The cold wasn't really a problem. Hiccup and Toothless were both used to it, and Hiccup's suit was fairly well insulated. The lack of air, however, was an issue. It was only magnified by the fact that Hiccup was straining to get the bulkhead doors to move. The only reward for his efforts was an audible grinding noise, and no movement.

"They're not gonna budge. We," Hiccup tried to take a breath, getting less air than he really needed, "need to *gasp* find another way out."