I insulated my 3×3.5 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).
Best Answer
Sorry, qtron. You really should've mentioned you were using this shed as a workshop or something & trying to cool it. I realize you might say "why would I have insulated it otherwise". We have no way of knowing unless you say, we see many weird actions taken by people. Cooling changes everything!
In that case, you have the foil of the adequate insulation facing the wrong way, it's currently retaining any heat that literally seeps inside. You do want it facing the metal to literally reflect heat back out as soon as it tries to enter, that's actually the specific purpose of the shiny side.
You do still want some minor venting & a small air-space between the insulation & the shed walls (just a few mm's), to let any heat out as quickly as possible. Venting could be a series of small holes drilled directly under any roof edge or lip that you might have & would only be for venting the insulation air-space & not the entire shed's volume. You may also have a roof ridge cap where the same could be done. If these areas or opportunities don't exist then you'll need to attach a roof vent box & wall grills.
As well, do definitely fill or cover that door gap, even with just a sweep type strip will make a big difference. Please explain the cooling unit if it's not a rolling portable or standard window type unit that's dumping heat & humidity directly to the outside & bringing in fresh air during the cooling process.