As it has been pointed out, it may be true that if one examines the total heat loss/retention of a naked person, the head may (or may not, as I still do not see their published data, just their statement) only account for approximately 10% or the BSA of the head. However, if one is clothed and of course heat loss/retention will vary with the amount and type of clothes one is wearing, an uncovered head may account for 50%+ of the total heat loss/retention of the body.

Pete