Hats are completely unnecessary: simply remove your head and carry it in your rucksack 
<Edited for Added Silliness and Fun.
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I have just completed testing of Head Sack performance and have a few observations I can share. The Moderators do have my OK if they want to pin it as a 'sticky note' in the Equipment Section of the Forum.
Head Sack Testing, plus Recommendations for Head Sack Use
As a traveling visitor while on Camino, I want to be as courteous as possible. My detached head laying loose in my backpack, makes a fairly audible thump-thumping noise caused by it rolling around inside the pack. This can be irritating for people who have to listen to this. That is why I developed a system which solves a frustrating problem: How do I protect my head from direct sun exposure by having it inside the backpack, keep it secured from rolling around, yet be able to quickly retrieve and reattach it to my neck as needed.
Part of the solution came from the development of my Head Detachment System (HDS). It incorporates the use of wide velcro strips. The 'fuzzy' part of a velcro strip is attached to the Atlas bone of the Craniocervical Junction (the top of my neck). The 'toothy' part of the velcro strip is attached to the Occipital bone of the skull (the base of the head). This allows for a super quick, but very secure, disconnection and reconnection of my head.
I found that a self-stick adhesive on the back of some brands of velcro provided a quick and convenient method for positioning the velcro into place. The adhesive could loosen over time, so a secure and long-term attachment method was needed. What ended up working best was the insertion of staples that snugly pinned the velcro into place.
This was done
only after verifying that proper head-to-neck placement and centering had been achieved. The medium-duty staple gun, commonly found in home toolboxes, uses a heavier-duty staple that works best. The staple gun also does a superb job of inserting the staples through the velcro and into the bony structures.
Upon entering a village or town, my HDS allows me to quickly retrieve and reconnect my head. This eliminates an irritating noise that could disturb the residents. I have to admit there were a few times when I was deep in thought and not paying attention and suddenly found myself entering a town.
It is a challenge to simultaneously deal with the noises produced by my head
and my trekking pole tips. It is worth the effort, though, to keep villagers from seething from hearing 'clickity clackity, thump-thump. click clack, thump, click, thumpity-clack. . . '
I tested Head Sacks manufactured by Aussie Ultralight, Zpacks, Osprey, Sea to Summit, Gossamer Gear, REI, Montane, Montbell, RAB, ULA, and Decathlon, among others. Most of the manufacturers used an ultralight nylon-weave, non-waterproof material for their Head Sacks, a few used other fabrics.
I tested each Head Sack based on several Priorities which are discussed below. When testing the Head Sack for attachment methods on the outside of a backpack, I looked at the ability to attach the Head Sacks to the waistbelt
or the shoulder harness. Most manufacturers had some variation of an adjustable velcro loop sewn to the back of the Head Sacks.
There were three priority issues I specifically looked at when testing Head Sack performance.
The First Priority: When my head was detached and kept inside the Head Sack, did I find it easy to take in air while breathing? Passing this test was an absolute must; if a manufacturer's Head Sack failed this test, it was immediately eliminated from further testing or consideration.
The Second Priority: How well did a Head Sack perform? Was a Head Sack's motion-control force
inside the Head Sack adequate to keep the containerized head comfortably positioned while inside a backpack? Did the Head Sack prevent itself from rolling around the main compartment of the pack while containing the head?
The Third Priority: Did the Head Sack's design allow a quick and easy access when putting a detached head inside? When needing to reattach the head to the neck, was retrieving the head from inside the Head Sack quick and easy?
It was during the Third Priority testing phase when I realized that for quick and easy storage and retrievals of my head when it was detached, I needed to move the Head Sack to the outside of my backpack. This saves time by not requiring a Pilgrim to stop, remove the backpack, then search for the head inside the main compartment, then putting the backpack on again.
Fortunately, a goodly number of gear manufacturers have created some great looking and performing Head Sacks. They usually come equipped with adjustable velcro straps sewn to the back of the Head Sack allowing the Head Sack to be securely attached to the outside of the pack.
PRO: Once attached to the backpack, the location of the Head Sack's opening allowed for an easy reach. Access was very quick for either the insertion or the retrieval of my head. PLUS, I found that I could actually breath fairly well due to the weave of the nylon fabric.
CONS: Even though the Head Sacks were tightly secured to backpacks, the Head Sacks would still slightly sway and swing from my body's movements as well as the slight movements of the Head Sacks themselves. So while my head would remain securely in place within the Head Sack, the sack could slightly sway while I walked. This caused some minimal thumping and bumping against the hipbelt or shoulder straps. This simultaneously gave me headaches
and motion sickness. Ibuprofen helped deal with headaches, but anti-motion sickness medication made my body too sleepy to walk a straight line.
A funny thing happened after I had taken some Dramamine. A local parks department employee saw my body laying down. I was so sleepy from the sedative effects of the Dramamine that I hastily picked a spot to nap under the shade of a large Maple tree. In my haste, I neglected to retrieve my head and reattach it.
The parks employee found me lying there, sans head. Talk about being startled!! Fortunately, my head was able to rouse itself awake just long enough to explain why I was laying there, and how my Head Detachment System (HDS) functioned.
Recommendation:
- The most important steps in choosing a good Head Sack is determining its degree of breathability as well as the correct Fit-N-Feel that it has around your head.
- Following a few simple guidelines, spend a bit of time carefully attaching a Head Sack to either the shoulder straps or hipbelt. Taking your time and exercising a bit of patience will give you a big payoff in comfort and usability of your new Head Sack.
- Sales Associates employed at Brick and Mortar outdoor stores, like REI, can often assist you with proper Head Sack adjustments. I also know that by taking my time and following some simple guidelines, I can achieve even better results. So can you!!!
Final Thoughts: Many folks do not have a decent, local outdoor equipment store to shop at. While I prefer that folks 'shop local' to support their local community, it is not always possible. If this is you, do not fret. Use online shopping to purchase a Head Sack. Then follow the simple steps in the tips above for fitting and attaching a Head Sack. You will see that you can achieve great results by doing the fitting and adjustments yourself.
When purchasing a Head Sack online, please make sure that the online store's policies allow for returns.
It is much lighter than a hat and on fountains you can soak it with water 
in order to cool your head.
