Rick Dipper wrote:

I have worked out the U values of the insulation I intend to use in my
cavity walls, I have 2 solutions

1) Blocks -celotex-rockwall = 0.224 W/m2K
2) Blocks -2 * rockwall = 0.255 W/m2K

I did not take account of the external wall, plaster, mortar or wall
ties in working out these figures. The external walls are 600mm stone
walls.

Normally I would go for option 1, but its arround twice the cost of
option 2, a whole lot more difficult to install (I am building the
inner wall second) and there is not a big difference in the insulation
values.

Any ideas on how I work out if its worth it ?

Is it as simple as working out the temperature difference, the surface
area of the wall, to work out the loss via the wall, and then mutiply
by the cost of a watt of heat ?

My thoughts right now are to save the money, and use it to put extra
into the roofspace.

Thanks
Rick


I went with 2x rockwool purely on cost. It was good enough.

Despite IMM's blatherings, once you meet building regs U values, you are
already into the law of diminshing returns as unless you put in heat
recovery ventilation, and have no windows, the majority of the heat loss
will be via the windows and ventilation - which you need to avoid steamy
damp atmospheres in winter anyway.