Have used HJC for 3 years!! And have always used motocross style all year for years. Scott, Smith, and my new 509's all seal up with no gaps, always warm, and never to hot.
Biggest thing with motorcross helmets i find is on cold day -15/-20 and colder they dont do much for protection. Frost bit can set in as the googles dont complete cover your face, pretty safe to say this happens with all motorcross setups.