How to install hydronic radiant heat over concrete slab
From Radiant Engineering, thanks for discussing our ThermoFin U product. The preferred method is to install the plates with the fins down. For a ThermoFin U installation on concrete, you would build a surface to be able to screw the plates into by first putting down a layer of Atlas Rboard®, recommended 1/2", which you should be able to get from your local building supplier.
A Google search on Atlas Rboard® will bring up their site and the product info.

On top of the Rboard® you would put down a layer of plywood. The fasteners will hold best if you can use 3/8" plywood. It will work with 1/2" plywood if necessary.

Keep in mind that you want the heat energy to be going into the plywood sleepers and the finish floor, which is why we recommend the Rboard®.

The energy without the layer of insulation will be drawn into the concrete, and the response time would be longer in heating the floor. Heating up the mass of concrete is not necessarily a good thing when you want the entire surface floor to be warmed. The ThermoFin U heat transfer plates are easier to install if the fins are down against the subfloor and plywood sleepers on the fins. See the PDF multipage spec sheet that is linked on the U-fin page of our site. You are making a sandwich of aluminum and plywood to evenly heat the structure and provide a friendly surface for the flooring installation. You can call or email us for more help. We also have routed 180 degree return bends for the PEX tubing when using ThermoFin U.