How thick is your armor (5e D&D)?

Here is something I've found out: 

If your Dexterity is greater than your Armor Class, you are wearing "lighter" armor. If your Dexterity is smaller than your Armor Class, you are wearing "heavier" armor.

Sounds quite obvious, but it took me a while. 

Now, here is an explanation, and why this is useful.

I have often considered playing around with armor in my 5e D&D games. For example, having certain weapons work better against certain types of armor, and so on.

It is not easy to do, however. AC 15 can mean that someone is really agile, or that he or she is wearing chain armor. Likewise, when a monster has AC 15 (natural armor) it could be anything - from the tiny stirge (AC 14) to the large troll (AC 15).

Certainly the troll's skin is thicker than the stirge's?

Having a specific type of armor (leather armor, plate armor) indicated is a bit better, although you'd have to create specific rules for each kind of armor (there are twelve, plus shield), or memorize in which categories each armor fits (light/medium/heavy) or reverse-engineer the monster's AC to find out how heavy the armor is*. 

* Notice that someone with Dex 18 might have AC 16 using light, medium or heavy armor - although in this case you might assume the armor to be light. Curiously enough, one of the main downsides of heavy armor (and some kinds of medium armor) is giving you disadvantage to stealth - but, again, you'd have to memorize them to to know if a monster has disadvantage.

But if we use the formula indicated above, things get a lot easier. Now it is obvious that the stirge's natural armor is lighter than the troll's, and the Tarrasque's carapaces in a lot heavier than both. 

With this formula, we can create easy house rules, such as "slashing weapons deal more damage on a critical hit against light armor". This is just a start.

It doesn't solve all our problems - there are still a few outliers, and some instances where Dexterity is equal to AC (but not enough to create a "medium armor" category by itself), oozes might be a problem, etc. But it is simple and efficient... which is all I need right now.

Images: copyright Wizards of the Coast AFAICT.