Helmets

Helmets are the one armour slot that never charges you for wearing it. No helmet in the game carries a movement penalty, while a heavy breastplate or a pair of greaves costs 5% each.

Protection

Pros

No helmet costs movement speed. The only head pieces with any movement figure are the Fenris Hood at +3% and the Crown of Valheim at +5%. Every point of armour in this slot comes free, which is not true of the chest or legs.
It is the lightest real armour in the game. A Flametal Helmet weighs 3.0 for its 38 armour, while the breastplate of the same set weighs 10.0 for the same 38.
Set bonuses hang off a head piece: Troll (+15 Sneaking), Root (+15 Bows), Fenris (+15 Fists), Bear (Berserk), Vilebone (Vilebone Wrath), Ask (Ask's Endurance) and Vanguard (stamina and regeneration).
There is a helmet at every station, so the slot never stalls while you wait on a building: the Workbench, the Forge, the Galdr table, the Black forge and the Frost Foundry all make one.

Cons

Armour is all most of them do. Most helmets carry no effect whatsoever, so once your tier is settled there is no choice left to make.
Some helmets have no upgrade ladder at all: Hildir's caps and headscarves, the Midsummer Crown, the Yule Hat, the Dverger circlet and the Crown of Valheim. Most of them are clothing with an armour figure.
No helmet resists frost. Freezing in the Mountain is answered by a cape, not by anything in this slot, which is why a Drake Helmet does not make the biome survivable on its own.
You cannot have the Dverger circlet's light alongside armour. It takes the head slot, cannot be worn with any other head piece, and cancels any sneak bonus while it is on.

Builds

Take the highest-armour helmet of your tier, then decide whether a set bonus is worth more. Because the slot costs no movement, the usual heavy-versus-light argument does not apply here. The only question is whether the 3 pieces of a light set together beat a heavy helmet on its own.

Mages answer differently. The Eitr-weave Hood and the Hood of Embla trade armour for +20% and +30% Eitr regeneration, and a caster with no eitr has no attack at all.

Every craftable piece gains +2 armour per upgrade level. A helmet is therefore 6 points better at quality 4 than at quality 1, and that ladder is the same 6 points whichever helmet you upgrade.

Helmets come from Hildir, the Workbench, the Forge, the Black forge, the Frost Foundry, the Galdr table and Haldor.

Best Helmets

Build	Name	Stats
Most armour	Crown of Valheim	50 armour at quality 1
Longest lasting	Carapace Helmet	1800 durability at quality 4
Lightest to carry	Hood of Ask	1 weight
Most armour without a forge	Fenris Hood	10 armour at a Workbench

Notes

Armor reduces physical, fire, frost and lightning damage, but it does nothing against poison or spirit damage.
Against a hit more than twice the armor value, armor simply takes its own value off the damage. Against weaker hits it blocks a growing share of the damage instead, and every extra point of armor adds as many survivable hits as the one before, so stacking armor never stops paying off.
The armor rating is the total of the helmet, chest, leg and cape armor worn, so going without a helmet throws away a large part of it.
Upgrading a piece that is being worn takes it off, so check the gear before heading back out.
Every craftable armor piece except Rag Armor gains 2 armor with each upgrade.