Is this homebrew approach for helmets that grant an AC bonus a viable option?

Question

My players like to look cool wearing the helmets but wanted some function to them as well. Basically I was thinking about giving helmets a +1 AC to players. It will only cost 10-20 gp, so there will be drawbacks.

Helmet
+1 AC
Anyone who wears one gains a point of exhaustion for every 3 hours of continued use without taking it off for at least a few minutes. This doesn't apply when they are sleeping during short or long rest.

I was also thinking of applying 1 or 2 of these drawbacks appropriately to the type of helmet they wear. If the helmet gives +2 AC, all of these will apply:

1. -3 to -5 on Perception rolls of any kind (depends on type of helmet)
2. -3 on Investigation rolls of any kind
3. Very susceptible to flanking maneuvers in combat

Is this homebrew approach for helmets a viable option?

Answer 1

As people's tastes for rules are super subjective, and I upvoted jgn's fine answer, I am going to throw my hat (helmet?) in the ring here. I will be taking more a simulationist view on this and upping the functionality on them for those who want to run with this more. In short, while using this answer, your mileage may vary and you should find the balance that is right for your group.

Penalties/Bonuses Do Not Align with 5e Philosophy

As jgn said in his answer, D&D 5e uses disadvantage and advantage for this sort of thing. Keeping with 5e philosphy, helmets should impose disadvantage on checks, not an actual penalty. (Although you can argue the bonus/penalty thing has been with D&D so long that this makes sense. I am not here to argue this point.)

Simulationist Point #1: Helmets Are More Important than +1 AC

You can find plenty real-world examples of people living without limbs, but an amazing dearth real-world examples of people living without heads. Historians and HEMA-ists agree that head protection is among the most important things to protect: it is often considered the best place to hit. Of course, the various editions of D&D and many players consider helmets part of armor in D&D, so any bonus from headgear is "baked in" to armor's AC.

In short, protecting your head is first priority so consider more than a +1 AC. If you do so, maybe consider reducing armor values accordingly. (Such as: +1 Ac for helmet + +2 for other bits of armor = +3 bonus described in the player's handbook)

Simulationist Point #2: All Headgear is not Equal

A great helm, visor down, protects more than a WWI Brodie Hat. Consider upping AC bonuses but pairing that with disadvantage on things. Perhaps:

• +1 to AC for a basic, relatively open helmet, such as a silk turban or Kettle Hat.
• +2 AC but disadvantage on hearing-based perception for something more like a Roman Galea.
• +3 AC but disadvantage on all perception and potential fatigue for visor-down great helms or Frog-Mouth Helms

Simulationist Point #3: A Worlds of Hats

Headwear can be important. Consider adding advantage or disadvantage or social checks based on the helmet style. For instance, some helmets can be considered more intimidating than others or identify you as part of a group that people respect/disrespect.

Have Fun!

This is all assuming you want something a little more in-depth than your initial rule and this kind of thing is something your group wants. Remember the most important thing is to have fun while plying D&D!

Answer 2

The Quick Solution

If your players want a helmet to look cool and also have some function, the simplest solution is to say that the 'function' the helmet provides is already part of their armour. For example, you rule that Scale Mail comes with a helmet, you only get the AC 14 base when you are wearing the full kit. This also avoids a potential future problem, if there is a player who wants to wear a helmet purely for aesthetic purposes, you aren't forcing penalties/bonuses on them that may interfere with their build and play style.

The Homebrew Solution

Shields

If you want to create a new item type, balanced with the rest of the game, I'll take a quick look at shields first, which function in a similar way.

A shield gives +2 AC, at the cost of being unable to dual-wield, or use a two handed weapon.The highest damage dice on a one handed weapon is 1d8 for an average of 4.5 damage. For a two handed weapon the highest average damage roll is 7, from 2d6. It is restricted by proficiency, a character without shield proficiency cannot effectively fight while wearing a shield. It is worth noting which Unarmored Defence class abilities are restricted by shields. A barbarian can wield a shield and still gain the benefits of Unarmored Defence, but a monk cannot.

Shield Summary:

• +2 AC
• -2.5 average damage, per attack.
• Barbarians can wield a shield and benefit from Unarmored Defence, Monks cannot.
• Requires Proficiency.

Proficiency

Starting with proficiency, I suggest splitting your proposed +1 and +2 helms into light and heavy. Light helmets require proficiency with light armour. Heavy helmets require proficiency with heavy armour.

Exhaustion:

As it stands, your proposed rules for exhaustion are weak, and finicky to track. To avoid it a player has to simply remove the helmet for a few minutes every three hours. A two minute break a character takes when there is no immediate threat is not normally something tracked at the table. As such it is unlikely to have any impact on the game, unless you are springing encounters on them whenever they take their helmet off, which will feel cheap. If your table likes keeping track this level of detail, go for it, but I don't consider it to balance +1 AC.

Perception and Investigation Penalties:

As others have pointed out, penalties applied to ability checks in 5e normally take the form of disadvantage, roughly the equivalent of -5.

I have never needed to do an investigation check in combat. Taking the helmet off for investigations, and putting it back on for combat does not balance the +1 AC. Perception is similar, except it has some chance of being used in combat, if you need to perceive a hidden enemy. Both of these checks are more likely to be used just before combat breaks out, which means the donning/doffing rules should be considered, and their impact on the action economy.

If the donning/doffing is the same as a shield, one action, this prevents the helmet from being an off/on switch for exploring and combat. If attacked while investigating, the player now has to decide to fight without the helmet, or use their action to put it on. But, even taking an action to don/doff it, the amount of in-game moments that will make this an interesting choice are very few, DM dependant.

Flanking

This penalty isn't well defined in your question, but I think it has the most potential to make helmets interesting items to serve a unique niche in character choices.
These is the variant flanking rules:

Flanking on Squares. When a creature and at least one of its allies are adjacent to an enemy and on opposite sides or corners of the enemy's space, they flank that enemy, and each of them has advantage on melee attack rolls against that enemy.

When in doubt about whether two creatures flank an enemy on a grid, trace an imaginary line between the centers of the creatures' spaces. If the line passes through opposite sides or corners of the enemy's space, the enemy is flanked.

If you are already using these rules, "Very susceptible to flanking manoeuvres in combat" could mean removing the restriction of being in opposite sides or corners to the character wearing the helmet. This is effectively giving all enemies pack tactics against that one character, getting advantage on attacks if another ally is in melee range. If you are not yet using these rules you could implement them with the addition that a character can only be flanked if wearing a helm.

In summary, a helmet grants +2 AC, with advantage on attacks against the wearer if surrounded. This gives helmets a unique niche. Characters have better survivability against ranged attacks when not in melee, against a single melee character, but worse against a group of melee characters. Characters who stay out of melee will also benefit the increased AC, if they are willing to gamble that they won't be flanked.

If advantage is the equivalent of +5, and bonus to enemy attacks reduce your effective AC for that attack, the helmet offers +2 AC in the right circumstance, but -3 in a bad one (2 - 5 = -3). This is a simplification, advantage isn't a true -5 and it also means enemies have nearly double the chance of scoring a crit against you, from .050 to .098 chance. Here are more details on the effect of advantage.

I'm going to denote the situational bonus/penalty as +X/-Y for the rest of this post, but don't forget the crit chance as well, especially since it doesn't matter how high your AC if one gets scored against you.

Is this final version balanced? +2/-3 seems kinda of harsh, especially since most combat encounters are created in such a way that melee characters are often surrounded by more than one enemy. If not used tactically the helmet will be a greater hinderance than benefit to its wearer.

tl;dr Give me the New Rules

Vulnerable to Flanking:

When a creature and is surrounded by at least two enemies, all melee enemies have advantage on attacks against it.

Light Helmet

Light Armour
10gp
+1 to AC.
One action to Don/Doff
The wearer is subject to the Vulnerable to Flanking Rules if the Variant Flanking rules are in use. If the Variant Flanking rules are not used in regular play, the wearer can be targeted by those rules.

Heavy Helmet

Heavy Armour
300gp
+2 to AC.
Str requirement 15
Disadvantage on Perception
One action to Don/Doff
The wearer is subject to the Vulnerable to Flanking Rules if the Variant Flanking rules are in use. If the Variant Flanking rules are not used in regular play, the wearer can be targeted by those rules.

Reasoning:

Light helmets provide +1/-4. This is a terrible choice for anyone who is regularly in the middle of the melee, but for characters who tend to stay on the edge of the fight, it may be a tempting gamble. The Heavy Helmet is at +2/-3 making it more attractive to melee fighters in the middle of it, especially if facing down a single enemy, or if the party has good battlefield control. I modelled it against the Plate heavy armour, which is where the high cost, Strength requirement come from, with disadvantage on Stealth switched out for Perception, per your original idea. Finally as types of armour, proficiency requirements are added, and monks and barbarians cannot use a helmet and benefit from their unarmored defence. The question was whether to make the same exception as shields for barbarians, but if the penalty for wearing a helm is attacks made against you with advantage, barbarians would get to use reckless attack at no extra cost when surrounded.

Final Things To Consider:

+1/-4 and +2/-3 Seem like a stingy tradeoff to give the characters. Playtesting would be required, but bumping the light and heavy helm up to +2 and +3 might be better balanced. This makes the tradeoff +2/-3 and +3/-2. +3 AC in the right circumstance is a a pretty big buff though.

This is also makes the effective AC of a character pretty swingy. A helmeted character might find themself untouchable one moment, when engaging on their terms, and a punching bag the next when the get ganged up on. These rules place a lot more emphasis on battlefield control, and letting helmeted characters choose when and how to engage with enemies. If your players aren't used to playing tactically, this will make the game more difficult. Sometimes the best tactical choice will be to not wear the helmet.

Answer 3

In my table, we handle three types of helmet:

Light helmets or helmets that protect only the top of the head

They offer little effective protection, so I just ignore them on combat. For role playing effects, they may protect from things from hitting the top of the head and the player has to handle one more item, specially when it is not being used.

Heavy helmets

Helmets that cover the top, back, sides and maybe the nose, but they are not closed. They give +1 to AC, but gives disadvantage on all perception checks and reduce initiative by 1 (the awareness is reduced).

Great helmets

They are metal helmets that fully cover the head and have just a tiny space for seeing and maybe small holes or grating for breathing. They give +2 AC, but gives disadvantage to all perception checks, reduce initiative by 2 and also gives disadvantage on dexterity saves (it's difficult to detect and quickly react to a challenge). If the Great Helmet has a visor and it is opened, it is treated as a Heavy Helmet. The visor can be opened/closed as a free action.

About heavy and great helmets, they have also another two big mechanical issues:

1. They are heavy, bulky and very uncomfortable. If characters use it for a long time (variable according to the context), they get 1 point of exhaustion.

2. And when the characters are not wearing the helmet, they need to worry how to carry it. They can be carried on the weak arm and hand, preventing the character to use this arm and hand for other tasks. This allows the helmet to be put as a free action, but as RAW rules allow the interaction with only one object for free, if you also draw your weapon on the same round, it costs an Use an Object action. You really need to tactically think before going into a combat.

If the characters decide to carry the helmet any other way than on the hand, they would need a full action to reach it before using the free action to put it.

These are the rules we use and they seem to balance well the AC bonuses.

Answer 4

+1 AC for a perception penalty is a fair trade.

Investigation is not commonly used, and susceptibility to flanking would add a whole new system. The exhaustion ticking is too easy to counter and difficult to track.

Perception is useful and important

However, perception is extremely important both in and out of combat. Being forced to take a penalty on perception would mean that donning a helm is a choice with advantages and disadvantages. Even during combat it may be better to remove your helm to locate a hiding foe, or listen for approaching foes.

Use disadvantage instead of a penalty

Having modifiers to abilities isn't common though. Usually 5e relies on disadvantage.

Consider the action economy penalty

Out of combat you probably want to take your helm off so you can keep your perception at a reasonable level. You may even want to stow your helm so you have a hand free to use a torch, interact with switches and other objects, or whatever else.

When it's time to fight you will have to get your helm back on your head. If it is in your hand, that requires the free object interaction for the turn. If you stowed it, then it takes free object interaction to unstow it, and then an action to put it on your head.

This could have a big impact on action economy and the way that your character behaves out of the fight. Normally you would draw your weapon with object interaction, and then attack (or cast a spell, etc) with your action. However if you use your object interaction when putting on the helm, then you will need to use your action to pull out your weapon. Worse still, if your helm was in your pack, you will have to wait until turn 2 to even draw your weapon.

If, during the fight, you need to take off the helm to locate a hiding foe, or listen for extra monsters that may join the fight, then you need to use your object interaction, and you need to have a free hand.

Suggested stats

You could implement a simple helm that seems balanced like so:

Helmet

While wearing a helmet, you gain a +1 bonus to AC, but you have disadvantage on Wisdom (Perception) checks that rely on sight or hearing.

What if you aren't playing much of anything other than combat?

However, if you are playing a hack 'n' slash style game that is purely straight up meat-grinder combat where perception is not used, and you can walk around with your helm in one hand and your sword in the other, then it's a no-brainer +1 AC.

No-brainers are generally bad. Using your brain is fun!

I go into more detail here.