I have a Foam Helmet, made for climbing. Its kinda old and looks like it needs a wash. The Helmet is old, and so is the foam. Roughly a bit more than 3 years now.
With other foam-made things that are a bit old, I have a personal experience that when I washed them, the foam got screwed, like lost its shape and cushion.
How to effectively wash a foam-helmet and not kill the cushion?
P.S. depending upon the solution provided, I may consider using the same technique to wash my biking helmet.
In all likelihood, you just need to replace the helmet. Nearly all helmets, climbing and bicycle helmets included, need to be replaced once they become worn or older than 5 years. Ideally, you would track the age of each helmet. If you don't know how old one is and it looks well used, I wouldn't risk it.
Stick to gently wiping it with a soft dry cloth. Trying to clean a helmet with water or most any kind of soap only risks damaging the foam. If a helmet is so worn that a dry cloth doesn't restore it, it might be too worn. Consider how important your head is before try to cover it with compromised foam ( or compromised plastic, etc ).
I will not address the issue of whether the helmet in question is still usable or should be retired but instead answer the main question: How to wash a foam helmet?
Generally water must not damage any kind of helmet you want to use for climbing, as it has to be usable in long constant rain. So rinsing with water without additives is certainly save. The manufacturer of my helmet (Kong Scarab) advices to use warm (<40degC) water with a mild detergent to clean the helmet and even describe a procedure to disinfect it: Put the helmet for an hour in a water bath with sal ammoniac in it. Not that I can ever imagine me having the need to disinfect my helmet, but it clearly states that it does not hurt the helmet.
In short: In general a wet cloth should be enough, but you can rinse with or even submerge in water if it is very dirty. If this screws up the foam, it had to be replaced anyway.
