Is it ideal to have the helmet shell as close to your head as possible when fitting a bicycle helmet? Some retention systems can have a snug and apparently secure fit to your head while still allowing a little bit of room between the shell and your head. Is it best to have the shell as close to your head as possible, or does it not matter as long as the retention system is keeping it secure?
AFAIK there are no cycle helmets on the market that are form-fitted to the outside of your head - everyone has a different topology to their skull so any such hat would have to be made custom. That's prohibitively expensive and would be limited to aero helmets for elite-level time-trialists.
Every helmet has some kind of strap system used to hold the helmet to the wearer. This always entails some kind of chin strap - there are no rated helmets that don't have a chinstrap.
That chinstrap must be an integral part of the helmet - not just secured to the lower rim. So it goes through the helmet and secures at multiple points. This takes space between you and the foam or it may be embedded in the foam.
Finally, modern helmets come with a system called MIPS which is a secondary layer allowing the helmet to slide around the head some, minimising shear forces to the brain. This is often a separate lattice inside the helmet, and takes space. This layer would be the closest surface to your scalp and also holds comfort pads, and takes some space to function as designed.
You might be thinking of construction hard-hats which are restrained using a low-rider strap around the rear of the head. These are not a positive retention system and while elements of this are in modern cycling helmets they still require a chinstrap.
Also, hard hats have enormous gaps between the hard outer layer and the inner support system. They generally lack EPS foam.
But the most important reason for some space there is ventilation and airflow. My first helmet in 1987 had no air vents, and it got very hot. So you need some space for air to cool your head. Too close-fitting and you'll sweat more and overheat sooner.
A bicycle helmet is designed to destructively absorb the impact of your head hitting another solid object at velocity. The expanded foam frame will crush and absorb energy. It’s why it’s important to discard a crash damaged helmet. It’s a single shot device.
As long as your helmet is CE certified for your flavour of riding, fits you well and is secure when fastened any other considerations will be personal comfort or budget decisions.
A helmet with pads attached to the shell where your heads rests against the shell may be less comfortable to some people than a helmet with a inner cradle which holds your head a few mm away from the shell. The gap allows air to flow inside and over your head better. More air equals more cooling.
Is it ideal to have the helmet shell as close to your head as possible when fitting a bicycle helmet?
A helmet too close to the head means every rotation of the helmet causes an equal rotation of your head. Crashes often happen at high speed, creating rotational forces. There's actually a theory that most brain damage is caused not by linear acceleration but rather by rotational injury.
You should select only helmets having MIPS, a system for preventing (or at least reducing) rotational injury.
Other features you should look for in helmets are in-mold technology (where a polycarbonate shell is molded to the expanded polystyrene) and the best possible certification. This would mean Snell B95, but oops, you can't buy reasonable Snell B95 helmets at all. Some time ago you could buy Snell B90 helmets by buying only Specialized helmets, but oops, Specialized has stopped B90 certifications and every helmet in Specialized lineup probably has been renewed after the stop of B90 certifications, so I guess this means reasonable helmets can't be bought anymore.
I still have one helmet that is from the time before the stop of Specialized B90 certifications. I have absolutely no idea what helmet I would buy today.