Lordy. We take in the outdoor air and add more vapour from breathing,
cooking and washing. Do you think that dries the air?

You are forgetting that outdoor air is normally cooler than indoor. When
it enters the house, it is heated and, what was a high RH outside,
quickly becomes a low RH inside. Buildings with modern standards of
heating and insulation will invariably be too dry in winter. Ask anyone
who owns a piano. The RH in my house is between 40% and 60%. Lower than
that you get static problems, and higher than that you can get thrush
and similar ailments.

Whether a home stays dry is more to do with its design &
construction. Only a minority of buildings require significant care
over interior activities.

On that we agree! The only insurmountable problems I have encountered
were in a 60s concrete north facing flat that was in constant shade and
should not have been built in the first place. Low RH is a major problem
in offices, despite the occupants breathing in and out all day. It's
only a slightly lesser problem in modern houses where kitchens and
bathrooms raise moisture levels, and it probably isn't a problem at all
if you live in a listed building on Dartmoor.

You continue to adopt this high handed and patronising attitude to
anyone that disagrees with you, which I don't find at all
constructive

Well, you demonstrate a determined failure to grasp the basics, you
can't be bothered to read up on the subject, yet you insist your
confusion is right. Lets just agree to disagree, its easier.

The lack of comments from anyone else suggests that is probably a good
idea. Actually it's QI that there is no interest from a group that will
argue the toss about virtually anything :-)