This might not be a concern to most bike riders but if you live in a hot sunny place with high UV and skin cancer it can matter,
If you also find you are one of the 1% of the worlds people also subject to a helmet law this might be an interesting project to consider.
Horse riders often add a brim to a helmet but the problem with the brims available is they dont handle the wind too well. The idea is to find a compromise with material that is stiff enough but wont affect the helmets function.
So far the best is a closed cell foam used as underfoot padding or for a yoga mat.
Step 1: Material.
So far this seems to work the best though I have seen something used that comes from a suitcase and looks even better but is harder to find. Only about 10 mm thick so looks alright.
Step 2: Cutting.
This is the helmet for this exercise though I have tried this on different types also.
Mark out the outside brim that feels appropriate and draw a line around the helmet.
Most importantly, cut slightly inside the line drawn around the helmet. 3mm or 1/8th".
Step 3: Glueing.
Spray glue and clear kwik grip will not harm the foam of your helmet whereas most glue will.
But if glueing straight to the helmet foam exterior PVA will work well also and give plenty of time to adjust.
Best to test the glue on an old helmet first.
Step 4: Shaping Brim.
This gives a bit of character to the brim.
Simply use a heat gun thats a bit hotter than a hair dryer. I borrowed mine but was assured that are really cheap to buy.
Step 5: Finish.
Well, that's it!
If anyone's interested I might post a few other images of other attempts to add a brim to a bike helmet that look ok.