Not Too Hot, Not Too Cold...
One misconception is that temperature is a critical consideration with the preservation of artifacts. This is not necessarily true. Seasonal museums regularly allow their collections to dip to relatively low temperatures in the winter or rise to high temperatures in the summer. Many artifacts will tolerate extreme cold (-30C) and low winter temperatures indoors can reduce such problems as chemical self-destruction, pests, mould, and energy consumption. Extreme low temperatures, however, are a problem for plastics and paints as these materials may become brittle at low temperatures. The biggest danger with high temperature is that the humidity commonly drops below 40% and drying occurs. Often, drying of the helmets results in overall shrinkage making them seem extremely small today. But the most critical aspect of dealing with temperature is to prevent constant fluctuations. Fluctuation in temperature causes most materials to expand or contract. As discussed in the section on humidity, the lacquered finish on some artifacts may crack on the surface which results in "crazing". Preventing rapid fluctuations and maintaining a comfortable temperature will help to prevent this effect.
Lacquered surface on a Pickelhaube that has been stored correctly over the years. Very few lacquered helmets from this era have been this fortunate.