I insulated my 3x3.5 m metal shed with 55mm thick foil lined "space blanket". Foil inwards, of course, is, not facing the metal. The metal walls & roof was initially & thoroughly lined with Ametalin moisture barrier first.(4mm thick).
There is a gap under the only door allowing air in. There are no ceiling vents (yet).
On a nice cool ish sunny 22 degree day, the shed rises to 27 degrees. Previous night was 15 C. Shed was 17C at 11pm, the night before. This was disappointing. Why the poor thermal "lag"?
Floor is concrete with 19mm thick chipboard on top, so no heat source there! So, I don't understand why with meticulous insulation, the shed temp rose 5 degrees above the ambient air temp, especially on a day that was only mildly warm.
In contrast the house remained 2 C below the ambient air temp. (I am unsure how else I can clarify this question).