I’d like to back up comments re fit and comfort being as important, maybe more important.
Comfortable, well fitting helmet may be unpleasant when it’s hot, and you may forget you are wearing it the rest of the time.
Poor fitting helmet will be unpleasant, noticeable and likely annoying all the time.
As for testing ventilation, or finding tests of ventilation, I suspect you are out of luck. It’s increasingly understood that aero is different for everyone. Helmet ventilation is all about air flow, so it will also be personal. For example, how long is your hair, as that will have a large effect on the airflow through the helmet and therefore the cooling.
Best bet in my view is go find a good local shop to try different helmets. Work out what brand fits your head, and then work from there.
Surely everybody riding long brevets would select shoes, clothing and helmets that are comfortable and fit well? Choosing a well-ventilated but ill-fitting helmet while riding for many hours and days would be as absurd as riding shoes a size too small for reduced weight.
So anybody’s helmet recommendations are probably meaningless because of differences in airflow between individual riders? Nice to know. I guess multiple recommendations for the same helmets suggest that some designs work for more than just one rider.
I have the Specialized S-Works Prevail 3 and the Propero 4. Because I am bald, on sunny days, even though the Prevail 3 might provide better airflow than the Propero 4, more often than not, I put on the latter because doing so means I won’t have to put sunscreen on my head that will inevitably drip down into my eyes. Everyone is different and as someone who sweats bucket and does not handle the heat well, even wearing aero helmet with few vents like the S-Works Evade, I don’t think I have ever got to the point where I told myself that I wished for a helmet with more vents.
I sweat like mad, not bald but hair cut fairly short. I use Halo Headbands to channel sweat away from my eyes and to help hasten evaporation - they work great! Tech fabric skullcaps are also a good call (Halo makes those, as well).
As far a helmets with ventilation, my oval-shaped head likes lids from Abus (the Stormchaser is nice and cool), Kask (Vallegro or Protone), and Giro (the Cinder is what I have now). I’ve taken them on long climbs and rides in the Alps and Rockies and have never overheated. Even on LoToJa, which was on a hot day in the mountain west when I did it in 2024, my Giro Cinder kept my head comfortable.
I handle dry heat rather well, and all the heat I’ve experienced in Europe is a lot easier to cope with than the humid heat of the Mid Atlantic and southern U.S. For me it’s all about wearing kit that is properly cut (i.e. not baggy/flappy) and wicks well, hydrating properly, and reading body signals well. I’m also a fan of sleeveless base layers to help wick from the skin, as well as lightweight sun sleeves to act as both SPF and extra evaporation surfaces (skin cancer runs in my family, so the cover-up is nicer than tons of sunblock on super long days).
I’ve not ridden helmet-free in over 40 years, and the modern helmets ventilate so much better than the original Bell V1-Pro and the like, even at low speeds. I don’t feel like I’m wearing anything that causes undue heat retention, though my profuse sweating probably compensates for anything the helmet does to trap heat.
But the lids I mention breathe well. If I had to add additional suggestions, the POC Cytal is airy, as is the Met Trenta. Bell, Lazer, and POC tend to fit rounder heads better, while Met seems to aim more at the oval shaped set.
I guess multiple recommendations for the same helmets suggest that some designs work for more than just one rider.
Agreed, that is likely, unless they all have the same head shape. And still not guaranteed that it would be a good helmet for you or me.
for example, POC are/were everywhere in the MTB world, at least round here, and so when I came to needing a new helmet, I went for a POC. It was horrible, and really uncomfortable; it moved around on my head on every bump, unless tightened it up, but tight enough to stop it moving was so tight that created headache inducing pressure points. It’s possible it was that particular model, but it’s put me off POC. Giro on the other hand I find fit me consistently well.
The Prevail 3 is the most open to the air helmet I’ve ever seen – so much so that it is good to be mindful of sun damage if you don’t have a full head of hair. Considering the original question about slow (and fast) speeds – I find something like this to be better at ventilating, while my POC ventrals are good at higher speeds because the channels do seem to flush air through (while preventing as much the sun exposure).
Another vote for the Valegro (I really prefere the fit of the Protone but it’s less light and airy). I also have a Protone (and long concussion effects) that prove helmet’s efficiency at least in some cases - and some is enough for me to never ride without one.
Shape really does matter! I used to be okay with Bell, but later on they became shorter fore/aft, which seemed okay at first but gave me headaches. Giro are good for my head shape, and it’s useful to know that POC are different - I will avoid them.
Re cooling, IME internal channels do bugger all if you have any hair, but big holes everywhere definitely work (to a degree) on slow climbs. In my case heat buildup is the main factor that limits climbing speed, but YMMV.