External cavity wall now internal - cold bridging/other issues?

[NandM]

As part of our proposed extension work, a current external cavity wall will become an internal wall.  The current brick wall will have a 100mm slit to act as a thermal break, but I will still have a approx 4m length of cavity that is still "connected" to the outside. Do you see this causing any issues with heat loss? I can wet plaster both sides to help with airtightness - but I really don't want to add insulation to the walls on both sides (although I could fill with eps beads).

 

The one idea I have in mind is to start the new block work from the old block work by having an approx. 250mm opening down the entire side of the wall and joining the two. This will result in a separation between the" internal" cavity and the external one. There are two challenges I see here: 

 

1) with the 250mm opening, could it be done from bottom up without the need for any lintels. Google tells me this should be OK. Otherwise, we could do it top down.

2) the new block that will connect to the old would have to use whatever the current foundation depth is - at least for 150mm of the block.  Would this be a problem?

 

 

[Oz07]

Is it single storey? Even if its 2 storey if existing is 50mm cavity you might be overthinking this. It's not like the existing building is at passive levels of insulation. It's likely to not be the only cold bridge in the house whats the existing floor insulation? A 4" break should be fine

[MikeGrahamT21]

Definitely fill the cavity which will end up internally, which will help to prevent the chimney effect of heat loss