Armor Class Penalty For Not Wearing a Helmet

TL;DR: -2 AC penalty.

As I revealed in my storyline, Elmö’s Obligation, Xenograg has a taboo that his head must be bare to perform sorcery. Obviously, this includes helmets. So I have always been curious about how Dungeon & Dragons Armor Class (AC) is/should be adjusted when a combatant is sans helmet.

I never agreed with Advanced D&D’s no-called-shots-except-in-the-case-of-bare-heads rule. This is literally not exceptional enough of a case.

The house rule I have seen most often is a -1 AC penalty. It is one I have used, but it feels insufficient.

Then the answer came to me while I was thinking about the difference between a (chain) mail shirt, which is medium armor in D&D, and (chain) mail armor, which is heavy armor. What is the difference?

How much of the body is covered by the armor.

Therefore, foregoing head protection lowers your Armor Class one category. In Old School D&D clones, this equates to a -2 penalty.

It is the simple elegance and clarity of the explanation that tells me this is the correct answer.