Timber frame VCL detail query

[Annker]

Appreciate any ideas on the following issue to overcome.

When constructing my stick build TF extension I mistakenly left out a strip of VCL that I believe is now (at fit out stage) needed to link together with the VCL proper.

The design for the VCL was either going to be the PIR foil layer taped as required, or the addition of a sheet VCL.

 

My designed TF build up is broadly as section below:

 

 

I'm at the stage now of measuring up insulation and looking back at an old thread I see I missed installing a strip of VCL around the joist ends as per image below:

 

 

 

 

So now I am wondering how best to detail/install the wall VCL at the ceiling/floor joist to wall junction as its now built

Do squares of VCL need to be cut in around each individual joist void pocket and taped to the joists or can the VCL be simply taped along the continuous top plate?  

  

    

 

 

Similarly, at 1st floor ceiling/roof level I'm unsure how the wall VCL should be terminated.

I have a VCL installed externally above the osb, so again I will not be now able to link it to the internal wall VCL.

 

[Redbeard]

44 minutes ago, Annker said:

I'm at the stage now of measuring up insulation and looking back at an old thread I see I missed installing a strip of VCL around the joist ends as per image below:

 

Yes, that's effectively the 'Tony Tray' that many on here have discussed.

45 minutes ago, Annker said:

Do squares of VCL need to be cut in around each individual joist void pocket and taped to the joists

 

Arguably yes, though it won't be as perfect as a Tony Tray

 

46 minutes ago, Annker said:

or can the VCL be simply taped along the continuous top plate?  

 

Which continuous top plate? Not sure I get this bit. Can you explain, perhaps with a felt-tip annotation?

[Annker]

3 minutes ago, Redbeard said:

 

Yes, that's effectively the 'Tony Tray' that many on here have discussed.

 

Arguably yes, though it won't be as perfect as a Tony Tray

 

 

Which continuous top plate? Not sure I get this bit. Can you explain, perhaps with a felt-tip annotation?

Yes the "Tony tray", I had it well researched but must have been rushing and its inclusion was forgotten.

 

By top plate I'm referring to the horizontal top plate of the studded wall.

Basically just end the VCL along the ceiling line, don't fit it in around the joists and tape it along the top plate of the studded wall.

   

The option to have the VCL cut in around the joist will be a bit of a faff alright, however if its required I will do it. 

[Nickfromwales]

23 hours ago, Annker said:

 

The option to have the VCL cut in around the joist will be a bit of a faff alright, however if its required I will do it.

You can spray this with a closed cell foam, and it'll do a better job than trying the PITA and knuckle-scraping with membrane and tape.

 

https://insulation-more.co.uk/products/soudal-soudafoam-maxtwo-polyurethane-spray-foam-insulation-kits-complete-sets

[Annker]

14 minutes ago, Nickfromwales said:

You can spray this with a closed cell foam, and it'll do a better job than trying the PITA and knuckle-scraping with membrane and tape.

 

https://insulation-more.co.uk/products/soudal-soudafoam-maxtwo-polyurethane-spray-foam-insulation-kits-complete-sets

Looks like a good solution but as I've only about a dozen pockets to do it's probably too expensive an option. 

[Annker]

I'm still trying to figure out the best option to resolve the problem of the missed VCL installation at joist level.

My issue is struggling to understand the princple of where the VCL should be in the assemblage, therefore I'd appreciate any comment on the two sketched sections below.

 

My understanding is that ideally the entire joist would be internal side of the VCL.

That cannot now be achieved here, however, as per below I could continue the VCL up the wall, cut in-between the joists, taped back to the rim joist and around the perimeter of each individual joist pocket perimeter, then fit insulation within the joist pocket. 

In this arrangement the VCL is cold side of the insulation layer.

 

 

 

An alternative sketched below is where the 60mm PIR board (installed over the studwork) is cut around the joists and continued to the underside of the plywood subfloor, then similarly the VCL is continued to the underside of the plywood subfloor and taped accordingly around the joists.

In this arrangement the VCL is warm side of the insulation layer.

 

 

 

The first arrangement seems to closely mirror other details I have seen online, however does it not pose a condensation risk, where internal born warm moist air may condensate on the VCL behind the insulation?

The second arrangement seems a safer in terms of at least limiting the amount of internally born warm moist from entering the insulation layer, and what does get through can still dry into the cavity.

However I have not seen any similar detail online.

So thats my hurdle in a nutshell, any comments are greatly appreicated!

 

 

   

[Iceverge]

Airtight layer placement isn't as important as making sure it's continuous. 

 

It's drafts that carry vapor laden air, not diffusion etc. 

 

I would just do what's easier to install at this stage. Probably something like FM330 foam to fill any big gaps in every "box" at the end of the joists and then some airtight paint over the top. 

 

Airtight tape is also an option. The important thing is that is joins the layer above and below the ceiling. 

 

I'm slow to rely on the foam long term for airtightnss as it shrinks. 

 

 

[Annker]

@Iceverge Once again thanks for your response.

I like airtight paint, I had its inclusion in mind for the first option above and I think it would be the best option to confidently seal all gaps. The joints in the timberwork are all tight (my own work) so FM330 would not even be necessary to fill any gaps. 

 

"The important thing is that is joins the layer above and below the ceiling."

If I understand you correctly, joining those two layer together will not be possible as the plywood subfloor runs continuously between the lower and upper stud wall, thus physically separating the VCL above and below the ceiling.

 

I can ensure that the separate VCL below and above the ceiling are fully sealed & bonded to the respective underside/topside of that plywood subfloor but short of cutting a track through the plywood they wont be physically connected to one another.

[Iceverge]

6 minutes ago, Annker said:

I can ensure that the separate VCL below and above the ceiling are fully sealed & bonded to the respective underside/topside of that plywood subfloor. 

 

This will work perfectly.

 

Controlling vapour is largely nonsense. 

 

The aim is to control the flow of air and in doing so not create any vapour traps. 

 

 

 

 

[Annker]

2 minutes ago, Iceverge said:

 

This will work perfectly.

 

Controlling vapour is largely nonsense. 

 

The aim is to control the flow of air and in doing so not create any vapour traps. 

 

 

 

 

Great thanks for clarifying.

I'll update the post with photos in due course for anyone following with a similar issue,

[Annker]

Very useful video on some of the design issue relvent to the situation being discussed here in this thread.