Fine tuning my IWI Solid wall (Warm Batten) design

[Redbeard]

1 hour ago, Iceverge said:

Maybe @Redbeard has more experience than me. 

 

Hello! Not with mineral wool, I am afraid. I have not used mineral wool as an internal insulant. I think I would be a little uncomfortable using it without a VCL, as even the 'rigid' grade of rockwool used for EWI is not as dense as 'basic' (140kg/m3 - 180kg/m3) rigid WF in terms of moisture-buffering ability, although at apparently 110kg/m3 it's not so far away). If you are finishing with plasterboard you could (at a cost) use an intelligent membrane such as Pro Clima's 'Intello'.

 

Yes, for reveals use as much insulation as you can get in without 'losing' the window or door frame.

 

If you were taking the risk of no VCL then you could buy the EWI grade of rockwool and plaster directly to it, with the necessary mesh reinforcement, effectively as per thin-coat EWI.

[torre]

On 15/10/2024 at 13:18, ab12 said:

some sort of ventilation system put into the property... PIV or decentralised MHRV etc

I would avoid relying mostly on PIV in an old property and use MVHR or continuous DMEV instead.

 

If there's a lot of moisture internally, PIV has to push that out somewhere, and that's likely to be into any gaps etc, where it may condense, potentially making any damp problem worse, whereas MVHR or continuous extract will help ensure more of the moist air is pulled out of the property.

[lookseehear]

23 hours ago, Redbeard said:

 

Hello! Not with mineral wool, I am afraid. I have not used mineral wool as an internal insulant. I think I would be a little uncomfortable using it without a VCL, as even the 'rigid' grade of rockwool used for EWI is not as dense as 'basic' (140kg/m3 - 180kg/m3) rigid WF in terms of moisture-buffering ability, although at apparently 110kg/m3 it's not so far away). If you are finishing with plasterboard you could (at a cost) use an intelligent membrane such as Pro Clima's 'Intello'.

 

Yes, for reveals use as much insulation as you can get in without 'losing' the window or door frame.

 

If you were taking the risk of no VCL then you could buy the EWI grade of rockwool and plaster directly to it, with the necessary mesh reinforcement, effectively as per thin-coat EWI.

 

Thanks for this. What attracted me to the method of parge for airtightness and rockwool/service void/plasterboard finish is that it decouples the airtightness from the services (no taping socket backboxes etc). It seems simpler from an execution perspective and I think this would work well with our builder and help me stay on top of the quality of the work. It isn't that I don't think he'll do a good job, but I think that we still have work to do when it comes to selling the importance of airtightness. I'm also wary that the more rigid the insulation board is, the less able it will be to match the uneven shape of the stone walls, and the more likely we are to have voids.

 

If we were to use a parge coat of NHL 3.5, rockwool within a CLS timber 'batten' frame, Intello Membrane then Plasterboard (and I assume a skim of gypsum), does that mean that all sockets/holes need to be taped to the Intello membrane?

 

Our project is a combination of extension, reorganisation and renovation, so anywhere we can create a simple solution will be big positive.

[lookseehear]

On 27/10/2024 at 13:49, torre said:

I would avoid relying mostly on PIV in an old property and use MVHR or continuous DMEV instead.

 

If there's a lot of moisture internally, PIV has to push that out somewhere, and that's likely to be into any gaps etc, where it may condense, potentially making any damp problem worse, whereas MVHR or continuous extract will help ensure more of the moist air is pulled out of the property.

 

Thanks for this - I think we're likely to use a combination of centralised and decentralised MVHR, as we can likely get ducting to 2/3 of the house relatively easily, but the final 1/3 will be more tricky. This definitely makes sense to me.

[Redbeard]

6 hours ago, lookseehear said:

If we were to use a parge coat of NHL 3.5, rockwool within a CLS timber 'batten' frame, Intello Membrane then Plasterboard (and I assume a skim of gypsum), does that mean that all sockets/holes need to be taped to the Intello membrane?

How about the above but with added 25 x 38 (?) counter-battens over the Intello, forming a service void so that the elecs are inside the air-tightness/VC layer?

[Annker]

Posting a revision to my thinking on my particular warm batten arrangement.

 

Detail aside there is a potential installation issue that has been at the back of my mind, and that is the fixing of the battens through the PIR sheets and into the brick wall.

There are a few tricky steps with this:

• Quality of the fixings.
  • I had planned to use concrete screws which depending on substrate can either get a good fixing, a loose fixing or blow out the substrate; dry clay bricks are particularly prone to blowing out and that is the substrate in my situation.
• Positioning of the fixings:
  • Ideally the fixings are centred on bricks, this will be difficult to achieve given the PIR sheet will be installed against the brickwork hiding all the joints and bricks 
• Compromised VCL 
  • Drilling through the batten and PIR with a hammer action masonry bit will certainly blow out the back of the batten and PIR. I suppose this damage to the VCL could be reduced by drilling a pilot hole with timber spade bit until the brickwork face is reached but regardless the drilled hole through the VCL is a potential moist air route.
•  Plumbing the battens 
  • Where some battens need to be pulled out from the wall to plumb this again will contribute to the likelihood of moist air routes

Giving the above I'm considering how this alternative arrangement may work:

• PIR is fixed to brickwork face solely with low expansion foam.
• Metal studding (stood away from the PIR by 10mm or so) provides the framework to carry the plasterboard.
• Metal studs are fixed solely with a floor track and head track (perhaps a strongback is added) thereby no fixing penetrate the VCL.

Appreciate any critique of this method.

The PIR has been purchased (bought some months ago during some manner of flash sale) so even though some disagree with its use it has not to be used, plus the fact that the job needs to get done asap now.

 

 

 

   

Edited November 16 by Annker

[BadgerBodger]

@Annker

 

I am planning a barn conversion with IWI. I think we are considering similar strategies. 
 

Architects preference was for natural breathing solution but my preference was for a more commercial bag it up and stop it moving solution. 

In the most extreme case the external walls deviate 200mm from plumb…. We have an internal steel frame and are looking at  currently looking at a minimum 50mm ventilated cavity, treated timber stud with PIR infill between. Likely 120mm. VCL to the inside face counterbatten and then PIR backed board to the front which will also be used to cloak the steel frame (potentially with and addition 25mm board to further limit cold bridging). Window openings will have VCL dressed through to the frame and the PIR bonded board will form the reveal which will vary I depth but be a minimum of 350mm deep. 
 

I’m considering the use of a breather on the outside face of the stud to protect the timber better but I think it might be unnecessary and make buildability challenging. Currently planning erecting stud frames at ground level and offering up for fixing to steel. 
 

If I were in your position, and wanting square wall I’d batten up at fixed centres and plumb with packers before affixing the next product. IMHO VCL could then be either to front or rear of PIR and dare I say it if you’re just driving improvement but without a specific figure you could just use foil faced PIR and tape with alutape… that’s the rougher side of me. Compromised VCL is almost always inevitable and I take one of two approaches I see as being practical and pragmatic. Either tape VCL to timber battens using some butyl tape strips then PIR backed PB could be directly affixed with the butyl tape “self healing) the fixing point OR (and this is how I always approach it) sandwich the VCL between two sheet products. In this case PIR and PB. The “trapped” nature of the fixing point and sandwiching between two impermeable products means leakage is minimal. The first option is a belt and braces version of the latter. 
 

Also might be worth considering a gypliner system… these can form a void of 30mm if I recall. Quick, easy, cheap and maintains a continuous cavity…

 

https://www.british-gypsum.com/Systems/wall-linings/gyplyner-single#description

 

Other variants exist

 

Edited November 16 by BadgerBodger
Addition of gypliner

[Annker]

50 minutes ago, BadgerBodger said:

@Annker

 

I am planning a barn conversion with IWI. I think we are considering similar strategies. 
 

Architects preference was for natural breathing solution but my preference was for a more commercial bag it up and stop it moving solution. 

In the most extreme case the external walls deviate 200mm from plumb…. We have an internal steel frame and are looking at  currently looking at a minimum 50mm ventilated cavity, treated timber stud with PIR infill between. Likely 120mm. VCL to the inside face counterbatten and then PIR backed board to the front which will also be used to cloak the steel frame (potentially with and addition 25mm board to further limit cold bridging). Window openings will have VCL dressed through to the frame and the PIR bonded board will form the reveal which will vary I depth but be a minimum of 350mm deep. 
 

I’m considering the use of a breather on the outside face of the stud to protect the timber better but I think it might be unnecessary and make buildability challenging. Currently planning erecting stud frames at ground level and offering up for fixing to steel. 
 

If I were in your position, and wanting square wall I’d batten up at fixed centres and plumb with packers before affixing the next product. IMHO VCL could then be either to front or rear of PIR and dare I say it if you’re just driving improvement but without a specific figure you could just use foil faced PIR and tape with alutape… that’s the rougher side of me. Compromised VCL is almost always inevitable and I take one of two approaches I see as being practical and pragmatic. Either tape VCL to timber battens using some butyl tape strips then PIR backed PB could be directly affixed with the butyl tape “self healing) the fixing point OR (and this is how I always approach it) sandwich the VCL between two sheet products. In this case PIR and PB. The “trapped” nature of the fixing point and sandwiching between two impermeable products means leakage is minimal. The first option is a belt and braces version of the latter. 
 

Also might be worth considering a gypliner system… these can form a void of 30mm if I recall. Quick, easy, cheap and maintains a continuous cavity…

 

https://www.british-gypsum.com/Systems/wall-linings/gyplyner-single#description

 

Other variants exist

 

Not sure if you missed it but my initial design is warm batten, meaning the battens are fixed on the warm side of the insulation layer.

I wouldn't be too keen having the battens cold side of the insulation as I think you are suggesting, correct me if Im wrong!

 

I'm familiar with gyplyner however the issue with it in my situation is that it is required to be fixed to the wall; in contrast to my proposal of metal studs which don't necessarily need fixing to the wall; floor and head track fixing will suffice.

 

That looks like an interesting project you have there, spacious and I think the arrangement you have designed will work well. Ventilated cavity is a good option although one I dont have space for on my own project.

 

[BadgerBodger]

@Annker I see.
 

I can’t tell, are you planning to use a VCL or use the PIR as the VCL?
 

If the latter… Would the infilled stud option with insulation between not be an option akin to mine then? The difference being your VCL sits outside in the “cavity” and the stud frame is on the “dry” side. An infilled stud allows for better optimisation of space if it is scant.  Service routes could the. Be routed in the PIR or you could use a nominally deeper stud to allow the PIR to sit at the back and have service void to the front? 
 

I appreciate that may not result in a “warm batten” design but I would consider it as an option. 

 

 

Edited November 16 by BadgerBodger

[Annker]

1 hour ago, BadgerBodger said:

@Annker I see.
 

I can’t tell, are you planning to use a VCL or use the PIR as the VCL?
 

If the latter… Would the infilled stud option with insulation between not be an option akin to mine then? The difference being your VCL sits outside in the “cavity” and the stud frame is on the “dry” side. An infilled stud allows for better optimisation of space if it is scant.  Service routes could the. Be routed in the PIR or you could use a nominally deeper stud to allow the PIR to sit at the back and have service void to the front? 
 

I appreciate that may not result in a “warm batten” design but I would consider it as an option. 

 

 

Yes, I meant to clarify that the PIR will be the VCL.

And yes insulation infill between the studs is an option but then continuity of the insulation layer is lost and its also more faffing to fit and fit well.

My vote would be for a continuous uninterrupted, say, 50mm insulation layer; rather than a, say, 90mm insulation layer between stud that may also be a bit gappy. Although I have made no calculation to support that opinion!   

 

A word of caution regarding running services (specifically cabling) within PIR. Anecdotally at least I hear that running electrical cabling within PIR may present a fire risk.  

 

I've researched the hell out of IWI in a solid walled building, and the one thing I'd say is take absolute opinions and desk study theory's with a pinch of salt, there are many ways to skin this cat.

No two properties are the very same and there is clear evidence that an approach that worked/failed in house A has been shown to work/fail in house b.

For example, site location and aspect play a significant role but are frequently overlooked 

A house built on a well sheltered site on free draining sand in Kent is going to be less sensitive to dampness than one built in a marshy Welsh valley. 

[BadgerBodger]

Yes, I’m inclined to agree. There doesn’t seem to be a single solution for all scenarios. 
 

interesting observation regarding the PIR and wiring. I imagine it’s a retained heat issue, something that can likely be overcome by oversizing the cabling for experience with other cable overheating issues. Something I’ll discuss with my architect for sure. 

[Iceverge]

11 hours ago, Annker said:

Appreciate any critique of this method.

The PIR has been purchased (bought some months ago during some manner of flash sale) so even though some disagree with its use it has not to be used, plus the fact that the job needs to get done asap now.

 

Sell the PIR again or put it in the floor. 

 

Airtight the house rigidly, (can be done room by room)

 

Parge for the walls, tape the windows, airtight paint for joist ends and anything tricky.

 

Ensure you return the parge to all internal walls too by at least the thickness of the wall. 

 

 

Build 63mm CLS studs internally independent of the wall. Fit mineral wool batts and plasterboard. 

 

Provided yiu do a good job with airtightness its's a very safe, cheap and easy install. The PIR is really not a good idea. 

 

 

[Annker]

10 hours ago, Iceverge said:

 

Sell the PIR again or put it in the floor. 

 

Airtight the house rigidly, (can be done room by room)

 

Parge for the walls, tape the windows, airtight paint for joist ends and anything tricky.

 

Ensure you return the parge to all internal walls too by at least the thickness of the wall. 

 

 

Build 63mm CLS studs internally independent of the wall. Fit mineral wool batts and plasterboard. 

 

Provided yiu do a good job with airtightness its's a very safe, cheap and easy install. The PIR is really not a good idea. 

 

 

Just wondering if some specific instance has come about that has you against PIR?

I ask because previously your opinion was that PIR would be fine in this situation.

 

Appreciate that you are just offering an opinion that I'm asking for, and its welcomed!

 

[Iceverge]

1 hour ago, Annker said:

because previously your opinion was that PIR would be fine in this situation.

 

Don't think I said this. Remind me. 

 

1 hour ago, Annker said:

Just wondering if some specific instance has come about that has you against PIR?

 

Have a look at this recent thread. 

 

The walls were previously done with kingspan PIR and plasterboard. I very much doubt anything was done about Airtightness. 

 

Look at how damp all the structure is behind the removed boards. Similarly note the rot in the timber lintels. 

 

A house that was internally dry for at least a century suffered these kinds of problems in just a few years because of inappropriate internal insulation. 

 

Edited November 17 by Iceverge

[Annker]

6 hours ago, Iceverge said:

 

Don't think I said this. Remind me. 

 

 

Have a look at this recent thread. 

 

The walls were previously done with kingspan PIR and plasterboard. I very much doubt anything was done about Airtightness. 

 

Look at how damp all the structure is behind the removed boards. Similarly note the rot in the timber lintels. 

 

A house that was internally dry for at least a century suffered these kinds of problems in just a few years because of inappropriate internal insulation. 

 

A had a previous thread where I outlined my general design.

This was your comment I was referring to:

That's an interesting project/thread you'd linked above, I hadn't come across that one.

To me the source of the dampness seems to have been the condition of the roof line and those cracks in the render, and the cement render largely contributing to its persistence.

Would a mineral wool or natural fiber based IWI fared any better in that situation? I cant see that it would myself.

Regardless insulated plasterboard dot and dabbed on to sand and cement walls and a lack of any airtightness (as you point out) I feel is pretty far from my own build up.

And on top of it all the house is in Kerry, Kent were I am is the Costa de Sol by comparison, (and I say that as a Galway man!)

 

The risk of damage to timber within the wall is something I'm concerned about, I guess its a risk all IWI solutions to varying degrees, seeing as they all lower the existing wall temperature.

I do like your suggestion to seal in the joist ends with airtight paint, possibly airtight tape wrapped around the joist could perform similarly well should someone indeed use PIR!🤠

Do you think it would be wise to also inject boron into the joist ends, and timber lintels as I also have a serval of these present. 

 

       

     

 

Edited November 17 by Annker
Spelling

[haythorn_1]

On 16/11/2024 at 20:12, Iceverge said:

Parge for the walls, tape the windows, airtight paint for joist ends and anything tricky.

Interesting. I would have presumed you would do the whole thing.

 

Does it make a difference what kind of internal wall it is? Say if it’s a cavity?

[Iceverge]

11 minutes ago, haythorn_1 said:

Interesting. I would have presumed you would do the whole thing.

 

Does it make a difference what kind of internal wall it is? Say if it’s a cavity?

 

A Cavity wall is far less risky as the cavity will be either ventilated or insulated. 

 

A ventilated cavity will be excellent for allowing drying towards the outside of the inner brick. 

 

The insulated cavity will keep the inner leaf far warmer dissuading condensation. 

[ab12]

On 17/11/2024 at 01:12, Iceverge said:

 

Parge for the walls, tape the windows, airtight paint for joist ends and anything tricky.

 

Ensure you return the parge to all internal walls too by at least the thickness of the wall. 

 

Build 63mm CLS studs internally independent of the wall. Fit mineral wool batts and plasterboard. 

 

Provided yiu do a good job with airtightness its's a very safe, cheap and easy install. The PIR is really not a good idea. 

 

 

Hi Iceverge

Can you please elaborate a little bit on your method of choice for IWI.

Am I correct in thinking all the walls whether internal or external facing have a parge coat of plaster (lime?) and then for externally facing walls CLS stud work is applied to the wall? and filled with a breathabl material.

Is the 63mm CLS Stude essentially a 3inch by 2 inch stud?

Also what centres would you go for?

Would you fit the studs independant of the wall or attached to the wall? If independent is there niot a change that the studs may not be stable?

 

Novice here so apologies for the basic nature of the questions.

 

Thanks

[Iceverge]

The parge is a mix of sand lime cement in the ratio 3:2:1.  It is applied to make the interior face of your external walls as your airtight layer.  You should return it on any internal walls by say 3-600mm  to avoid any leaks at this junction. 

 

I choose 63 mm CLS timber because it is the thinnest studwork lightly to be any way straight. You could screw it to the wall but it'd need a lot of shims and drilling to get it straight and fixed. If you feel it us still wobbly you could fix it through with a hammer in fixing here and there to the wall. You'll need to shim it to avoid it being pulled out of line though

 

Fix a top and bottom plate using a vertical laser level, mark your centres and cut your studs to fit and you'll be through it in no time by comparison. 

 

I would probably use 400mm centres but for the most efficient use of labour I would make sure its a snug fit for whatever your preferred batt insulation is. 

 

Normally 12.5mm plasterboard is best kept to 400cc.

 

 

Edited November 20 by Iceverge

[Annker]

57 minutes ago, Iceverge said:

The parge is a mix of sand lime cement in the ratio 3:2:1.  It is applied to make the interior face of your external walls as your airtight layer.  You should return it on any internal walls by say 3-600mm  to avoid any leaks at this junction. 

 

I choose 63 mm CLS timber because it is the thinnest studwork lightly to be any way straight. You could screw it to the wall but it'd need a lot of shims and drilling to get it straight and fixed. If you feel it us still wobbly you could fix it through with a hammer in fixing here and there to the wall. You'll need to shim it to avoid it being pulled out of line though

 

Fix a top and bottom plate using a vertical laser level, mark your centres and cut your studs to fit and you'll be through it in no time by comparison. 

 

I would probably use 400mm centres but for the most efficient use of labour I would make sure its a snug fit for whatever your preferred batt insulation is. 

 

Normally 12.5mm plasterboard is best kept to 400cc.

 

 

@Iceverge I'm trying to get my head around the parge coat, specifically its position in your build-up.

Isn't it "cold side" of the insulation layer and therefore internally borne, warm moisture laden air can past through the mineral wool and potentially condense behind the mineral wool on the relatively cold surface of the parge coat/brickwork?    

 

Or perhaps the ideas is the breathable mineral wool allows that condensate to evaporate back into the room?

 

Edited November 20 by Annker
Adding Clarity

[Mike]

1 hour ago, Annker said:

I'm trying to get my head around the parge coat, specifically its position in your build-up

A parge coat is used to make the wall airtight, for three reasons. First, it stops stop cold outside air from reaching the insulation, which would degrade the effectiveness of the insulation and potentially lower the temperature within the insulation to the point where condensation forms - which is why it needs to be on the outside of the insulation. Second, it stops the wind from blowing outside air in and drawing inside air out. Thirdly, it stops air escaping due to the 'stack effect' - that is, even on a day without wind, warm moist internal air is more buoyant than the outside air, so will rise and escape through any gaps, drawing outside air in as it does so.

 

1 hour ago, Annker said:

internally borne, warm moisture laden air can past through the mineral wool and potentially condense behind the mineral wool on the relatively cold surface of the parge coat/brickwork?    

If the building is airtight, the air pressure inside the house and inside the insulation will be more-or-less identical, so there's nothing to drive that warm moisture laden into the insulation. So, in answer to your question, no, it won't.

 

That leaves only differentials in relative humidity as the means by which moisture (moisture vapour) can move through the materials and into / out of the insulation. Which is why, if one has been specified, a vapour control layer will always be on the the warm side of the insulation (or at least not more than 1/3 of the way into the insulation, according to a well-known rule of thumb).

 

Edited November 20 by Mike

[Annker]

26 minutes ago, Mike said:

A parge coat is used to make the wall airtight, for three reasons. First, it stops stop cold outside air from reaching the insulation, which would degrade the effectiveness of the insulation and potentially lower the temperature within the insulation to the point where condensation forms - which is why it needs to be on the outside of the insulation. Second, it stops the wind from blowing outside air in and drawing inside air out. Thirdly, it stops air escaping due to the 'stack effect' - that is, even on a day without wind, warm moist internal air is more buoyant than the outside air, so will rise and escape through any gaps, drawing outside air in as it does so.

 

If the building is airtight, the air pressure inside the house and inside the insulation will be more-or-less identical, so there's nothing to drive that warm moisture laden into the insulation. So, in answer to your question, no, it won't.

 

That leaves only differentials in relative humidity as the means by which moisture (moisture vapour) can move through the materials and into / out of the insulation. Which is why, if one has been specified, a vapour control layer will always be on the the warm side of the insulation (or at least not more than 1/3 of the way into the insulation, according to a well-known rule of thumb).

 

Thanks for the explanation Mike, useful.

 

So the parge coat primary deals with internal born moisture issues.

It stops moisture transportation (by airtightness), limiting internal born moisture being drawn into the wall build up?

 

The preference of mineral wool/fibre insulations over PIR, is primarily associated with external born moisture issues.

Mineral wool/fibre insulation's provide better drying to inside of any external borne moisture that makes its way to the interface of internal face of the brickwork and insulation layer?

 

Am I on the right track with thinking those are the principals?   

 

  

[Iceverge]

 

It's easy to do something like draw a layer of PIR taped correctly but another thing altogether to try to do it in reality. 

 

I built a passive house, really understood my junctions and airtighness layer, and did the necessary work to get it airtight. Having seen the level of detailing and labour required I think that attempting to do so in an old house with internal boards is absolutely futile. You will have leaks, you will have gaps behind the boards and you will get interstitial condensation. The impermeable nature of the PIR will prevent the wall from drying apart from the miniscule cracks where the vapour got in in the first place.

 

My main preference for a parge coat and mineral wool is that good real world performance is actually achievable.

 

Parge and airtighess paint is very forgiving on wonky surfaces and corners around timber etc. The parge is vapour permeable so all the wall can dry. The mineral wool can flex to old wonky walls so there's no gaps for condensation to occur. 

 

 

 

 

[lookseehear]

Would it be fair to say that flexible wood fibre would be a step up from mineral wool here from a moisture buffering perspective?

[Annker]

1 hour ago, Iceverge said:

 

It's easy to do something like draw a layer of PIR taped correctly but another thing altogether to try to do it in reality. 

 

I built a passive house, really understood my junctions and airtighness layer, and did the necessary work to get it airtight. Having seen the level of detailing and labour required I think that attempting to do so in an old house with internal boards is absolutely futile. You will have leaks, you will have gaps behind the boards and you will get interstitial condensation. The impermeable nature of the PIR will prevent the wall from drying apart from the miniscule cracks where the vapour got in in the first place.

 

My main preference for a parge coat and mineral wool is that good real world performance is actually achievable.

 

Parge and airtighess paint is very forgiving on wonky surfaces and corners around timber etc. The parge is vapour permeable so all the wall can dry. The mineral wool can flex to old wonky walls so there's no gaps for condensation to occur. 

 

 

 

 

I'm coming around to mineral wool option, it would mean selling the PIR but that can be done at a minimal loss.

 

The brickwork doesn't take fixings too well, so the studwork will be fixed at floor and ceiling level, no intermediate fixings to the brickwork, also the floor to ceiling height is ~3m, so that all points me towards using metal studding.

 

With that in mind I'm unsure how mineral wool batts could be kept in place, tight against the face of the brickwork.

Fitted in between studs will mean an absence of insulation behind the studs; hence a gap for condensation to occur (this will be the case with either timber or metal studs). 

I suppose mineral wool could be simply packed behind the studs or perhaps first install a full layer in landscape format, fix studwork and then infill studwork.