In the void between the ground floor ceiling and the first floor floor boards many problems exist. Basically air or wind from outside can blow through small gaps and cracks to the sides and above floor joists, these result from the fact that the joists shrinks by up to 10mm in height and 4mm in width. The blockwork is not always fully filled with mortar, there is often none between the good end of a block and a joist, some missing from perp-end joints and gaps around service penetrations. It is easier to see this from the following set of pictures with descriptions.
1980’s house near Eastbourne, how much energy has this wedge cost? and how many saw it and ignored it during construction?
For the the very technically minded among us we will realise that air and wind can still enter the void between the plasterboard and the blockwork, sadly it can exit through socket outlets, go round corners and get behind internal wall linings, we have even seen it come out over internal door heads 🙁