MarkMc wrote:

1. Can anybody tell me the name of the solid insulation which is a
solid foam, backed on both sides with foil?

Polyisocyanurate (PIR) or urethane foam typically.

The most well known brand names being Kingspan, Celotex, Ecotherm etc.

2. I'm about to start building a downstairs WC in the under the stairs
cupboard (has plenty of standing room) of our house, which is
victorian. I want to use this insulation on the inside of an outside
wall as it's very cold under there. I was thinking of say 50mm (2")
studs on the wall, with the solid insulation fitted between, and then
plasterboarded over the top. Seems like a good place to conceal the
copper pipe from the hot water tank almost directly above. Does this
sound sensible, or will I be causing myself damp problems, building
code issues etc?

Should be fine.

You could use two layers of 25mm, and that way the second layer could
cover the studs as well (i.e. make the studs only 1" deep). That will
stop there being any noticeable cold bridging from the wood.

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/