It's really the air tightness that causes CO2 buildup, rather than insulation. "Insulation" tends to mean materials which reduce thermal conduction - e.g. rockwool, PU foam etc. You can have a very well insulated but also well ventilated space which doesn't have CO2 buildup (obviously at the cost of some thermal performance). Likewise you can have a very airtight but poorly insulated space which does suffer from CO2 buildup (the most extreme example being an inflated plastic bag). The shed may be air tight, but the presence of insulation isn't causing that.
As others have said, the solution is a heat recovery ventilation system. These use counterflow heat exchangers to move almost all the heat energy from the outgoing air to the incoming air - generally they have an efficiency of about 80%.
As others have said, the solution is a heat recovery ventilation system. These use counterflow heat exchangers to move almost all the heat energy from the outgoing air to the incoming air - generally they have an efficiency of about 80%.