Why bother selling argon-filled units, if there is massive gaps in the
roof?

All rooms needs ventilation, or the occupants would get ill and condensation
would get everywhere.

The reason for having good insulation, but still allowing ventilation is
that heat lost through walls/windows is entirely wasted. However, heat lost
through air at least has the benefit that pollutants (i.e. carbon dioxide
and excessive water vapour) can escape, allowing fresh air in.

Even this heat lost can be recovered, using heat recovery ventilation
systems, strangely enough. However, these would be overkill for a
conservatory.

Christian.