Vapour-open plasterboard alternatives that *don't* need to be skimmed?

[YodhrinForge]

Still trying to figure out the specifics of my eco retrofit of a circa 1900 stone cottage, but I've been slightly buggered by the WUFI modelling. Based on what I'd read previously I believed taped & jointed - American style - thin gypsum plasterboard with vapour open paint would be sufficient to allow the internal wood fibre insulation and the stone wall to breathe properly, but Computer Says No apparently. Previously planned buildup was external lime harling > original stonework > up-to 30mm Diathonite levelling & airtight layer > 100mm rigid wood fibre > ~40mm stud wall to create service cavity with spaces infilled with flexible wood fibre > 12mm plasterboard > clay paint, however the modelling suggests this would accumulate moisture over time despite proper MVHR to manage internal humidity - slowly, but in a few years or certainly a decade or two I'd be having issues. The two suggested alternatives suck for different reasons; either adding an Intello membrane between the rigid wood fibre and the stud wall, but this would require dropping the flexible wood fibre filling out the spaces in the stud wall which is a noticeable reduction in u-value; or switching to clayboard with clay plastering for the interior instead of plasterboard, which has sufficient permeability to avoid the buildup issue but uses up a bit more internal space given it's thicker and would require me to fork out for a specialist plasterer to do the mesh-reinforced binder coat and the skimmed finish coat, which ain't cheap.

 

I found *one* company offering what appear to be lime-based plasterboard substitutes designed to be very vapour open but still work pretty much exactly like common gypsum board, but they're only just starting to roll out the product and aren't selling to the public yet, and I can't imagine that a small house renovation will get through their application process to be a demo project(not to mention that as it's brand new there's no track record to look at, could be a complete dud in practice). Are they genuinely the first to come up with such a thing? Standard plasterboard was appealing exactly because with the tape & joint method I could do it myself and achieve pretty nice results for just material costs, while still maximising insulation, so any alternative that let me do something similar would be ideal.

[JohnMo]

31 minutes ago, YodhrinForge said:

read previously I believed taped & jointed - American style

Not really American style, been done in the UK since the 70s, almost all homes in Scotland are done that way - dry lined, or Ames taping is what it's called. Just make sure you use taper edge plasterboard.

 

Discussion here on plasterboard 

https://www.greenbuildingadvisor.com/question/gypsum-board-vapor-permeable-or-what

 

 

[YodhrinForge]

1 hour ago, JohnMo said:

Not really American style, been done in the UK since the 70s, almost all homes in Scotland are done that way - dry lined, or Ames taping is what it's called. Just make sure you use taper edge plasterboard.

 

Discussion here on plasterboard 

https://www.greenbuildingadvisor.com/question/gypsum-board-vapor-permeable-or-what

 

 

I refer to it that way mostly because most of the youtube tutorials I found were yanks and they rarely do anything else by all accounts, whereas all you hear about on UK property shows when I was growing up was plastering. Anyway, I appreciate the link but that's actually one of the things I read that encouraged me that plasterboard would work in the first place. Unfortunately the modelling of my intended buildup says that even the small additional resistance of gypsum board over clay or similar is enough to cause a slow accumulation of moisture in the wall over time even with fully vapour open paint used internally, so I'm not sure what to believe except that if the modelling is right then in a few years time I'll have to rip out all the work I'd done and do it again so it seems safer to look for alternatives.

[nod]

You will have to ask your taper not to seal 

As it’s normal to seal after sanding which is a sealer and hardener 

[jfb]

Is any of this work done yet?

 

why bother with the studs for a service void? You can embed electric cables into the wood fibre board.

then finish with a number of different breathable renders (I just used sand/nhl lime). Would save a lot of time.

[SteamyTea]

Rerun the WUFI model with some full fill mineral wool insulation, a VCL and a normal plasterboard wall.

It may be that WUFI does not like the 'moisture buffering' of wood fibre insulation.

[Mike]

On 26/04/2025 at 18:40, YodhrinForge said:

the modelling suggests this would accumulate moisture over time despite proper MVHR to manage internal humidity

Selecting the correct internal environment in WUFI to model that is not at all straightforward, as mentioned in my post on the topic from last year - there seems to be an absence of accurate information on which to calculate it. And, If they have erred too much on the side of caution (which I suspect), the results will be overly pessimistic.

 

If you're able to post the details of the internal environment that they've used, that may give us a clue. if you don't have it, it would be worth asking for it.

 

On 26/04/2025 at 18:40, YodhrinForge said:

either adding an Intello membrane between the rigid wood fibre and the stud wall

If that's what WUFI suggests, that increases my suspicion that it's inaccurate. I'm not a WUFI expert, but based on my own WUFI experiments, adding Intello membrane to a building that has WUFI is likley to make the moisture build-up within the wall somewhat worse (in winter), not better. That's because standard MVHR is likely to result in a lower internal relative humidity than would otherwise be the case, which, in turn, would remove moisture from the wall structure. A VCL, even a smart one such as Intello, will tend to block that drying action for part of the year.

 

On 26/04/2025 at 18:40, YodhrinForge said:

switching to clayboard with clay plastering for the interior instead of plasterboard, which has sufficient permeability

If that's also based on WUFI, that also seems counter-intuitive in view of the previous suggestion. Option 1 (Intello) suggest that slowing drying to the inside (in winter) is better; Option 2 (clayboard) suggests that increasing drying to the inside is better...

 

Edited 3 hours ago by Mike