Airtightness barrier across terraced wall cavity

[Crispy75]

I'm adding external wall insulation to my 60's terrace and would also like to improve airtightness as much as possible too. I can't afford the disruption of removing all internal wall finishes & fixtures, so the boundary has to be at the outside surface of the brick. This will be a parge coat and taped external window support frames.

 

The party walls are 215mm brick plastered both sides so reasonably airtight to start. I don't mind squirting stuff under skirting boards to fill any cracks there.

 

But how do I bridge the two? Or is this a futile hope? The best I can think of is drilling some holes from the outside and squirting in expanding foam.

 

 

 

[MikeGrahamT21]

If the cavity isn’t already filled with something this will need doing prior to EWI.

 

otherwise I don’t see a need for any airtight works in the cavity itself (and it would be almost impossible to do successfully), it won’t be open to the room, the cavity should be closed at the reveal, if it isn’t this needs doing using a cavity closer.

 

remember your airtight layer is in most cases the inner most layer of your fabric buildup.

[Crispy75]

The cavity has blown insulation already yeah.

 

I understand the logic behind having the AT layer right on the inside, especially due to the risk of interstitial condensation. But once the additional EWI is added, even the outside face of the brick is "inside" from a thermal gradient POV. My calcs say it'll be 15° at that surface on a 0° day, so relative humidity would need to get up to ~70% indoors and stay there long enough for moisture to get all the way through the wall. MVHR will keep that in check.

 

All that being said, it feels more like I shouldn't expend so much effort if there's going to be such large gaps impossible to seal.

Maybe I'll rig up a DIY blower and see how bad a single room is.

 

The rooftop extension (full footprint) that's also part of this project: that I will make airtight.

 

I'll be putting in MVHR regardless, just for the air filtering. My wife's allergies demand it!

[MikeGrahamT21]

Best thing you’ll ever do, MVHR