On 12 Nov 2008 07:06:51 -0500, wrote:

wrote:

... The cold gravel floor causes condensation when warm damp air contacts
it, the moisture drops through the stone to the vapour barier, and cannot
get away. Earth warms up a bit - becomes warmer than the air above,
moisture leaves and condenses on the cold car above.

This could also happen with concrete, which absorbs about 1% moisture
by weight as the room RH rises from 40 to 60% and gives it back
as the room RH falls.

Nick
But only 1%, not 15 or more.