I originally tried cork board and then exhaust wrap but over time but the base of the y carriage would still get quite hot from the heated bed. I found a supplier on ebay from the UK which sells an aerogel based product called spaceloft which has an r factor of 10x (I think) that of cork, Fortunately they sell 200mm square samples which coincidentally fits under most standard heat beds! After using it for a few hours the base only gets a warm whereas with cork it would be quite hot.
As the insulation is fibre based it IS an irritant so I wrapped it in kapton as well. Hopes this helps!
