Kieth,
I agree, media grow beds transfer a lot of heat/cool to the surrounding air, especially in flood and drain systems where the water draining pulls in the surrounding air down into the media. I am planning a DWR system, the troughs will be insulated on bottom an sides and a 2" raft on top will actually provide insulation for all 4 sides of the grow area.
Sorry, I may have used the wrong terminology and miss led someone into thinking I was planning a media bed system.