Both Kaga and Akagi were the IJN's guinea pigs in their attempts to figure out an exhaust system for carriers (since, well, the usual 'chimney in ther middle' design would be in the way of the flight deck). Both were given different exhaust designs to test out under real-world conditions. Akagi's worked, Kaga's 'revolutionary' design... not so much. Especially since some of the crew quarters were near the funnels (angled smokestacks), and are pretty much inhabitable during flight operations.