Back in February we took note of the Agency 360 designed Overade folding helmet, which reportedly went into production. But for those looking for a compact helmet we recently came across the Carrera foldable helmet, which can reduce size by folding up like an accordion.
The helmet has no solid structure going across the front or back so it should provide good air circulation. We were worried that it might not do all it could to protect the head, but it apparently meets the EN 1078 safety certification requirements so that is good enough for us! It should enter production later this year and be ready to unfold when you need it!