Insulation under concrete

[Shox Dr]

Don't use that (expanded polystyrene) whatever you do, it's worse than useless for your application. It has very little if no compressive strength and poor insulation values. Use the Kingspan extruded polystyrene that clift-d linked to early in this thread. Go to a specialist insulation stockist and buy from them, not Wickes.

You dont sell kingspan do you? What do you think we've been using for the last 40+yrs. Beides keeping buildings insulated, other uses have been pontoons for boats, and used as sub-base rafting over marshland. And thermal quality is just as good as KP

 

[Burdekin]

Heat only moves from hot to cold so a big cold concrete floor will soak up a lot of heat. Insulate it and keep it warm then you have a nice thermal mass in winter.

 

[mangocrazy]

You dont sell kingspan do you? What do you think we've been using for the last 40+yrs. Beides keeping buildings insulated, other uses have been pontoons for boats, and used as sub-base rafting over marshland. And thermal quality is just as good as KP

No, I don't sell Kingspan. I'm just a user of various forms of insulation and I don't understand how anyone can tout the advantages of expanded polystyrene over extruded. Extruded beats expanded hands down in every metric and every application, with the exceprtion of cost. Expanded polysyrene is cheap. And for a reason. I couldn't care less what make of insulation anyone uses, but I couldn't hand on heart recommend expanded polystyrene for the purpose that the OP intends to use it for. Extruded polystyrene of a suitable compressive strength for the intended application, from the manufacturer of your choice, is so obviously the correct material to use it's a no-brainer.