One type of building envelope filter with great aesthetic appeal is the ‘half door’ or ‘Dutch door’: an external hinged door with independently openable upper and lower halves. The two halves may be completely separate leaves, with each leaf hinged directly to the door jamb; or the upper leaf may be a ‘door within a door’ hinged to the stile of the main door, a type most common in Ireland.

In terms of its filtering functions, the half door can work either as a normal door, or, when the top half is left open, as a gate, allowing light and air to pass while keeping farmyard animals from entering the house, keeping children from wandering outside, and preventing dust and debris from blowing in. The half door also acts as a social filter at the threshold of public and private: it allows comfortable, direct social exchange with a degree of separation, be it with salesmen or other strangers, or even with friends, when the visit is of the ‘I won’t come in…’ variety. In this role it is analogous to the Japanese genkan, where the social ‘gate’ is the change in floor level.

Half doors were never common in Australia, perhaps because the screen door performs much the same function, with the added (and absolutely essential in rural areas) role of keeping out flies.