We've got a 300mm cavity wall being built on our extension (cavity
contains 50mm air and 50mm Ecotherm). We want to install a UPVC sliding
sash window (probably quickslide) in a checked reveal to match the
existing house, which also has cast concrete sills like the ones in this
pictu

http://i704.photobucket.com/albums/w...ndowSill-2.jpg

Does anyone have any idea for how the detail where the sill and the
window meet works? I presume with a checked reveal that the sill needs
to be somewhat deeper than normal so that the bottom of the window sits
upon it, but we are worried about damp proofing, cavity closing and
insulation, I can't find any information on the web about it, and our
architect has neglected this corner for some reason.