Thanks for all the info,

The drainage problem was a blocked storm water drain caused when whoever
put up the fence drove a post through it and then set it with concrete.
This allowed the roof run-off to overflow and enter the crawl space.
Judging by the age of the fence it must have been at least 20 years ago.
The previous owners seemed to go to considerable lengths to control the
resulting rising damp (its an old house with inadequate or no damp
course) but never seemed to attend to the drainage problem.

The average afternoon relative humidity here is about 55% (SE
Australia). Closing the crawl space really goes against the grain as
everything I've come across regarding rising damp or rot suggests
increasing ventilation but I guess if the outside air is carrying more
moisture than the crawl space air it makes sense! In any case I will
never be able to get access to all of the crawl space (without ripping
up all the floors) to lay poly effectively so I'm thinking of relying on
low voltage sub-floor ventilation fans that turn on and off depending on
the atmospheric conditions eg. turn on if outside air is drier than the
crawl space air.... or perhaps let natural ventilation take it course
and move on to the next project..

thanks again everyone!



Jay Stootzmann wrote:
Check out
http://www.advancedenergy.org/buildi.../crawl_spaces/

for more info on crawl spaces

wrote in message
oups.com...

You're correct to be concerned about the difference in level between
inside and outside.
You don't say what "the drainage problem" was.
Certainly, as Rick says, it's a good idea to leave the working space
below the floor.
As Bob notes, the problem is to keep water out of the space.
As long as you think you can keep the crawl space dry, you are o.k.
In addition to directing roof run off well away from the house, sloping
the yard surface away from the house, sodding for improved run off, and
making sure of the water table height, you might consider directing
some conditioned air into the crawl space. This can be done with
passive registers or a fan.
In the South East, outside air is often too humid to be useful in
lowering the relative humidity in crawl spaces.
TB