Installed on the cold side of the insulation -The inside face of the Breather Membrane contains pores or holes which allow water vapour and other gas molecules to pass through from inside to the outside of the building.
Because these gas molecules are incredibly small (less than 3 millionths of 1mm) the membrane can appear solid to the naked eye but will still allow large quantities of water vapour to pass through.
The outside of the membrane stops liquid water ingress (rain/snow) from the outside of the building to the inside, serving as an extra layer of protection within the roof or wall when the cladding material itself may not be watertight (e.g. tiles)