What is the material used in this GossamerGear equipment?

3 common insulation choices would be reflectix, insulbrite or 2 or 3mm cc foam. Weight wise 2mm foam is the lightest with insulbrite (+80%) and reflectix (+120%).  While these weight differences look high you are only talking 15g odd for a cozy. You also need to factor in that insulbrite in particular and I expect the foam needs to be in a fabric case such as the GG Warm Sack you mention.

Ta. Any idea as to order of efficacy?

I am building a stuffsack for my pot etc similar to the GG Warm Sack – I didn’t know they existed. I did a rather crude experiment using the three materials without fabric backing as pot lids. They all worked identically with temperature dropping 11 degrees C over 10 minutes. (Variation +- 0.1 degrees.  By contrast no lid gave a drop of 24 deg C. I am not happy with the validity of these observations and feel I must do the same thing but with the pot fully insulated with each material to get a proper feel of how the insulations work as I am sure the great majority of the heat dissipated through the uninsulated sides and base of the pot.

I just ordered the 1mm thick neoprene. Just to explain. I have the 2mm version and it does a superb job at keeping my Foster’s Pot can contents warm for the duration of me wanting to enjoy a coffee or soup. I wanted to experiment with the 1mm stuff so will sew a potcozy out of that as a mate of mine has a 1mm neoprene cozy that came with his anodized alu cup and he seems to be very happy with it.

Ta for the info. I had an inkling that the neoprene stuff was going to be the lighter option so thanks for confirming that :-)