Heating a well-ventilated building reduces the interior relative humidity
(RH), because the saturation water vapour pressure increases with temperature
but the
actual partial vapour pressure remains the same as that outside.
Since RH is defined as
the actual partial vapour pressure divided by
the saturation vapour pressure, its value
falls as the inside temperature rises.
The name 'conservation heating' arises from the assumption that artifacts
need a constant
RH rather than a constant temperature, in contrast to
humans, who need a temperature
within a narrow range but are
indifferent to RH variation, as long as they are not hot or
exerting themselves.
Conservation heating warms a building to achieve a constant RH,
regardless of human comfort.
In rare instances, in a desert where the daily average RH can be as low as
20%, conservation heating may actually require cooling.