Kev prompted me to say a bit more about airtightness...
The covers hanging over the walls are to prevent the airtight membrane being punctured.
In conventional homes ventilation comes through opening windows and doors or through the draughts that creep in through floorboards, gaps and airbricks in walls, through loft spaces and so on. And this ventilation is a good and necessary thing because it moves air around the house, ensuring that damp doesn't build up.
It does, however also, quite literally, come at a cost: as the cold air whistles in out goes expensive heat.
Our aim is to create as airtight a home as possible but to combat the heat loss of necessary ventilation by having a mechanical heat recovery and exchange system. This is an exceptionally clever bit of kit, expelling stale air from the home but capturing and returning 94% of the heat back into the house.
Unfortunately, our progress with airtightness has come to a grinding halt because we've run out of the special adhesive tape!! No one's fault but our own. It should be here by next week.