My experiences of keeping piggies in sheds is that once it gets much above 20 degrees, sheds can very quickly become dangerous. They are usually a good 5-10 degrees hotter than the outside air temperature so having a thermometer inside so you can keep an eye on things, plus a spare cage to bring them out is essential.
It is 20 degrees outside here today but not sunny and even today my shed thermometer is showing 25 degrees, so my piggies have had to be removed from the shed and are on the lawn for today. A 20 degree but sunny day will make my shed a dangerous 30 degrees within an hour easily.
They’ve spent all of the last week (up until today) living in the house and not able to go into the shed at all due to the heat (the shed wasn’t even really cooling overnight). They’ve moved back out to the shed now but within an hour the shed was too hot again so are now on the lawn for the day.