@tsarstepan,
I studied thermodynamics in a previous life when I was trying to be an engineer, and it is my assumption that a steam radiator can never exceed 212 degrees F and that is far below combustion rates for all materials and insulation, and virtually all materials will work okay for you, or at least be unaffected by the steam heating appliances, or baseboard, as long as the radiator or appliance is not leaking or venting in a way that might swell wood based materials. The foam is very messy, but after proper curing, you can trim it easy with a sharp knife. They have foam that is highly expansive, and foam that is mildly expansive. This foam is famous for expanding in window frames and door frames so much they can warp the frame or break the glass even. You did not describe the holes depth or under materials ... depth. I am curious what is under it, in essence, where is it going to expand. In summary, 212 degrees is not going to bother this foam, if you trim it