I like the idea.
Seems useful. Armor is something unrelated so I will not provide an opinion on this.
Crafting recipe should be as simple as helmet and a beanie. Anything beyond this would be ultimately unreasonable as the two components should be separate. There should also be a backwards craft to receive a helmet and beanie item, in the event the weather warms or you wish to divide the whole for any reason.
Insulation would be reasonable to reduce Extreme to Heavy, as metal conducts the cold and could potentially serve as incentive to decide what you will wear, should you not be min-maxing on all insulation factors. These are my thoughts on the matter. It is a good idea.
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The following is unrelated. I am a hypocrite.
The current price of the helmet serves as unreasonable. The whole raw cost being somewhere in the area of around twelve Metal Scrap, seven Boxes of Screws, or so far as I remember the values whilst away from the Crafting screen. Two Iron, perhaps one Box of Screws(though the Iron likely serves a decent cost enough), using a Metal Workbench would be more reasonable in my opinion. That amount totals in raw materials to eight Metal Scrap, and three Boxes of Screws.
It still serves a cost, but it is more reasonable as the general cost is reduced and Rivets are no longer included. It most *especially* pales in comparison when you note that an item detailed as providing armor costs *less*, in the form of Resistance Attire, serving a cost of one Iron, a purchasable clothing item, and what I remember to be one Adhesive, being cheaper in total raw resource cost. I will receive some hatred, especially taking into account the fact I am unaware of the total posited armor provided by said Resistance Attire(If any, should the recipe description lie), but it should cost more if it does indeed provide armor. I was frankly surprised by the cost compared to the helmet, since it supposedly has utility.
However, I will note this does pose the risk of far more people crafting a helmet due to the cheaper cost, and thus higher appearances of helmets. To that issue I address as such-
Posers will stay posers. A cheaper helmet won't change that.
Fashion before function, bitch.
Thank you for your time.