Thanks for your question. The low air exchange is achieved through attention to details both at the design stage and by the trades on site. This includes for example considering the practicalities of window fitting long before windows are purchased, allowing space between the building structure and the window frame to allow the fitter to get his hands in. Having a bigger space may initially seem counterproductive but this means that insulation can be fitted and the gap filled properly rather then squeezing something in a confined space. Hope this answers your questions let us know if you want any specifics.