I've removed the ceiling in our lean-to extension to form a nice airy/open kitchen/family space. I plan to insulate between the rafters with 140mm flexible wood-fibre and then over the top with 60mm wood-fibre t&g.

On top of this, for VCL/airtightness, I was thinking of using Passive Purple liquid membrane in white which would also provide a decorative finish. This should avoid the need to scim plaster and paint.

Does anyone have any comments/see flaws in this plan?

Can't see that looking good at all, I'm afraid. And you've the issue of penetrations for lights etc that won't be sealed. You could use PIR under the joists , tape, the. Batten out and board under. Gives you a void to run your wiring etc.

If you have no penetrations I would rather render directly to the woodfiber as an airtightness layer rather than purple passive. A a clay or lime render may be needed if you wish to retain the  breathability of the wood fiber. 

 

Having used a fair few tubs of the Soudal airtight paint it yellows in sunlight and has a texture a bit like fiberglass so I don't think it's a suitable final finish. 

 

Caution of blocking any airflow gaps with the batts if you have under sarking ventilation in the roof. 

 

 

 

 

That's helpful, thanks Iceverge... 

10 hours ago, Iceverge said:

If you have no penetrations I would rather render directly to the woodfiber as an airtightness layer rather than purple passive. A a clay or lime render may be needed if you wish to retain the  breathability of the wood fiber. 

 

Having used a fair few tubs of the Soudal airtight paint it yellows in sunlight and has a texture a bit like fiberglass so I don't think it's a suitable final finish. 

 

Caution of blocking any airflow gaps with the batts if you have under sarking ventilation in the roof.