cold air coming through right behind my steam radiator when it is off. Here is a picture of the radiator with the cover on.
Below is a picture with the radiator cover off. I can feel a ton of cold air coming through the vertical gap that you can see in the picture. How can I insulate the gap while making sure that any insulation is heat resistant due to the proximity of the steam radiator? For any solutions proffered, should the metal grate be removed first? It is held in place with nails which I would rather leave in place if at all possible.