When I play Skyrim, I like to actually see my character's face. But I dislike losing the benefits from armor perks that require wearing a helmet. I realize there are other mods that provide the benefit of these perks through invisible helmets or armored circlets. However, in my opinion, both of these approaches are immersion breaking. This mod is my attempt at a balanced and immersion-friendly solution. 

What it does:
Armor perks that normally require wearing a full set of armor will give a slightly decreased benefit when not wearing a helmet. 

• The Well Fitted/Custom Fit perks now give a 15% armor increase when wearing all heavy/light armor and a helmet isn't worn. The normal 25% armor increase still applies when wearing a helmet. 
• The Matching Set perks apply a 15% armor increase when wearing three pieces of a matching set without a helmet, and the normal 25% armor increase when wearing four or more matched pieces of armor. This means that if you use an armor mod which has more than four pieces of armor in a set, you will get the full 25% increase not only when wearing the full set, but also when wearing any four pieces.
• The Wind Walker perk gives 40% faster Stamina regeneration when not wearing a helmet, and the normal 50% faster regen otherwise.
• The Deft Movement perk gives a 7.5% chance to negate damage when not wearing a helmet, and the normal 10% chance when a helmet is worn.
• The Cushioned perk only blocks one third of falling damage when a helmet isn't worn, and half damage when one is worn.
• The Reflect Blows perk provides a 7.5% chance to reflect damage if not wearing a helmet, and the normal 10% chance otherwise.
  

The Diadem of the Savant is a special case, and is handled as if it were a normal circlet with no armor stat. This means, for example, that when wearing the Diadem you will only receive a 15% armor rating increase from the Well Fitted/Custom Fit perks. This is a change over Vanilla where it is possible to receive the benefit of most of the armor perks that require a helmet while wearing the Diadem. The Diadem itself remains unchanged.

Compatibility:
Two versions exist: one for vanilla Skyrim, and another for Dawnguard/Dragonborn. USKP is required for both, including the Unofficial Patches for Dawngaurd and Dragonborn if you use the DLC version. UPDATE: a version compatible with USLEEP is now available!

This mod should be compatible with any mod that does not change the armor perks listed above. The perks should work as expected for any armor mod, including the Matching Set perk as described above, as long as the mod properly classifies the cuirass, boots, gauntlets, and helmet, and also sets the armor material to a material existing in the base game or DLC.