Under slab insulation helps during heating months but is a detriment during cooling months. I've lived with a copper radiant system without under slab insulation for 30 plus years and have been very happy with the system. Mid Michigan. New systems use plastic instead of copper. Systems using plastic seem to require AC. Will plastic systems work without under slab insul?

No insulation under slab is a gargantuan net energy waste, regardless of what your radiant pipes are made out of. Radiant systems have used PEX (plastic) for decades now. Cooling your home via an uninsulated slab has limited utility.

ICF has it right. The slab doesn't care if you use PEX or copper heat or cool. Insulation is frankly a no-brainer in most residential slab-on-ground radiant systems as long as you don't confuse the various blankets and bubble-foil for accepted rigid insulation such as EPS and XPS.

Cooling in most homes means a significant amount of latent energy is taken out of the air i.e. de-humidification. A cold slab does nothing of humidity unless you like wet feet. Were you in a hot climate we may consider variations.

Yes, ICF has it right...but no surprise there...

However, I am surprised and happy that you got 30 years using copper pipe. A chemical reaction between metal pipe and concrete often leads to corrosion and failure. Very likely your installer knew what he was doing and put a protective coating on your metal pipe.

Posted By bert seyfarth on 02 Mar 2014 10:12 AM
Under slab insulation helps during heating months but is a detriment during cooling months. I've lived with a copper radiant system without under slab insulation for 30 plus years and have been very happy with the system. Mid Michigan. New systems use plastic instead of copper. Systems using plastic seem to require AC. Will plastic systems work without under slab insul?

The amount of "free" sensible cooling you get from an uninsulated slab is pretty limited, and by letting the slab run at the soil temps you end up with a higher potential for mold growth in the basement during the summer in a mid Michigan location due to the fact that your outdoor summertime outdoor air dew points  are well above the deep subsoil temps, and the likely temperature of an uninsulated slab.  Any air infiltration/ventilation into the basement ADDS moisture during the summer, and the cool air films at the slab are at or near saturation. 

Mold-susceptible materials resting on the slab suffer the inevitable, unless you air condition or mechancally dehumidify the basement. In my case the uninsulated slab costs me on the order of 400-500kwh/year in dehumidification energy use, and in some years it's been as high as 700kwh. My summertime outdoor humidity isn't dramatically higher than MI, though a fraction of the humidity load is from ground moisture, since there isn't a full vapor barrier. Set to control the basement to 60% RH, the humidifier's power use still tracks the weekly average latent load, and goes to near-zero when outdoor dew points are under 55F.