Why isn't shed insulation working as I expected?
Sorry, qtron. You really should've mentioned you were using this shed as a workshop or something &amp; 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 &amp; a small air-space between the insulation &amp; 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 &amp; would only be for venting the insulation air-space &amp; 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 &amp; 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 &amp; humidity directly to the outside &amp; bringing in fresh air during the cooling process.