Insulatle is a recurring spell in the Dragon Quest series. It decreases the damage dealt by breath attacks by a substantial amount in all appearances, with the exact percentage varying between titles.
Insulatle is learned by Priests and Sages at level 32, and costs 6 mp to cast. In the original version the spell will reduce damage by 33%, overriding any additional reductions provided by armour. This bug is fixed in all subsequent versions, and the spell will last 6~9 turns in all versions.
Insulatle is learned by Meena at level 29, and costs 6 of her magic points to cast. It reduces damage by 50%, setting the precedent for nearly all future appearances.
Interestingly, Insulatle decreases breath damage by a massive 70% in the original Super Famicom version before being nerfed to 50% in all subsequent versions. This is likely the reason why armour resistances are weaker than usual in V.
Insulatle features the same traits as in the previous game and is once again learned by reaching rank 2 of the sage, but in the 3DS remake it can also be attained by reaching rank 3 of the Armamentalist, rank 3 of the Monster Masher, rank 1 of the Druid, rank 1 of the Hero, rank 4 of the Vigilantern, and rank 5 of the Gripevine vocations.
Insulatle is learned by Jessica at level 25 and cuts damage by 50%.
Insulatle is learned by rangers at level 26 and priests at level 43. It reduces breath damage by 25%, capping at 50% when cast twice, and costs 8 mp to cast. The effects of the spell will last for 6~9 turns.
Insulatle (then Barrier) is learned by monsters beginning at level 18 if the monster has at least 76 MP and 70 Intelligence. It is learned naturally by AmberWeed, KingSlime, MistyWing, WeedBug, and WindMerge. 
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