One of our bathroom toilet tanks 'sweats' badly at this time of year -
the outside of the tank is covered in condensation and it puddles on the
floor beneath. I was thinking of retro-fitting some insulation to the
inside of the tank (I don't want to mess with adding a warm-water feed,
and the tank's a nice old decorative one, so I'd rather not replace it
with a modern one that has a double wall or built-in insulation)

Questions:

1) how thick does the insulation need to be? Is 1/4" probably enough?
(that's perhaps something of a "how long is a piece of string" question;
I think I'm dealing with a max temperature differential between water and
air of 35 degrees)

2) What adhesive to use? Needs to be water-resistant, obviously, but also
something that's suitable for styrofoam and won't destroy it...

(alternately, I could just buy a kit, I suppose - they seem to be about
$30 online. Not sure if HD etc. carry them...)

cheers

Jules