Airtight around existing joists in retrofit? Tony tray option?

[low_and_there]

Looking for wisdom from this forum on how to maintain an uninterrupted airtight layer between the ground floor and the first floor in our retrofit (solid brick mid terrace). 

 

The joists run front to back and on the front we have to do IWI (On the rear we’ll be doing EWI so my assumption is that we have a thermal bridge issue to solve on the front; but the rear won’t be such a challenge).

 

I have seen a few different solutions proposed for this:

 

i) rehang joists on joist hangers

ii) create a new wall plate on which to rest the joists

iii) support from below, leaving a complete gap between floor and wall

iv) employ the “Tony tray” method

 

Options (ii) and (iii) aren’t really suitable for our small footprint, bay window shape and structure. I don’t much fancy option (i) with a brick wall… so am extremely interested in the Tony tray option but haven’t been able to find an example of how to use it in a retrofit…  is it possible? Can the airtight (breathable) membrane be wrapped around a joist in situ? And if so how…? Any pointers to diagrams, photos or videos extra appreciated!

 

I can accept maybe not being able to achieve an airtight layer in it’s entirety, but what worries me is leaving the end of the joist getting damp in the brick area, which is beyond the insulation and airtight layer. I plan to use only natural breathable insulation, tho, so maybe my concern is unwarranted? 
 

thoughts extremely welcome!

[markc]

Easiest was to make airtight between ground a first floor would probably be to install a membrane on the walls and ceilings as a continuous layer. 
removing cold bridges in an old property is more difficult due to the construction methods and joint details.

why between ground and 1st floor? A flat I’m assuming?

[low_and_there]

Hi @markc, it's a whole house retrofit we're doing.

 

The ground floor airtight layer is starting above the Joists so nothing to solve there; the connection between the GF and the FF is what I'm looking at in this thread, and my question is how to make it airtight when the Joists are already placed on the masonry (bricks) and can't be easily moved to put the airtight membrane between them... 

[markc]

26 minutes ago, low_and_there said:

Hi @markc, it's a whole house retrofit we're doing.

 

The ground floor airtight layer is starting above the Joists so nothing to solve there; the connection between the GF and the FF is what I'm looking at in this thread, and my question is how to make it airtight when the Joists are already placed on the masonry (bricks) and can't be easily moved to put the airtight membrane between them... 

ahh, yes so it’s not that you want to seal the downstairs from the upstairs but rather looking to stop cold air coming into the floor void. Tony tray is out of the question so I think you are down to spray foam around the joists/walls or tightly packed insulation. 
the danger here is trapping moisture against the joist causing rot to set in.

[low_and_there]

Yes, that’s precisely it. How can the risk of rot and decay be mitigated or avoided entirely in a retrofit context like this?
 

Someone must have solved this… 

[Russell griffiths]

You want a liquid. There are two on the market I know of. 
BLOWER PROOF MEMBRANE 

and

PASSIVE PURPLE. 

I have used both either spray or paint brush. 
 

The blower proof is thicker and has some fibres , it’s easy to paint on , but a bit expensive.