The so called thermal looping effect in cavity walls is a common problem, wich lowers the insulation performance of the complete wall structure.
As a short explanation: cold air infiltrates the cavity in the winter from the outside and circulates inside of the cavity. As a direct result, the insulation performance is worse than calculated on the paper.
KORE Insulation is explaining the thermal looping effect much better in one of their previous blog posts. The only thing, I can´t agree, is their suggested solution.
Coming from my experience, there´s is the same thermal looping effect in cavities, "fully filled" with insulation beads. I had plenty of buildings so far, where I noticed a draught coming out of the cavity through the reveals of the windows. The cavities have been filled with beads already and were blocked by using PIR board as cavity closers.
As soon as the gap between the window frame and the inner leaf is air tight sealed in an appropriate way, you blocked the access of the cold stream to the room. But she is still there and will circulate in the cavity further.
That´s why I would prefer the rigid cavity wall insulation, bonded to the inner leaf without any gap between the insulation board and the wall, or - even better - to consider another wall structure instead of the cavity wall.