This topic makes me squirm a little.  I am in England in a house built
in 1900 with solid 9-inch brick walls (no cavity gap).

We have been here 10 years and have no air-conditioning which is usual
in almost all private homes here in UK (and cars).  When the windows
are closed during the winter months I run two de-humidifying machines
24 hours a day to suck the moisture we breath out from the air.

My meter always shows 65 to 85 percent humidity, all through the year
and I don't have any problem with my pianos or Aeolian organ.  I would
_never_ think of adding more water to the atmosphere here in case it
actually rains indoors!

I have certainly noticed original rolls in varying condition depending
on which of the USA states they have spent most of their life in.
Talking just about Aeolian 58-note rolls, some are brittle, some are
quite supple and some are really dis-coloured at the start from
atmospheric pollution.

Humidity is very different here in UK.

Kevin McElhone