The window sits on the cill so it's not connected as such. Because I wanted my windows as far back as possible the cill needed to bridge my cavity. The white board (orange arrow) was some offcuts of plastic soffit, it is just a formwork to hold the concrete until it went off but I chose that as it was waterproof and will never rot, black tape was just duct tape to seal all the edges.    One bit you may not be able to see is I fitted some draught excluder on the back of the mould t