Fine tuning my IWI Solid wall (Warm Batten) design

[Iceverge]

On 25/07/2025 at 16:18, Annker said:

Yes similar setup to your arrangement.

The brackets are needed for that particular system though, as the studs are gyplyner type not  C or I type. 

 

I actually called Rockwool tech dept with a couple of questions.

The fella I spoke to recommended/was insistent that in IWI usage the rockwool batts are friction fitted between studs and that the studwork (and batts) are both stood 25mm off the existing brick wall. In effect creating a 25mm (unventilated) cavity space.

 

@Iceverge in the example you shown earlier I understood that the rockwool batts were pressed up tight against the existing brick work.

That arrangement makes sense to me, as otherwise the unventilated cavity is created and that seems like an environment where condensation could potentially form.

Whenever I speak to product tech depts I invariably think I should not have bothered calling!

 

 

   

 

Correct, no gap. 

 

It's a bad idea as it will allow internal air to circulate freely and condense on the internal face of the existing wall causing damage. Espically at joist ends. 

[Annker]

8 hours ago, Iceverge said:

 

Correct, no gap. 

 

It's a bad idea as it will allow internal air to circulate freely and condense on the internal face of the existing wall causing damage. Espically at joist ends. 

Understood, that makes sense to me.

[mistake_not]

Slight tangent. What paint are you / folks finishing IWI like this with? I'm looking at using something breathable which rules out the normal vinyl Matt etc I think.

[SimonD]

1 hour ago, mistake_not said:

Slight tangent. What paint are you / folks finishing IWI like this with? I'm looking at using something breathable which rules out the normal vinyl Matt etc I think.

 

I've used mainly Auro clay paint as it's ultra breathable, but there is also Earthborn clay and I've recently found a new company that does clay pain called Coat (not tried them). Auro clay paint is sprayable so very helpful in that. The normal Auro paints are also breathable but nowhere near as much as the clay versions.

[Redbeard]

If low cost is of the essence then 'contract matt' (once used by volume builders, AIUI, to allow painting a bit 'too early' over plaster - perhaps less so now with taped joints) is probably the one to go for. As @SimonD said, avoid anything with Vinyl in the title (and I think, but don't have evidence to prove, that anything 'silk' may/will be less breathable.

[torre]

We've used Keim paints for breathability - excellent but expensive - with lime plaster. As it sounds like breathability will be limited here by gypsum boards then as  @Redbeard suggests, trade matt is probably a decent choice

[Iceverge]

I'm a fan of this for allowed walls to dry. 

 

 

Resembles a thin layer of chalk almost when dry. 

 

Easy to touch up too and it's dirt cheap. 

[mistake_not]

 

3 hours ago, Iceverge said:

I'm a fan of this for allowed walls to dry. 

 

 

Resembles a thin layer of chalk almost when dry. 

 

Easy to touch up too and it's dirt cheap. 

Before I have just used contact matt as a mist coat, but realised with some modern paints to need to use the right base coat / primer as some folks have had issues with contact matt as a base coat and paints peeling.

[Mike]

10 hours ago, SimonD said:

I've used mainly Auro clay paint as it's ultra breathable

I'll be using clay paints too, for the same reason.

[Iceverge]

3 hours ago, mistake_not said:

 

Before I have just used contact matt as a mist coat, but realised with some modern paints to need to use the right base coat / primer as some folks have had issues with contact matt as a base coat and paints peeling.

 

True, I reckon you could literally scrub it off the walls if you had a scourer. 

 

 

[Annker]

OP here. Just asking for a sanity check.

I have a number of various gaps and penetrations that need to be air tighten prior to IWI.

Some are large and will be awkward to detail with tapes, membranes or even airtight paint.

 

I am wondering if I could just bulk fill them out with Illbruck FM330, it certainly would be much, much easier to apply and have confidence that it seals up.

I imagine where the gaps are simply missing bricks or where steel is adjacent FM330 is a suitable choice.

Question is will FM330 also be suitable to use where timber joists are pocketed into the wall, or may it cause a dampest issue against the joists?  

 

 

   

[lookseehear]

What I've been doing is using lime hemp plaster (pre bagged from Ty Mawr) to fill these gaps with some appropriately sized stones. Basically find a stone that nearly fits, put some lime plaster in and then push the stone into place, then cover the lot with more lime plaster around the joists. I'm then going to airtight tape the joists back to the plaster.

 

It's been a lot harder than I thought in an old house. You take the ceiling down and there are voids in the wall all over the place.

[Annker]

@lookseehear That sounds like a good solution, I believe lime against timber shouldn't encourage any decay. Would airtight paint not be a better solution vs using a tape?

I imagine slapping on paint is much easier way to close up the small gaps the lime plaster misses.  

[lookseehear]

I have got some airtight tape and haven't got any airtight paint which is steering my decision! Here are some pics of what I've done.

 

[Mike]

9 hours ago, Annker said:

Some are large and will be awkward to detail with tapes, membranes or even airtight paint.

 

I am wondering if I could just bulk fill them out with Illbruck FM330,

In some situations I've done similar to @lookseehear, in others (particularly for some odd timber-to-timber junctions) I've used FM330 as you proposed, sometimes with anti-rodent mesh embedded in it.

[Annker]

36 minutes ago, Mike said:

In some situations I've done similar to @lookseehear, in others (particularly for some odd timber-to-timber junctions) I've used FM330 as you proposed, sometimes with anti-rodent mesh embedded in it.

Thanks for the response Mike, I have a few cans of FM330 in the store so these voids will get filled in the morning👍

[Annker]

 In the past week or so I have noticed seemingly damp patches showing through the parge coat at the base of the ground floor walls, pretty much around the entire external perimeter of the house. 

The damp patches do not feel noticeably damp to the touch, however they are certainty visible and register ~25% on the damp meter, as opposed to ~12% where dry (I am aware that this damp meters measure salts as a proxy)

Points to note:

• There is a reasonable dry cellar below the ground floor, it is damp but certainly not flooded.
• External ground level surround the house ranges between 1 foot to 2 foot below internal floor level and is freely draining. 
• Brickwork external face does not appear to be wet, certainly no obvious correlation with the patches   
• Underground drainage is in good shape
• Roof line is also in good shape.    

So the issue is I cannot see any obvious source of this apparent dampness, could this be a case of the much debated rising damp?

I'm concerned that if I just crack on and install the IWI as is, I may have mould issues down the line.

Appreciate to hear if anyone else has observed a similar situation.

I imagine the IWI can handle some moisture but how much?

Current IWI design is Rockwool batts within a timber stud perhaps using Steico flex wood fibre boards instead of Rockwool may be a safer in these seemingly damp areas?

 

TIA

 

[Mike]

On 18/09/2025 at 12:40, Annker said:

So the issue is I cannot see any obvious source of this apparent dampness, could this be a case of the much debated rising damp?

The corner of a building is often where rainwater pipes were fitted, and a leak in one of those could have been another source, even if subsequently removed. Past sources of moisture are relevant as they could have caused salts to migrate through the brickwork and be deposited on the interior as the moisture evaporated. And salts also absorb moisture from the atmosphere and so can appear damp - which could be what you have, even if the source of moisture was removed years ago.

 

To know for sure, you'd need to get some tests for dampness and salts. It's possible to DIY them, but easier to get someone in; the first such company in my Google search appears to do that https://www.coreconservation.co.uk/technical-page/the-effect-of-salts-in-old-buildings/.

 

One solution to salts is to apply a temporary coat of lime plaster ('lime poultice') to the wall, which will draw the salts out of wall and into the plaster, which then gets hacked off. However it will take time and possibly multiple applications - so not ideal. However I see that Core Conservation also sell a breathable salt-resistant lime plaster - I've not come across it before, but it looks a potentially quick solution.

 

On 18/09/2025 at 12:40, Annker said:

Current IWI design is Rockwool batts within a timber stud perhaps using Steico flex wood fibre boards instead of Rockwool may be a safer in these seemingly damp areas?

I wouldn't use either until this is resolved.

[Annker]

On 23/09/2025 at 15:19, Mike said:

The corner of a building is often where rainwater pipes were fitted, and a leak in one of those could have been another source, even if subsequently removed. Past sources of moisture are relevant as they could have caused salts to migrate through the brickwork and be deposited on the interior as the moisture evaporated. And salts also absorb moisture from the atmosphere and so can appear damp - which could be what you have, even if the source of moisture was removed years ago.

 

To know for sure, you'd need to get some tests for dampness and salts. It's possible to DIY them, but easier to get someone in; the first such company in my Google search appears to do that https://www.coreconservation.co.uk/technical-page/the-effect-of-salts-in-old-buildings/.

 

One solution to salts is to apply a temporary coat of lime plaster ('lime poultice') to the wall, which will draw the salts out of wall and into the plaster, which then gets hacked off. However it will take time and possibly multiple applications - so not ideal. However I see that Core Conservation also sell a breathable salt-resistant lime plaster - I've not come across it before, but it looks a potentially quick solution.

 

I wouldn't use either until this is resolved.

Thanks Mike, useful information.

Seems to be a lot of research on that website will have a proper read through later on.

 

Your point on RWP positions is spot on. There were leaky RWPs adjacent to the damp patches, now no longer an issue but likely they were leaking for years prior to I repairing so it's entirely likely that was the initial source of the dampness.

 

I know from past experience that saturated brickwork can take an age to dry out. The bricks certainly aren't saturated now and I'm happy my vapour open buildup will handle any remaining dampness.   

Additionally, I wonder if salts would cause a problem for me, as my newly finished wall surface will be separated from the salt affected patches on the wall. 

[Iceverge]

What is under the timber floor?

 

Has this change only come about since you applied the parge coat? 

[Mike]

On 25/09/2025 at 13:34, Annker said:

Your point on RWP positions is spot on. There were leaky RWPs adjacent to the damp patches... I know from past experience that saturated brickwork can take an age to dry out. The bricks certainly aren't saturated now and I'm happy my vapour open buildup will handle any remaining dampness.   

One thing I might consider doing then is to apply some cork insulation to the wall over that area, fixed with (NHL-based) Isovit E-Cork; cork isn't harmed by moisture but it, and the E-Cork, are breathable.

 

I've used that solution on my chimney breasts, partly due to the higher risk of moisture ingress there (using 1 or 2 layers of 20mm cork, with additional hemp insulation in places).

[Annker]

On 29/09/2025 at 22:26, Iceverge said:

What is under the timber floor?

 

Has this change only come about since you applied the parge coat? 

Below the floor is a cellar, damp but certainty not flooded and is it well ventilated.

I have installed an additional 6 or 8 wall vents and have good cross ventilation throughout the cellar.

Note. Prior to erecting IWI studwork I plan to run a rips of Intello AVCL along that gap for airtightness, although I'm unsure which side should face the room or cellar or if that matters

 

@Mike, just to copy you in:

 I have a humidity meter in the house and have noticed that the patched darken when the humidity rises (no sh*t Sherlock!)

However, I think that observation is important in the absence of any sign of external source of water ingress to the building.

 

Yesterday was a particular humid day I had left a rear sliding door open for a while and the humidity meter was reading 95% (matching the weather report).

 

Is it plausible that the those patches were historically damp, and are now sensitive to changes in humidity. Perhaps due to a concentration of salts.

Just to reiterate they certainly do not feel damp to the touch, so rightly or wrongly I'm not that concerned about treating them.

 

 

 

 

 

 

[Annker]

I get so much valuable information from people on this forum and feel I contribute little in return so I'm going to post up photos of the IWI install as I go, it may provide some insight to others doing a similar install.

Below are the various methods I used to seal around embedded joist ends.

 

Where the background wall was reasonably sealed around the joist I used airtight paint.

It is easy to use, fast to apply and gives a comprehensive covering.

 

In instances where the background was gappy such as around joist hangers I started of just taping the hell of them.

 

 

The Tesco Vana tape is good, you can mould it around a little and it gets a good adhesion to the parge coat and especially so to timber. (I also primered backgrounds with Tesco primer)

However in some other instances the mortar joints in the brickwork between joists was very gappy and would require jointing.

And in some of these instances there are timber lintels in the mix so I decided to fit and and tape sections of Intello plus, this meant that the surfaces to be adhered to were predominantly timber which the Vana tape does a great job of.

 

Lastly there were a few instances where the joists ran parallel and too close to maneuver a tape in to seal around.

Here I just build filled with Illbruk FM 330. These void seem dry so fingers crossed this bulk filling wont end up trapped moisture against the joist.

 

[Mike]

1 hour ago, Annker said:

Is it plausible that the those patches were historically damp, and are now sensitive to changes in humidity. Perhaps due to a concentration of salts.

That's very likely and exactly how salts respond to moisture; the same as would happen if you were to leave out a heap of table salt on a plate.

[Annker]

Timber for the IWI studwork has now been acquired and loaded out, walls are going to get built, it seems like years getting to this point!

 

However, there is an airtightness detail I have yet to get my head around and resolve.

The parge coat, taping around joists and other penetrations has eliminated draughts that could potentially transport moisture into the IWI build-up.
I suppose moisture that is transported through the ceiling and into the loft will be ventilated away in the cold roof in the typical fashion.  

But what about the junction wall the IWI assemble meets the ceiling?

   

Should my airtight layer continue up from the external brick wall and across my 1st floor ceilings; or can it terminate at the top of my external wall?

I can think of three options to deal with this:

• Intello plus AVCL installed extensively across the entire ceiling and taped to the external brickwork
• A ~300mm rip of Intello plus installed around the perimeter of the ceiling again then taped down to the external wall brickwork
• No AVCL required along ceiling level at all; any moisture in the the upper section of the IWI being ventilated away through the loft  

I can justify all three options in my head for various reason so would appreicate to heard any suggestion

Edited October 4 by Annker