Fine tuning my IWI Solid wall (Warm Batten) design

[Annker]

Following on from my previous thread I'm planning to start IWI installation on my victorian renovation in a couple of weeks time.

My proposed  buildup:

• Existing Solid 9" brick external wall
• 60mm foil backed PIR (Will serve as VCL)
• 35mm timber battens/service cavity. (Fixed to brickwork with concrete screws)
• 12.5mm plasterboard 
• Gypsum skim coat  

 

I'm generally confident this arrangement will perform in my particular situation, however there are some junctions where ensuring vapour/air tightness is maintained at the VCL will be tricky. 

 

For example the VCL will be repeatedly penetrated by the batten fixings, furthermore the battens being fixed plumb may not be tight against the PIR along the entire length of the batten; my solution: Foil tape all batten edges to the PIR.

 

However there are some interfaces jumping out as needing attention that I'm not certain how to best deal with:

• external wall to internal cross wall junctions:
  • I imagine there is a tape by Siga or a similar manufacturer for such junctions, appreciate if anyone can recommend such a product?   
• bottom of external wall
  •  this will be somewhere within the cellar (I will be running the PIR down below GF joist level), again probably will seal the PIR to the wall with a suitable tape
• top of external walls
  •  here I'm sure if I should just terminate the PIR/VCL at the roof wallplate, (perhaps taped to the wall plate) or should it be connected to a VCL installed to the bedroom ceiling joists
• external wall to intermediate 1st floor/ceiling.    
  • I don't believe any addition VCL is required here as I have re-joisted much of the house and stood the joists 70mm of the external walls where they run parallel to them, to allow the PIR run unbroken from cellar to roof.

 

 

Any comments or critique greatly appreciated.

 

 

[jayc89]

Same build up as I did.

 

After I drilled through the battens and PIR into the brickwork, I squirted a glob of acrylic sealant into the holes before drilling the screw through to, hopefully, provide an airtight seal. 

 

internal wall joint - I used Pro Clima Contega PV which is a plaster-able tape - to provide an airtight joint between PIR and internal wall

bottom of external wall - I used hemp lime plaster to bring the floor void flush with the PIR and taped again, opted for hemp lime for its breathability around the joist pockets into the brickwork. 

top of external walls - I pulled the ceilings down, stapled an airtight membrane to the bottom of the rafters and taped the PIR to that, then fitted a service void to the ceiling and plasterboard it

external wall to intermediate 1st floor/ceiling - see "bottom of external wall"

 

 

Edited September 5 by jayc89

[Annker]

@jayc89 Thanks for your reply, your comment here and on previous threads have been very helpful

 

I will be plasterboarding and beading the entire house myself (I'm a carpenter by trade). I'm doing this as I want the plasterer on the job to solely focus on skimming, which will hopefully result in a good finish.

As such I will be dot and dabbing plasterboard to the internal walls, I suppose I can integrate the Contega PV behind that D&D boarding, sealing the tape to the wall by pasting board adhesive on and over it.

 

With the hemp plastering, am I correct to picture that you ran the PIR down/up to the top/underside of the joisting and then applied the hemp plaster between the joists?

That seems like a neat solution, my only question though is doesn't the hemp introduce a vapour open surface among a vapour closed solution? (I believe you had a similar query in your own thread)

 

Also, you had considered applying boron to the pocketed joist ends, did you do so? 

I google finds this product which seems suitable however I'm unsure how effective application can be as the joists ends, which are most in need of protection are buried with the walls.

Regardlesss perhaps just applying over the first couple of inches of the joist within the room will help.

 

[Trw144]

F

[ab12]

On 05/09/2024 at 15:31, Annker said:

Following on from my previous thread I'm planning to start IWI installation on my victorian renovation in a couple of weeks time.

My proposed  buildup:

• Existing Solid 9" brick external wall
• 60mm foil backed PIR (Will serve as VCL)
• 35mm timber battens/service cavity. (Fixed to brickwork with concrete screws)
• 12.5mm plasterboard 
• Gypsum skim coat

Any comments or critique greatly appreciated.

 

 

Hi Ankker. I'm in a similar predicament.

How are you proposing to attached the 60 mm PIR to the wall? By foam or dot and dab or mechanically fixed?

Also the service cavity created by the timber battens am I correct to assume this will be void space except of any services running through the void.

What sized batten do you intend to use and are you thinking of going down the route of using tanalised battens?

Thanks

[Annker]

7 hours ago, ab12 said:

Hi Ankker. I'm in a similar predicament.

How are you proposing to attached the 60 mm PIR to the wall? By foam or dot and dab or mechanically fixed?

Also the service cavity created by the timber battens am I correct to assume this will be void space except of any services running through the void.

What sized batten do you intend to use and are you thinking of going down the route of using tanalised battens?

Thanks

I'm yet to start this particular job.

The plan in my mind at least, is that low expansion foam will momentarily hold the PIR in position, the installed battens then primarily securing the boards in position.

The service cavity may as you describe solely accommodate cabling etc, although I believe it could packed with rockwall if targeted u-valves required.

IMO the batten thickness needs to be minimum 35mm. Where the batten it is positioned in the build-up (warm side of VCL)  I don't believe it needs to be tanalised, regardless I am using a tanalised batten.  

  

 

[ab12]

Thank you for the explanation.

Are you thinking of going to bare brick or installing the PIR insulation onto existing plaster?

I've come across recommendations of applying parge lime coat if bare brick and then having PIR on that to allow for levelling of the wall surface but also any moisture thats travelled through to the wall can be absorbed by the lime plaster.

Having said that I suppose its a bit of paradox to have breathable lime parge coat and non breathable PIR board on top.

 

There is so much info available about the breathable vs non breathable insulation options for solid wall IWI that it can make the head spin and create lots of doubts.

My main worry is interstitial condensation risk with non breathable insulation and the long term impact on the fabric of the building should the worst case scenario occur.

 

Can I ask what sawayed you to the PIR rather than breathable insulation?

 

Retro-fit insulation can be a conudrum to the point that it can cause paralysis in decision making

 

 

[Annker]

38 minutes ago, ab12 said:

Thank you for the explanation.

Are you thinking of going to bare brick or installing the PIR insulation onto existing plaster?

I've come across recommendations of applying parge lime coat if bare brick and then having PIR on that to allow for levelling of the wall surface but also any moisture thats travelled through to the wall can be absorbed by the lime plaster.

Having said that I suppose its a bit of paradox to have breathable lime parge coat and non breathable PIR board on top.

 

There is so much info available about the breathable vs non breathable insulation options for solid wall IWI that it can make the head spin and create lots of doubts.

My main worry is interstitial condensation risk with non breathable insulation and the long term impact on the fabric of the building should the worst case scenario occur.

 

Can I ask what sawayed you to the PIR rather than breathable insulation?

 

Retro-fit insulation can be a conudrum to the point that it can cause paralysis in decision making

 

 

Its a very difficult subject to find agreement on.

I've gone back to brick and will not be applying a parge coat.

 

A poster above suggested to omit PIR between the joists and apply hemp plaster, I'd like to know more on how that works but other than that I'm fixed in the spec.

 

I am satisfied, as much as I can be, in this approach for the following reason specific to my situation:

• Property is a semi-d situated in a well sheltered location on a well drained site.
• Over the past year even with some very leaky guttering I have not seen any visable internal penetrative dampness on the bare brick walls
•  Many houses have employed this method without issue and I imagine the majority of those without much consideration/research of the finer installation details.  
• I plan to ensure kitchen and bathrooms have top performing mechanical ventilation to at least remove this source of moisture to outside the building.

Also a main driver is I need to get this done now, I've spent too long mulling over the pros, cons and risks of IWI, etc

I have a wife, child, mortgage and ongoing temporary accommodation/rent, need to get the job wrapped up.

 

 

[ab12]

11 minutes ago, Annker said:

• I plan to ensure kitchen and bathrooms have top performing mechanical ventilation to at least remove this source of moisture to outside the building.

I'm in kind of similar situation and need to get moving have been dithering, thinking and over-thinking the past few months..... type of insulation, having some sort of ventilation system put into the property... PIV or decentralised MHRV etc lots of consequential decisions

 

You maker an interesting point above mechanical ventilation.

Are you thinking along the lines of decentralised MHRV or something else e.g a very good extractor fan

 

[Annker]

31 minutes ago, ab12 said:

I'm in kind of similar situation and need to get moving have been dithering, thinking and over-thinking the past few months..... type of insulation, having some sort of ventilation system put into the property... PIV or decentralised MHRV etc lots of consequential decisions

 

You maker an interesting point above mechanical ventilation.

Are you thinking along the lines of decentralised MHRV or something else e.g a very good extractor fan

 

I believe for MHRV to perform anywhere near its design it requires an air tight building envelope, I'm restoring my existing sash window incl draught proofing them however I don't expect they will be airtight and they would at least hinder the performance of a MHVR system 

I see targeted extraction working better in my house; good extractor hood over the hob, well thought out extraction in bathrooms and utility.

[Redbeard]

2 hours ago, Annker said:

Its a very difficult subject to find agreement on.

I've gone back to brick and will not be applying a parge coat.

Based on having done it the 'wrong way' - the way I would not do it now -  (no parge coat, and *adhesive dabs* (as spec'd by the manufacturer 30+ years ago)) and the 'right way' (warm batten method on a lime parge coat) and (IMHO) the 'right right way' (rigid wood-fibre on an adhesive layer of lime, on a lime parge coat and with mechanical fixings) my preference, in order, would be 3, 2, 1. I do acknowledge of course that wood-fibre will come out significantly more expensive than PIR, and it is a matter of personal choice.

 

EDIT:  Note that, although my preference is for a parge coat (which I either bed the boards into wet, or to which I add an 'adhesive coat' of lime) if you do full perimeter beads of adhesive foam or dri-wall adhesive and make sure that they are deep enough to 'close off' all undulations, then you may have avoided the worst risk of 'thermal decoupling' - cold air movement between the wall and the insulation. By doing the perimeter beads and cross-hatchings you may ensure that if any 'leakage' occurs it is confined to a small area rather than occurring over the whole 2.8m2.

Edited October 15 by Redbeard

[Annker]

2 hours ago, Redbeard said:

Based on having done it the 'wrong way' - the way I would not do it now -  (no parge coat, and *adhesive dabs* (as spec'd by the manufacturer 30+ years ago)) and the 'right way' (warm batten method on a lime parge coat) and (IMHO) the 'right right way' (rigid wood-fibre on an adhesive layer of lime, on a lime parge coat and with mechanical fixings) my preference, in order, would be 3, 2, 1. I do acknowledge of course that wood-fibre will come out significantly more expensive than PIR, and it is a matter of personal choice.

 

EDIT:  Note that, although my preference is for a parge coat (which I either bed the boards into wet, or to which I add an 'adhesive coat' of lime) if you do full perimeter beads of adhesive foam or dri-wall adhesive and make sure that they are deep enough to 'close off' all undulations, then you may have avoided the worst risk of 'thermal decoupling' - cold air movement between the wall and the insulation. By doing the perimeter beads and cross-hatchings you may ensure that if any 'leakage' occurs it is confined to a small area rather than occurring over the whole 2.8m2.

Thanks for commenting @Redbeard I've read your previous well thought out posts re IWI and can appreciate your preference for woodfibre however I have had to dismiss its use in this particular project.

 

Can you explain what mix is a parge coat in this instance and how to apply it.

I had previously thought of a parge coat to be similar to a sand and cement scratch coat (trowelled on to say a minimum 10mm thickness), however I believe I read somewhere that ia parge coat is a much wetter mix and is brushed on with perhaps a broom or bristled brush almost like a thick lime whitewash?

In any event I'm certainly onboard for full perimeter bead/cross hatching of the PIR boards.

[Iceverge]

Be very cautious of foil faced PIR as an internal insulation. 

 

It has very very low vapour permeability and will actively trap any moisture that is behind it permanently. 

 

Unless you are willing and confident to seal every single screw hole and edge of it I would stay away.

 

A more preferable method is a parge coat and a battered cavity full fill with mineral wool batts. 

 

A parge coat is a thin coat of wet plaster applied as an airtightness layer. You can use cement or lime but in this application lime would give more drying ability.

 

Use plastering sand and (NHL 3.5) at a ratio of 2 sand to 1 Lime. 

 

 

Mix it to a creamy consistency and use a trowel to apply a thin coat. Alternatively it can be applied with a sweeping brush but it needs to be watered into a slurry first. 

 

Floor to wall junctions, internal abutting walls, window and door junction, penetrations and joists ends should be airtighed to the parge coat with a plaster-able tape or airtight paint. 

 

 

 

So. 

 

1. Seal all windows and doors and junctions and penetrations. 

2. Apply a parge coat. 

3. Fix 70*42mm battens centres with hammer in frame fixings.

4. Install all wiring and services. 

5. Install mineral wool batts. 

6.Install sheets of plasterboard.

 

You can play with the batten thickness and spacing required to fit your batts and plasterboard and ceiling height. 

 

 

[Annker]

1 hour ago, Iceverge said:

Be very cautious of foil faced PIR as an internal insulation. 

 

It has very very low vapour permeability and will actively trap any moisture that is behind it permanently. 

 

Unless you are willing and confident to seal every single screw hole and edge of it I would stay away.

 

A more preferable method is a parge coat and a battered cavity full fill with mineral wool batts. 

 

A parge coat is a thin coat of wet plaster applied as an airtightness layer. You can use cement or lime but in this application lime would give more drying ability.

 

Use plastering sand and (NHL 3.5) at a ratio of 2 sand to 1 Lime. 

 

 

Mix it to a creamy consistency and use a trowel to apply a thin coat. Alternatively it can be applied with a sweeping brush but it needs to be watered into a slurry first. 

 

Floor to wall junctions, internal abutting walls, window and door junction, penetrations and joists ends should be airtighed to the parge coat with a plaster-able tape or airtight paint. 

 

 

 

So. 

 

1. Seal all windows and doors and junctions and penetrations. 

2. Apply a parge coat. 

3. Fix 70*42mm battens centres with hammer in frame fixings.

4. Install all wiring and services. 

5. Install mineral wool batts. 

6.Install sheets of plasterboard.

 

You can play with the batten thickness and spacing required to fit your batts and plasterboard and ceiling height. 

 

 

Iceverge, thanks for the comprehensive explanation of the parge coat much appreciated.

As you can imagine the external face of the brickwork being laid flat results in any discrepancies in brick dimension showing on the internal face.

In my mind to ensure that the parge coat is a continuous layer it would therefore need to be laid on quite thickly, perhaps in practice this wouldn't be an issue maybe a wet slurry mix would be the better option in that situation.

 

I'm aware that PIR has its risks and they (along with many other factors) have all fed into my decision to go with this method.

Understand the low vapour permeability of the PIR but don't agree that in my situation any moisture would be trapped behind the PIR permanently. The brickwork is bone dry, lime mortar pointed and I cannot see how that will change as it hasn't in the 1.5 that the brickwork has been bare, house unoccupied, windows missing, guttering leaking etc.

Therefore the brickwork will provide a drying face to wick away moisture should in develop behind the PIR.

 

I have considered a variant of your suggested design, are you familiar with the SWIP IWI product/method?

The issues I have with it is that the battens are cold side of the insulation layer and VCL; with the warm batten method I am using both those issue are removed.

 

Settling on a IWI design for this solid walled house has been the most difficult design decision. I'll be steering well away from pretty victorians for my next project. 🙁

 

  

 

    

[ab12]

23 hours ago, Redbeard said:

Based on having done it the 'wrong way' - the way I would not do it now -  (no parge coat, and *adhesive dabs* (as spec'd by the manufacturer 30+ years ago)) and the 'right way' (warm batten method on a lime parge coat) and (IMHO) the 'right right way' (rigid wood-fibre on an adhesive layer of lime, on a lime parge coat and with mechanical fixings) my preference, in order, would be 3, 2, 1. I do acknowledge of course that wood-fibre will come out significantly more expensive than PIR, and it is a matter of personal choice.

 

EDIT:  Note that, although my preference is for a parge coat (which I either bed the boards into wet, or to which I add an 'adhesive coat' of lime) if you do full perimeter beads of adhesive foam or dri-wall adhesive and make sure that they are deep enough to 'close off' all undulations, then you may have avoided the worst risk of 'thermal decoupling' - cold air movement between the wall and the insulation. By doing the perimeter beads and cross-hatchings you may ensure that if any 'leakage' occurs it is confined to a small area rather than occurring over the whole 2.8m2.

A timely response for me. I'm very much interested in your preferred breathable method of IWI. I need to go down this route very very soon but need to decide on the application of the breathable insulaton to the wall- so type of breathable (Inditherm batts, rigid fibre boards) and the method of fixing to the wall- between battens or mechanically fixed with plastic plugs

I got lost in the 3,2,1 bit of your explanation.

So just to confirm you would parge coat the wall with lime plaster (thickness of coat 6mm ?) then mechanically fix rigid fibre boards to the parge coat. What thickness of rigid boards would you opt for?

Theres mention of warm batten method on lime parge aswell. Can you please expand how this would play out- what centres between the batten would you opt for. I suppose this will be largely determined by the size of rigid fibre board?

What size of battens would you opt for? In my mind I'm overthinking that the depth of battens should be same depth as the rigid fibre board to create even surface which can be lime plastered finished.

Thanks

 

[Redbeard]

19 minutes ago, ab12 said:

 

Based on having done it the 'wrong way' - the way I would not do it now - (1). (no parge coat, and *adhesive dabs* (as spec'd by the manufacturer 30+ years ago)) and (2).the 'right way' (warm batten method on a lime parge coat)  and (3)(IMHO) the 'right right way' (rigid wood-fibre on an adhesive layer of lime, on a lime parge coat and with mechanical fixings) my preference, in order, would be 3, 2, 1.

 

23 minutes ago, ab12 said:

So just to confirm you would parge coat the wall with lime plaster (thickness of coat 6mm ?) then mechanically fix rigid fibre boards to the parge coat. What thickness of rigid boards would you opt for?

 

For rigid wood-fibre, yes, about 6mm (or the depth of the exg lime plaster if the house is old enough to have it. Would need to augment where, for example, skirtings and covings had been, and beware impervious paint. To fix the boards, apply a toothed trowel coat of lime to the back of the board and press firmly into place. Generally, and particularly if using T&G boards, make sure that the 'adhesive layer' does not get into the joint, or the boards will separate slightly and give you less-than-perfect insulation.

 

In a rush. More later!

[ab12]

On 15/10/2024 at 15:01, Redbeard said:

if you do full perimeter beads of adhesive foam or dri-wall adhesive and make sure that they are deep enough to 'close off' all undulations, then you may have avoided the worst risk of 'thermal decoupling' - cold air movement between the wall and the insulation. By doing the perimeter beads and cross-hatchings you may ensure that if any 'leakage' occurs it is confined to a small area rather than occurring over the whole 2.8m2.

Excellent. Looking for ward to the next read.

It's glaringly obvious i'm a bit of a novice, unware of the intricacies but ardent learner.

I didnt quite understand the above... am I correct in thinking the wall perimeter essentially needs to be sealed before the application of PIR or am I way off.

[Redbeard]

Answering the one immediately above: If you add the words 'on each board' after 'adhesive foam or dri-wall adhesive' it explains that you have to do this for every single (PIR)board or part-board. (That's where the reference to 2.8m2 (8ft x 4ft) came in.

 

More later!

[Redbeard]

3 hours ago, ab12 said:

Theres mention of warm batten method on lime parge aswell. Can you please expand how this would play out- what centres between the batten would you opt for. I suppose this will be largely determined by the size of rigid fibre board?

What size of battens would you opt for? In my mind I'm overthinking that the depth of battens should be same depth as the rigid fibre board to create even surface which can be lime plastered finished.

 

The Warm Batten method I was referring to was with PIR board (one I no longer use). It allows you to get excellent air-tightness and vapour control, and a service void (somewhere to run your cables at least, and possibly even plumbing, though I would not do the latter). I have never used, and personally would not use, the warm batten method with wood fibre, since it implies plasterboard over. Yes, if you have WF board only there is a bit of an issue with services. I happen to think that surface-mounted pattresses and trunking are fine, but it's not everyone's cup of tea.

[Redbeard]

3 hours ago, ab12 said:

What thickness of rigid boards would you opt for?

It depends on personal preference. It is fairly widely acknowledged that using more than 100mm may be risky from  the point of view of potential interstitial condensation. Some authorities and merchants suggest a maximum of 60mm (see, for example, 'A Bristolian's guide to solid wall insulation) while some suggest as little as 40mm. I always try to 'prove' (ideally by way of the WUFI dynamic condensation risk assessment model) that 100mm (which would give you a not-quite-compliant U value of 0.35ish) is 'safe'. Note that the target U value is 0.3W/m2K but for older houses you should be able to argue this OK with the BCO on the basis that the house was designed to allow evaporation and absorption of water (can't remember the exact wording in the Regs). I just try to get as close to the target U value as possible without inviting interstitial condensation risk.

 

Most probably know this already but just in case not:

 

Note that that requirement to apply for and comply with Bldg Regs may apply even when you are doing only one room. The Regs refer to Renovation (adding or replacing a layer) of a Thermal Element. If, viewed from the inside, you are doing more than 50% of a thermal element you are within the remit of the Regs and must pay and comply. Imagine you are doing the living room of an inner terraced house. There is only one external wall and you are likely to do 100% of that wall. You must therefore seek BC approval.

 

When you are doing one room, at a time always prepare the 'edges' for connection to the next room. In the case of the living room you would strip back the ceiling, apply a parge coat up to the floor above, prime and tape the joist ends, and continue the insulation up to the floor above. In x years time when you get round to thhe bedroom you can lift a floorboard, reveal the insulation from downstairs and connect tightly to it.

[Iceverge]

10 hours ago, Annker said:

Understand the low vapour permeability of the PIR but don't agree that in my situation any moisture would be trapped behind the PIR permanently. The brickwork is bone dry, lime mortar pointed and I cannot see how that will change as it hasn't in the 1.5 that the brickwork has been bare, house unoccupied, windows missing, guttering leaking etc.

Therefore the brickwork will provide a drying face to wick away moisture should in develop behind the PIR.

 

 

It's a very poorly understood topic. Unfortunately it's a common misconception that the moisture that causes an issue comes from outside. 

 

Assuming you have dealt with gutters + pointing etc this is never the case. 

 

It comes from inside the building, from breathing, cooking showering etc.  

 

This airborne moisture condenses when it reaches a cold surface. It is carried on drafts and air leaks around the house. This is why PIR is bad news unless you are prepared to hermetically seal it (Airtighess!!)

 

If you leave a hole the size of half a penny that allows damp air to leak consistently you will get dampness behind the boards. 

 

As it's cold and sandwiched between a layer of miniscule vapour permeability (foil) it'll be forced into the exterior walls. Bad news. The walls will be much wetter than before the interior insulation. 

 

A flexible more vapour open airtightness layer like lime parge will be much more resilient as the walls will still have a drying route into the room. 

 

Mineral wool and gypsum are still massively more vapour open than foil faced PIR. 

 

10 mm Lime render has a SD value of 0.1m.

12.5mm of gypsum is the same. 

Aluminium foil on the face of PIR is about 200m. 

 

In other words 2000 times worse. 

 

Lime parge, mineral wool and gypsum will allow the wall to dry inwards. Theres many way to skin cats and many wonder products but if you get a handle on the actual numbers the you can make an informed, financially balanced choice. 

 

 

 

 

 

 

 

[lookseehear]

On 16/10/2024 at 06:41, Iceverge said:

Be very cautious of foil faced PIR as an internal insulation. 

 

It has very very low vapour permeability and will actively trap any moisture that is behind it permanently. 

 

Unless you are willing and confident to seal every single screw hole and edge of it I would stay away.

 

A more preferable method is a parge coat and a battered cavity full fill with mineral wool batts. 

 

A parge coat is a thin coat of wet plaster applied as an airtightness layer. You can use cement or lime but in this application lime would give more drying ability.

 

Use plastering sand and (NHL 3.5) at a ratio of 2 sand to 1 Lime. 

 

 

Mix it to a creamy consistency and use a trowel to apply a thin coat. Alternatively it can be applied with a sweeping brush but it needs to be watered into a slurry first. 

 

Floor to wall junctions, internal abutting walls, window and door junction, penetrations and joists ends should be airtighed to the parge coat with a plaster-able tape or airtight paint. 

 

 

 

So. 

 

1. Seal all windows and doors and junctions and penetrations. 

2. Apply a parge coat. 

3. Fix 70*42mm battens centres with hammer in frame fixings.

4. Install all wiring and services. 

5. Install mineral wool batts. 

6.Install sheets of plasterboard.

 

You can play with the batten thickness and spacing required to fit your batts and plasterboard and ceiling height. 

 

 

 

This is incredibly helpful - thank you!

 

I have a couple of questions about this method if that's OK.

1. What would you do if the walls are uneven stone not solid brick. I'm imagining a parge over the rough surface, then battens create as flat a surface as possible for plasterboard, and mineral wool batts assumed to take up the uneven surface of the stone?
2. In this method there's no airtighness attempted between the room and the parge coat, so moisture is free to get behind the plasterboard and insulation onto the parge coat which (given it's on the cold side of the mineral wool) will condense. Assuming I'm right in this, is the thinking that it's all vapour open enough to allow drying inwards aided by PIV/MVHR?

Thanks in advance.

[Iceverge]

3 hours ago, lookseehear said:

What would you do if the walls are uneven stone not solid brick. I'm imagining a parge over the rough surface, then battens create as flat a surface as possible for plasterboard, and mineral wool batts assumed to take up the uneven surface of the stone?

 

You could lay top and bottom plates and build a stud wall detaches from the stone entirely. Something like 63mm CLS timber is probably the thinnest thing you would have a chance of getting. To stay straight. Insert 70mm or 75mm batts then. The mineral wool will take up the gaps. 

 

3 hours ago, lookseehear said:

1. In this method there's no airtighness attempted between the room and the parge coat, so moisture is free to get behind the plasterboard and insulation onto the parge coat which (given it's on the cold side of the mineral wool) will condense. Assuming I'm right in this, is the thinking that it's all vapour open enough to allow drying inwards aided by PIV/MVHR?

Thanks in advance.

 

It's air currents carrying moisture rather than diffusion through materials that causes issues. 

 

An excellent airtight of a well done parge coat, the elimination of any gaps behind the plasterboard with the flexible insulation and the good drying ability or the materials will prevent any buildup of moisture. 

 

 

 

[lookseehear]

Thanks - this all makes sense. What about window/door reveals? Maybe similar but thinner batts and thinner mineral wool insulation?

[Iceverge]

That'd be fine.

 

Maybe @Redbeard has more experience than me.