IWI & airtightness strategy

[OwenF]

Good morning,

 

After a recent question about a specific airtightness detail, I’m now debating the entire strategy I’ve adopted for my renovation🫣 Not ideal given I’m part way through it!

Layout below has existing wall buildup (out to in) as follows:

Walls A & B (previous garage external)

sandstone

50/100mm cavity (unfilled)

block

 

Wall C (remainder of wall dividing kitchen/garage)

block

50mm cavity

50mm EPS

block

 

Walls D (external)

sandstone

50mm cavity

50mm EPS

block

dotdab PB

 

Wall E (internal)

single block 

wet plaster both sides

 

EWI is discounted due to irregular sandstone exterior

Cavity fill installers won’t touch it because of irregular cavity

 

So I was left with IWI and came up with what I thought was a decent strategy:

 

Walls A & B (final build up)

sandstone

50/100mm cavity (unfilled)

block

90mm PIR between 38x98 CLS @ 400c-c

VCL/airtightness layer (Intello or similar)

25x38mm battens for service void

 

Wall C

same as above but only on utility side and reduce PIR to 50mm

(this is just to mitigate wall area directly exposed to cavity)

on ‘dining’ side there is 50mm EPS behind block so felt IWI would reduce floor area too much for limited benefit 

VCL/air layer will continue round from utility side

 

Walls D

no IWI but continue VCL/air layer from Wall C

 

Wall E

Lap VCL/air membrane coming from walls D and re-plaster join.

 
Wall F will be new internal timber stud wall

 

This all felt ok, as I was placing VCL on warm side of insulation. VCL also doubling as an airtight layer means reducing drafts into the habitable space.

 

But the chink in the armour I might’ve missed is potential for air movement in the IWI (I think it’s called thermal bypass). Seems like this could undo or severely reduce any gains from the IWI.

 

Would anyone be able to give feedback on this strategy or comment on the significance of air movement behind the air layer in my case?

 

P.S. all floor is suspended timber. To be filled with 100mm glass wool with breather membrane beneath and VCL above (taped to walls).

Equally ceiling on RHS is cold roof insulated between joists (300mm). VCL below (taped to walls) & battened for service void.

 

thanks

[ProDave]

Whatever you do, I would remove ALL plasterboard on all external walls.  I almost guarantee you will have a plasterboard tent.

 

So back to bare block, add your IWI then VCL / Air tight layer, battened service void and plasterboard.  Care with taping and sealing every single joint in the membrane and seal the life out of anything that has to penetrate the membrane. 

 

Make sure where the VCL meets the ceiling and floor, any penetrations up or down are well sealed.  There will still be cold air in the ceiling void unless you are continuing your renovations to that and the upper floor?

 

I have no idea why you propose to do nothing to wall D and seemingly leave the plasterboard tent in place?

[OwenF]

@ProDave thanks for feedback.

 

All existing PB will be removed. Wet plaster retained on internal wall except for where membrane gets lapped in (re-plastered after)

 

Good point re: Wall D. I hadn’t planned to IWI that wall, but yes clearly need to remove the ventilated PB tent. Perhaps I need to think again about either adding insulation-in-studs like Walls A/B. Or at least wet plaster D so the air layer continues all the way to internal wall E.

 

the GF/FF ceiling void is a tricky one. I actually have a different thread on that

 

Edited January 24 by OwenF

[Roger440]

Im not sure i understand why you think there will be aitr movement behind the VCL?

 

Only if you leave air paths open to it. Surely the answe is not to. As per ProDaves suggestion?

 

Ive just done similar in my workshop. 100mm PIR between batens, VCL over. But i sealed everthing before i started, which was onto bare block.

[OwenF]

I’m not sure myself, hence asking for opinions.

 

You say you sealed your bare block before you started. Was that a liquid membrane all over or just plugging gaps?

 

I guess where I think air movement happens behind the VCL is, for example,  between the suspended floor void and the vented attic. The wall insulation will be outside of the airtightness layer in my strategy (which I understand is a fairly textbook approach), yet is exposed to both those ventilated spaces.

This technical guidance is heavy going 

https://www.passivhaustrust.org.uk/guidance_detail.php?gId=55

but it seems to imply that any ventilation of insulation will result in reduced performance.

 

maybe I’m overlooking some nuance that exists across the myriad of possible build ups. Specifically, it seems attention to ensure no physical air gaps (E.g. insulation compressed between wall and VCL) goes a way to mitigating this ‘thermal bypass’ effect.

 

sounds like the method I’m proposing doesn’t ring alarm bells to anyone as a catastrophe in the making, which is reassuring.