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Hi @SuJay
You might like to start by having a read through some of the previous discussions on this topic. Our members have contributed a remarkable number of solutions and plenty of information to help you decide on the best way forward.
If your goal is to bring the shed closer to ambient outdoor temperature, ventilation is actually the most important part of the puzzle. Insulation helps slow the transfer of heat, but without somewhere for the heat to escape, it will still build up inside a metal shed. Since a whirlybird is not workable with the eucalypt overhead, wall-mounted passive vents are the next best option. Adding a pair of high-mounted vents on opposite walls will let hot air move out continually rather than being trapped under the roof.
Sheds are not constructed to the same standard as a home, so achieving a genuinely cool interior is very difficult. The Colorbond sheets sit in full sun and heat up all day, and that heat keeps transferring inward. You can reduce how much of it comes through, but you cannot eliminate it completely. This is where insulation boards come in. Foilboard-style rigid panels are one of the simpler options for a retrofit. You can adhere them to the walls and roof frame, then seal the joins with the appropriate high-temperature tape. That creates a reflective and thermal barrier which slows the inward heat flow. If you only install insulation without venting, the temperature inside will still creep beyond ambient because the trapped hot air has nowhere to go. Combining insulation with passive wall vents is what gives you the best chance of a workable space in summer, even if it will never be truly cool.
For the sliding doors, rigid panels again tend to be the easiest to work with. They can be cut to fit within the door frames and adhered with a suitable gutter silicone, then taped around the edges. Once you address the doors and add some form of venting through the shed, you should notice a meaningful drop in peak temperatures, enough to make short work sessions much more comfortable.
Please let me know if you have any questions.
Mitchell
A note to our New Zealand readers, the use of foil insulation products is prohibited.
It's great to hear you've had some success already @SuJay. Air gaps help stop heat transfer. In the case of Foilboard, yes, you add small blocks of the foam sheet, or the spacers, to ensure there is a gap between the boards and the shed wall/door/roof. Foilboard requires these gaps, but you might find some other types of insulation, such as batts, that don't stipulate this gap in their installation instructions.
Mitchell
