Warm flat roof airtightness

[mistake_not]

Been searching and thinking and can't think of / find / work out how do air tightness well for my extension other than taping everything in sight like @plockhart is. 

 

The extension is going to be a warm flat roof attached to an existing cavity wall, with a new full fill cavity wall. 

 

Airtightness on new walls achieved with parge coat and old walls with wet plaster.

Roof buildup so far is:

 

- plasterboard 

- joists

- OSB

- VCL

- Insulation 

- EPDM

 

Can see how I can easily join the VCL to either wall, and therefore am I better just omitting it, and ensuring the OSB is vigorously taped to all joists / itself / the walls, or doing something different? 

 

I don't like the idea of the VCL below the OSB, as the OSB is then effectively sandwiched between 2 airtight layers and has a moisture risk.

 

Grateful for wisdom / pointing me to the answer elsewhere.

[lookseehear]

If you search for a 'Tony tray' you'll see that a lot of people have wrapped the ends of the joists in 600mm+ DPC, which can be taped to OSB and into the parge coat on the walls.

[Nickfromwales]

On 21/03/2025 at 16:28, mistake_not said:

Been searching and thinking and can't think of / find / work out how do air tightness well for my extension other than taping everything in sight like @plockhart is. 

 

The extension is going to be a warm flat roof attached to an existing cavity wall, with a new full fill cavity wall. 

 

Airtightness on new walls achieved with parge coat and old walls with wet plaster.

Roof buildup so far is:

 

- plasterboard 

- joists

- OSB

- VCL

- Insulation 

- EPDM

 

Can see how I can easily join the VCL to either wall, and therefore am I better just omitting it, and ensuring the OSB is vigorously taped to all joists / itself / the walls, or doing something different? 

 

I don't like the idea of the VCL below the OSB, as the OSB is then effectively sandwiched between 2 airtight layers and has a moisture risk.

 

Grateful for wisdom / pointing me to the answer elsewhere.

Is the rest of the dwelling airtight? If not, just manage draughts and you’ll be fine without? If airtight, have you MVHR? 

[mistake_not]

13 hours ago, Nickfromwales said:

Is the rest of the dwelling airtight? If not, just manage draughts and you’ll be fine without? If airtight, have you MVHR? 

Not yet- but working on it as part of renovation/ extension. Aim to be when I'm done. Mvhr going in as part of the same work.