[MattSAU2XR8]

Sounds like you are thinking of making a container residence?

I think for the situations described the result would be largely similar whether its heat gain or heat loss, if the doors were wide open, certainly less than 5 mins to equalise either way

Although the interior walls and other fittings would take longer to warm up or cool down providing a sort of buffer or 'heat sink' if you closed the doors again the container would probably swing back several degrees toward the desired 20 as the walls took up or shed some heat.

If you were using a container for a home then I'd suggest:
- If it will be in a hot climate then paint it gloss white, at least on the north side since the heat load from direct sunlight can be up to 1000 watts per metre squared, way more than the aircon will be able to remove
- And line and insulate it
- And ideally install a simple frame with some steel roofing perhaps a foot above the container to prevent direct sunlight hitting it, or water pooling on the top
- And get some stumps or blocks poured so its up off the ground a bit, won't help with heating or cooling but will stop it rusting away and dirt blowing in and give drainage for shower and toilet and so on

The other thing that could be worth looking at, on the off change that its available, would be an old refrigerated container or truck body - much more suited to a dwelling...