Most gown preservation companies preserve wedding gowns in an acid-free box. Many have a window in which to see the gown. Acid-free tissue may be used to buffer the folds and a cardboard shape is often used to fill the bodice area of the gown. However, some companies actually seal the gown inside the box.
The assumption for sealed wedding gowns is that the dress needs to be protected from oxygen. However, unless you are NASA, it is nearly impossible to keep oxygen from getting in the preservation box. Sealed wedding dress preservation poses some risks for your wedding gown: