Sealed roof, but breathable walls. A bad idea?

[bodger53]

I’m building a small house in my garden with Isotex blocks. For various reasons I’m doing a lot of the detailed design as I go or just ahead of when needed - hence a bit of a late question.

 

It’s a one bed lodge with fully vaulted ceiling throughout and small mezzanine at one end. I’ve decided on an unvented warm roof, but would also like to use the breathable nature of the walls.

The walls are designed for this, but the roof construction I am proposing would be at risk of interstitial condensation without sealing. With a vapour barrier at the interface between the walls and roof I think the proposal below could work well.

I’m sure it must have been done, but can’t find evidence of it. Has anyone else on here done this or can see any massive pitfall I’ve missed?

Thanks in advance!

Details below.

 

Roof make-up

·        Slates

·        25x50mm battens

·        38x50 counter battens

·        Weather resistant, breathable membrane - Tyvek Supro Plus

·        100mm Cellotex

·        150mm rafters with 100mm Cellotex up against Cellotex overboarding)

·        Vapour barrier

·        Plasterboard

·        Plaster

·        Paint

Vapour barrier & weatherproof membrane would be joined – essentially sealing the entire roof structure from below and allowing any moisture that does enter to leave through the breathable membrane above.

I am aware this is a riskier construction. However, the additional space in the mezzanine is seen sufficiently beneficial and I’m confident there is sufficiently low chance of interstitial condensation with the above makeup.

 

 

Walls

·        Timber cladding

·        35x50 counter battens

·        Breathable membrane - Tyvec Home Wrap

·        Isotex blocks - woodcrete ICF; 170mm Neopor insulation outboard and 120mm warm concrete

·        Lime/breathable plaster (straight onto blocks)

·        Breathable paint

 

If I continued the vapour barrier from the underside of the rafters over the top of the wall (green line below), the only moisture bridge (is that a thing?) would be the wallplate to rafters which I think would be pretty negligible?