r.p.mcmurphy wrote:
need to get some windows made. joiner asked how thick a d/g unit i want to
fit. i want it to be as thin as possible, that the bco will allow.

anyone help?

You either need to use a FENSA (FENSTRA?) registered double glazing
firm, or else you or someone else needs to satisfy the BCO that the
neccessary U values will be achieved.

You can't decide what kind of spec window /glazing unit to have until
you know what U value you need to achieve, and this will involve
calculations.