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I suspect that thin "tin" walls cool quickly after the sun goes off them because of two factors. The first and most obvious, is because they don't have much thermal mass. The second factor has to do with the sky temperature and radiation cooling.
I bought an IR temperature "gun" from Bunnings when on special for $50-something recently. It's been invaluable in the observatory (still in late construction phase) to measure the temperature of the ground, the concrete floor, the concrete and steel pier, the dome and the "tin" walls. I've also pointed it at the sky numerous times, and have been fascinated by how sensitive the reading is to just whereabouts in the sky I point it. Even small patches of clouds change the reading significantly. During the day, the sky temp is quite positive, but as the sun sets, it plummets, especially if there are no clouds, in which case it has gone as low as -33 degrees C. (on a clear night late last year). Tonight, after a stinking hot day of 45 deg C, the sky temp is -7 deg C, and the ground temp is +24.
I wasn't originally going to insulate and line the observatory walls, but now have. I figure that if I can stop it heating up during the day (with it closed up), there won't be so much cooling to be done after it gets dark. Those parts that are insulated (it's a two-storey job) would, arguably, take longer to cool, but they shouldn't give off their heat very quickly. I plan on having a good flow-through system to aid cooling.
I think that your tin shed interior would cool even quicker if you could circulate the internal air to make use of the tin as a radiator.
Cheers,
Chris
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