Nice. But ReIg would, perhaps, be more practical in this case... since this, depending on the design, would only stop Ignem. Or magical dragonfire. Or infernal hellfire. Not all three, since the last two are non-hermetic.
If independent of penetration, this would combine with parma to make a hermetic magus rather untouchable. Low-level (for, perhaps, quite large values of "low") spells are unraveled, higher level doesn't have enough penetration to get through parma.