[Daox]

I am certainly no expert, but here is what I know. The temperature differential will be low with just cold water, especially if you are avoiding condensation. Therefore you need more surface area to get the same amount of cooling. This is kind of the same thing that the guys are doing with low heat (like GSHP and solar) heated floors. I don't think putting tubing into joist cavities is going to do very much cooling. I think you'd be much better off with a radiator and fan, or maybe a cooled wall for the convection benefits.