what is the special material to prevent heat from being lost??

Question

for example, "we've had the hot-water tank LAGGED"

Answer 1

There are many different types of pipe insulation on the market, such as styrofoam, fiberglass, mineral wool and formed perlite.
Lagging is the term used for placing a protective cover over pipe insulation, sometimes it is metallic lagging, such as aluminum, and sometimes it is plastic.

Answer 2

"...f you lag your hot water pipes for the whole of their length you should be able to turn the temperature of the hot water down a little, there will also be less of a wait for hot water comes out of the tap once it has been turned on. There are two areas you should consider insulating, between the boiler and the hot water cylinder and at least the 1st metre of hot water pipe going from the cylinder to the taps. The insulation should cost in the region of £12-£15 and will save between £7 to £15 per year depending on your usage of hot water. The easiest type of pipe insulation to fit comes in foam tubes that are pre-split along it's length and is about 20mm thick, and to insulate the pipes in your airing cupboard should take no more than 15-20 minutes...."