>>60Lost-foam casting is feasible with aluminum but you'll need to temper your expectations. While aluminum is hot enough to melt and ignite foam it is still relatively on the cool side, and doesn't quite have a high enough specific heat to ensure every little bit is burned off. To get a good cast you'll need to use a lighter foam.
The spray on foam insulation you're thinking of is a very poor choice for casting, it’ll be hard to get an accurate model due to the large cells, melts at high(er) temps, and polyurethane foam produces particularly noxious gases when burnt.
For casting purposes you need to go with a polystyrene foam and sculpt your part from there.
In my experience lost foam aluminum casts are really tricky to get right. There will almost certainly be pitting on the surface and with a part as large as you're making you will have issue getting the metal to fill out of your mold. Make sure to have a large head of aluminum up top to keep that from happening.
Given that, I recommend you start out smaller and try a practice pour before you even start making the helmet. This video
http://youtu.be/dNNdf0vlrWo gives a decent overview of the process and will get you on the right track. Once you feel competent enough to do the helmet make sure to prepare two for if/when the first one inevitably goofs up.