In Ars Magica a fire might not be considered magical (and therefore resisted by magic resistance) if it does not have fuel. Although it’s circumstantial, once an item is set on fire it may burn normally and harm everyone just like abnormal fire. A spell I wrote a little while ago to burn a castle (Immolation of Harrenhal) plays with this demonstrating how dangerous an entire building might be to both magical and mundane inhabitants. Once it is burning naturally the environment is deadly.
So the same is perhaps true of freezing things. Trapping a foe in a freezing chamber that they cannot escape from will chill, fatigue and maybe incapacitate them as long as it stays cold enough.
Freezing of Harrenhal
Perdo Ignem 35, R: Touch, D: Concentration, T: Structure
This spell cools an entire structure, or a sub-part chosen by the caster, and everything within it to below freezing, which creates a dangerous environment for the inhabitants and materials. All items inside will be affected either by the initial freezing or the remaining chilling temperatures which likely continue after the spell expires. Whilst active the spell inflicts +5 cold damage per round.
(Base 5 to freeze water, +1 Touch, +1 Concentration, +3 Structure, +1 Part flexibility)
These and others effects I’ve pondered over the past few years are part of the new spells grimoire for Ars Magica.