Having broken out the thermal camera, during the recent storms, I was shocked to find some internal walls getting as low as 5'C.

The worst side of the house is a long flat roofed dormer, housing two bedrooms and a bathroom.

I found a hardboard panel inside our bedroom cupboard and removed it to find an impromptu access hatch to the dormer and roof 'triangle' cavity. Some plumber or sparky must have been in there a long time ago.

This North side of the house is the coldest and the wind is whistling through the cavity, suggesting any insulation present is suffering from wind wash.

My first thoughts were to employ some air tightness measures to reduce the wind, but am I better focusing on insulating and adding a membrane to the living area side of the cavity and leaving the roof side free?

Existing loft insulation is poor and will be renewed however while Im in there should I add a membrane across the ceiling, draped over the insulation, to prevent further heat loss?

There is also the cavity under the bedroom floor to block - should I just foam in some PIR cut to size, to block the wind going through?

Insulation wise, would cavity batts be the best choice for the vertical stud walls? Currently some thin, 25mm, glass fibre is resting there (in places)

Just remember you if you put any membrane down, it’s a breather membrane for the cold side and a vapour check membrane for the warm side, as well as don’t forget to tape it also.
The breather membrane on the outside does the hard work of preventing wind wash, whereas the vapour check on the warm side prevents warm humid air crossing into the insulation and condensing. Ideally you need both. Or use foil backed PIR and foam and tape the area you install it. The foil prevents vapour moment, so getting a snug as fit as possible is key.