Construction under windows - how to insulate

looski

We bought an 80s (ish) house a few years ago and have spent the time trying to bring the insulation and general condition up to scratch. Thermal camera shows cold under every window in the place, which has cladding on the outside flush with the bricks (so obviously can't have the second layer of cavity walls there). 

After years of freezing while trying to use my home office I finally crowbarred off the window sill and had a look - there's an absolute gale coming from it now - it looks like the windows were originally floor to ceiling and someone has built a single skin breeze block wall underneath (not straight I might add). But the window doesn't actually sit on this - it floats - the wall is toward the inside of the house so then there is a large gap before the window I can put my hand in. If you feel under the window you can touch the cladding, presumably attached to battens elsewhere. The distance between the wall and the cladding is 5cm one end, 7cm the other, probably two thirds of that is between the wall and the window itself. There's also two small blocks of insulation which makes me laugh, presumably used for spacing rather than actual heat retention.
On the inside of the wall, a plaster board has been dot n dabbed on, leaving a one - three cm gap between the wall around the dots.

My plan/question:
For the outside I was going to remove the cladding, cut to fit a piece of kingspan thick enough to close the gap between window and wall and foam it in, then replace the cladding. Do I need to leave an air gap or use a vapour layer, or both? The unevenness presents another problem, if no airgap is needed I can fill one side with mineral wool (wall, board, wool, cladding) but would need the weight of the cladding to hold it in place.if I try and put the woop between the wall and the board I'm likely to create gaps. When putting everything back, how do I make sure it's waterproof? 

On the inside - it would be lovely to rip off the plasterboard and start again, with an insulation board and screws in place of the large gobs of adhesive. But because it's wonky and I have no idea how I'd true that up, I thought it might be easier to pour in loose polystyrene like a cavity wall insulation (it's sealed either side). I can't easily find the insulation stuff online, can I use the 'bean bag' variety? 

Last question - should I be getting a professional in to do this? I have enthusiasm rather than skill  

FreeBear

Another option is to demolish the <ahem> wall completely and rebuild properly. Not much more work, and you get rid of the wonkiness. As for the original plan - Strip off the cladding and use EPS/XPS insulation. It is waterproof and you shouldn't need any vapour control membranes. Internally, remove the plasterboard along with the adhesive dabs. Fix some battens to the wall horizontally (25x50mm will be fine) and use some scraps of wood to pack them out so that the wall is straight. Infill between the battens with 25mm Celotex/Kingspan and then cover the whole lot with a second layer taping the joints with aluminium tape as you go. Then fix vertical battens using 75mm screws in to the first set of battens you attached to the wall (you did make a note of where they were didn't you....). Infill with a third layer of Celotex/Kingspan, tape, and then cover the lot with plasterboard. Put 25mm or even 50mm of Celotex/Kingspan across the window sill before putting the board back, and that will kill the cold spot in that area.

If this window is flush with the rest of the house (i.e. not a bay), then you should consider insulating the entire wall - Material costs won't be much higher, although you will generate more waste and the job will take a little longer. but you will end up with a much warmer wall and cut your heating costs a little.

Tip - Use a strip of 12mm plywood over the top of the window, say 150mm high and extending 150mm either side. This will give you a solid surface to attach curtain poles to.

stuart45

Have you got a photo of the exterior?