All depends on the humidity there. This is what decals and also the carboard boxes don't like the most. However if the decal sheets are packed tightly , eg. wraped in in clear foil or hermetic foil bags, it can protect then against the dampness quite well.
In so far as the the moisture can affect the decals and boxes only , the temperature can have an impact on on the polystyrene parts of the kits. As a result it may cause them bending or getting warped. Additionally, the changing of the temperature from high to low usually works in tha same way like trempering. That means the plastic can be hardened firstly and then be loosing its hardeness. It results in growing brittle in time.
Of course , all that I wrote above may be avoided or limited considerably if the kits are storaged in a dry and quite good ventilated place. Also the airtight containers /fastenings of boxes or bags are very helpful with the long time storage.