Floor to ceiling window, closing cavity (at floor level)

Thanks, all these steps makes sense now overall. Ensuring airtightness did come to mind since the PIR will be butted to the cill and while the cill itself should be airtight but air could have a route up from the plywood but I thought that using airtightness tape between the cill and plywood and blocks and plywood at the sides should hopefully solve that.

I need to run all this past my architect anyway before I start work on it so will see what his overall feeling on it is.

Are you referring to the airtightness for the ground floor under the hollowcore? The hollowcore has solid ends on all section connecting to an outside wall and the top block has been plastered up to meet the hollowcore above the metal ceiling to address airtightness here. Where the hollow core sits on an RSJ (so not plastered) I have sealed around it with airtight tec7 sealant.

I also filled the majority of these with the airtight sealant, one or two bigger ones with expanding foam (which I will probably pair back and add sealant or airtight paint over when I have a chance).

I might add this was not something even suggested as being needed to be done I actually saw this as an issue myself. Plastering the top block was the only thing called out to be done.

I think the grouting of the hollowcore did a lot of the filling of these gaps which I guess is why no specific mention was made on filling it separately - I only needed a small bit of sealant in most places.