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👍