This is a very old idea, and I think Simon's actually coded it in before, but I have an idea that might make it sensible for gameplay.

1) Ambient heat would function by creating a 4x4 block grid not unlike the one used for air calculation where each block stores its own float heat value. When particles calculate their heat for each frame, lastly they check the ambient heat for the 4x4 square they reside under, and increment or decrement by 0.1. The exact number depending on the pressure. Higher with higher pressure, lower with lower pressure. This way nuclear explosions will transfer ambient heat very quickly, while the vacuum of space and normal room pressure will transfer heat rather slowly.

2) Ambient temperature does not exist inside of solid wall tiles, due to how they block pressure and presumably also air. This would make it possible to build a nuclear reactor without melting everything in the field.

3) The Heat Compatibility button on the powder toy GUI could have a third toggle added to it, so that the default heat setting is NOT Ambient Heat. This way a user can click it once to enable Ambient heat, click again to disable heat entirely, and then click again to have normal heat settings.

4) The outermost edges of the ambient heat grid would constantly average to a preset temperature that should be variable, and possible to set by the user without much grief. The console could be used for this at first, but perhaps the heat compatibility button could spawn a slider next to it that ranges from absolute zero to maximum heat? Not sure how problematic sliders are in C. If not that, then a query box could come up when the user enables ambient heat mode that requests a base temperature like for password entry.

With all this in mind, it would be easy to keep the base ambient temperature from having an effect simply by boxing the save's edges in with solid wall tiles. :D Does this sound sensible or would changes need to be made.

INSL has no heat conductive properties, so wouldn't conduct heat if ambient heat were not applied to it.

...However, for the same reasons as it's difficult to make a pressure-blocking element, a heat-blocking element would be similarly difficult to make in any less than 4x4 quantities.

Air is laggy in TPT because air is one of the most expensive calculations the game does.

Ambient heat however would just be a series of +/- checks with increments or decrements. It wouldn't require as much processing as normal heat does I would imagine.

Air actually is pretty laggy. :P Try switching to No Update mode and see what happens.