Old Computers and Moisture don't mix fairly OT 4

'Tony Epton' wrote, in part: I have moisture coming thru the cement floor right next to a row of lovely DEC cabinets.

If I knew something about the climate in Mundaring, West Australia, I might could offer more useful advice.

It seems to me that the ventilation and heating you describe is entirely inadequate.

I can give an example of humidity control for a middle Georgia climate zone.

Middle Georgia, USA - 50 inches rain per year, rain spread evenly through the year, clay soil (relatively poor drainage), average winter daytime high 38 F, nightime low 30 F; average summer daytime high 85 F, nighttime low 75 F

If the top of your slab is above grade by a few inches (the construction permit likely had that requirement since the property is zoned, also, building codes should require a membrane vapor barrier where necessary) then sealing the top surface of the concrete will help. DO NOT place a plastic sheet over the slab. Moisture will just collect there, and make controlling the humidity of the air in the structure much more difficult.

You need a very large amount of circulating air to control the humidity, and a way to extract moisture from the air.

A suitable dehumidifier (capable of removing 65 pints of water per day) might cost $200 US, and, depending on the rainfall and humidity in Mundaring you might need two. Dehumidifiers are a sort of air conditioner, but use much less power that units designed primarily for cooling. Units of this size can be plumbed to a simple gravity drain to the outside.

My brother has built a metal-woodworking shop of about 200 square meters size on the family farm; concrete block on concrete slab, standing seam metal peaked roof (insulation on the inside of the metal roof). He has a 65 pint per day dehumidifer that on ordinary days keeps the relative humidity to 60 %. At the moment, as hurrican Dennis pbuttes several hundred miles away, and rain has be falling at several inches per day, the single unit is struggling to keep the humidity to 80 %. In addition to the large iron machines in the shop he has a couple of PC compatible desktops and a ~ 40" HP plotter. In on end wall, in the peak, he has a powered louver, in the other a ~ 40" 1 HP fan on a thermostat. In the room where the computer equipment there is an overhead paddle fan (52", 60 to 20 rpm.) The combination keeps rust from forming, keeps his wood from warping, and maintains an adequate comfort level (winter requires quite a large propane fired forced air hanging heater.)

The other climate zone is Southern Florida (Miami-Dade County); the soil there drains VERY well, the temperature range is small (except for a few days per year, between 50 F and 80 F), the Atlantic Ocean and-or Everglades are within 10 miles, and the water table is only six feet or so below the surface. Some air conditioning is pretty much necessary year around, despite what the chamber of commerce would have you believe. Oddly enough, relative humidity tends to be higher in Middle Georgia than in Miami-Dade County.

Cooler air holds less water. Better soil drainage reduces diffusion of water vapor through the slab.

Perhaps you could find a HVAC engineer that could donate time to design a solution for you.