Solid wall Internal Wall Insulation (Warm Batten) design

[Annker]

Dear all,

 

I am looking for some design critique on a IWI design which I am planning to install in my Victorian semi-d renovation project.

The existing walls are 9" solid brick with original lime mortar jointing and the house is sited in a relatively sheltered urban location in Kent.

 

Following weeks of reading the wide array of opinions regarding installation of IWI to a solid walled house I have arrived at the buildup below with two different insulation options:

• Internal to external 
  • Breathable paint finish
  • Gypsum skimcoat
  • Gypsum Wallboard 12.5mm
  • Service cavity 38mm x 50mm battens @ 400mm c/c
    • Intello plus VCL
    • 60mm Breathable Insulation layer
  • OR
    • 60mm PIR foil backed Insulation/VB layer 
  • 225 solid brick wall *Existing 

 

The design follows the "warm batten" arrangement.

Preference is for a vapour open design and use breathable insulation, however I am unsure what specific type product could be used in that instance.

I imagine the insulation would need to somewhat resist compression from the battens so perhaps batt type format.

HAs anyone used a glass or mineral wool slab in a similar situation? Wood fibre is a candidate but it is pricey.  

 

I believe a vapour closed design has more risk in terms of trapping moisture within the buildup, however PIR insulation with its relatively high compressive strength lends its well to the warm batten design so this option is easier to spec material wise.

 

After much reading I am satisfied that a gypsum plasterboard and finishing coat can provide a satisfactory degree of vapour permeability, hence their inclusion.  

Some will disagree (some who worship at the Lime Alter will be horrified) however there are studies that suggest that gypsum is more "breathable" than lime.

I actually spoke to a tech/sales member of staff of a well known eco & natural BM and he (of his own accord) said he regards gypsum as entirely suitable in some IWI system build up, in fact they have it specified in some of their IWI solution on their website.

 

[jayc89]

The problem with gypsum isn't its breathability, but the fact it's hygroscopic. With a permeable construction you're allowing water vapour to travel as it pleases, but if that condenses, your skim and plasterboard is knackerd.

Either go with a IWI solution recommended by the likes of Mike Wye/Ty-Mawr which will include a breathable insulation layer (wood fibre, sheep wool etc) and a lime plaster, which requires a lime wash/clay paint, or go with a impermeable build up, like PIR and warm batten and plasterboard. Don't mix the two. 

 

There isn't enough data out to make informed recommendations either way. I've done sections of my house using both; vapour permeable in the cellar, PIR elsewhere, and I'm not seeing any ill affects, however someone else could quite easily come along and show you images of damp/rot/mould caused by the same, or similar, build ups. There are clearly a number of variables to consider before making your choice. 

[Annker]

Thanks for the response.

I know you have posted a lot on the subject and believe me I have read reams of material on the subject and still can't confirm my spec!

 

Agreed that hygroscopy may be an issue with gypsum and my initial thought was that gypsum could not be used in vapour open system but as my original post it is a recommended detail from Back to Earth.

 

Surely given where the PB and skim layers are located in the build up condensation should not be an issue.

Modelling shows the Dewpoint within the insulation albeit it is towards the warmside; perhaps you saying the concern is if the Intello membrane allows vapour to travel outsode to inside should relative conditions drive it so?

  

 

 

 

 

 

[Iceverge]

What standard of workmanship are you intending to employ?!

 

Anything can work. You just need more "drying" than "wetting"

 

The big one with internal insulation is airtighess, most moisture gets into a structure via airpaths from inside. Not diffusion through materials. 

 

You can do PIR internally and expertly isolated any part of the cold external structure from the moist warm internal air and you'll be fine. 

 

However do a slapdash job with insulated plasterboard and you're asking for trouble.

 

 

 

 

[Annker]

8 hours ago, Iceverge said:

What standard of workmanship are you intending to employ?!

 

Anything can work. You just need more "drying" than "wetting"

 

The big one with internal insulation is airtighess, most moisture gets into a structure via airpaths from inside. Not diffusion through materials. 

 

You can do PIR internally and expertly isolated any part of the cold external structure from the moist warm internal air and you'll be fine. 

 

However do a slapdash job with insulated plasterboard and you're asking for trouble.

 

 

 

 

Thanks for the response @Iceverge

I am fortunate with regard workmanship/QC as I am a carpenter by trade (consider myself to have an eye for detail) and I will be doing the work with my own team. 

And I also have an appreciation for the level of attention that correct installation of an airtightness membrane requires. 

 

As my original post I have endlessly read every facebook group posts, forums, etc in researching IWI options.

PIR got eliminated for consideration early on, almost entirely due to the absolute contempt the heritage/lime community have for it. Obviously they have a bias but it can be hard to ignore. 

 

That led me down the path of vapour open/breathable system, however not being a die hard conversationalist I still wanted to integrate the buildability of modern solutions into the mix.

I also follow a house renovation channel on youtube which recently uploaded a video of a hybrid system called SWIP, and that in short is how I ended up with the bastard solution above with a foot in each camp if that makes sense.

 

There is so much conflicting opinion regarding IWI that I'm ready to pull my hair out with analysis paralysis!  .

I personally have not had, or seen a PIR IWI cause damp/mould, and it is a system I would prefer. 

I feel there is too much broad strokes when discussion the various systems. The only differences debated are of the systems themselves; relevance of where (location, orientation, aspect) and how (workmanship) is ignored.  

So I must say its a relief to hear some experienced hands form this forum give a vote for PIR!

 

I have modelled the PIR solution on Ubakus, and to me it looks ok. 

Humidity doesn't approach saturation point which I imagine is the most important thing.

 

 

 

 

 

 

 

[Mike]

On 19/04/2024 at 14:14, Annker said:

Preference is for a vapour open design and use breathable insulation, however I am unsure what specific type product could be used in that instance.

I imagine the insulation would need to somewhat resist compression from the battens so perhaps batt type format.

HAs anyone used a glass or mineral wool slab in a similar situation? Wood fibre is a candidate but it is pricey. 

On my French renovation, I'm using hemp batts. There's at least one UK company producing an equivalent - Hemspan Bio Wall.

 

A key advantage of natural fibres is that (unlike other products) they are usually good at the adsorption / desorption of water vapour - that is taking water vapour from the air into the fibre's cell walls (often a considerable quantity) and releasing it again. There are several technical benefits to that, provided you have a construction that's vapour-open ('breathable') to take advantage of those properties:

•     it reduces the risk of condensation
•     improves thermal comfort by keeping the air within a comfortable relative humidity range for longer
•     improves air quality by keeping the air within a healthy relative humidity range for longer (at <40% RH respiratory infections are more likely; at >60% bacteria and mould thrive)
•     tends to reduce heating and cooling requirements (adsorption of moisture releases heat, desorption takes heat, though research into quantifying this in buildings is limited)

There are also non-technical benefits - they use renewable materials, have low carbon emissions, support (potentially local) agriculture, and tend to be pleasant products to work with.

 

On the other hand they take up more space than PU foam or the like for the equivalent u-value. My IWI is 100mm / 145mm thick depending on elevation & space available.

[Mike]

On 19/04/2024 at 14:32, jayc89 said:

The problem with gypsum isn't its breathability, but the fact it's hygroscopic. With a permeable construction you're allowing water vapour to travel as it pleases, but if that condenses, your skim and plasterboard is knackerd.

It's correct that condensation can cause problems with gypsum & plasterboard, but there's no need to worry at the exposed wall surface. What you don't want though is gypsum left on the original wall that you're insulating, as that's where condensation is most likely.

[Mike]

10 hours ago, Annker said:

I have modelled the PIR solution on Ubakus, and to me it looks ok. 

Humidity doesn't approach saturation point which I imagine is the most important thing.

Ubakus uses the Glaser modelling technique which, unfortunately, is too simplistic. The gold standard is dynamic modelling (WUFI being the main software used), which takes into account multiple factors - like those you mention (location, orientation, aspect) as well as climate data - and models the element over the course of several years. However if you're not going too low with the U-value then that's normally not necessary. Wall U-values down to 0.5 W/m²K are normally 'safe', while research in Scotland suggest that values down to 0.35 are likely safe too.

 

FWIW I have a very peculiar non-ventilated ceiling that I'd like to insulate to the max (with hemp insulation batts), so I have just commissioned someone to model that in WUFI to determine how low I can go.

 

Edited April 20 by Mike

[Annker]

@Mike Yes I agree that a vapour open system put together with natural fibres fairs better should significant amounts of moisture penetrate the external brickwork.

 

My hope is that that favourable site conditions (well sheltered semi-d, lime pointed brickwork, floor levels ~300mm to 900mm above external GL) and a conscientious install of the VB should eliminate or at least minimise moisture levels within the wall being raised to a level that will cause issues.

 

I have read elsewhere alright that Ubakus is not that comprehensive.

I played around with the outside temp & humidity values to somewhat compensate for that.

 

It was interesting to see that with the outside temp set to -10 degrees the model with 60mm PIR remined condensate free.

However condensation was indicated in the model with 60mm wood fibre (screen grab below)

Not sure if that observation has any validity/relevance.

    

 

  

[Annker]

11 hours ago, Mike said:

It's correct that condensation can cause problems with gypsum & plasterboard, but there's no need to worry at the exposed wall surface. What you don't want though is gypsum left on the original wall that you're insulating, as that's where condensation is most likely.

The existing internal wall lining on the externals wall is lath & plaster, all of that will be removed so the proposed IWI will be on to barefaced brick. 

[Mike]

6 hours ago, Annker said:

The existing internal wall lining on the externals wall is lath & plaster, all of that will be removed so the proposed IWI will be on to barefaced brick. 

Sounds good! You'll probably need to apply a parge coat over the brickwork to ensure all the joints are airtight.

 

6 hours ago, Annker said:

Yes I agree that a vapour open system put together with natural fibres fairs better should significant amounts of moisture penetrate the external brickwork.

You really don't want moisture (in the form or rain or the like) penetrating the external brickwork to any great extent. It's moisture vapour, coming (mostly) from within the house, that natural fibres are good at buffering (adsorption). You don't want them soaking up liquid (absorption). The U-values of 0.5 & 0.35 that I mentioned are only 'safe' if the wall is protected from rain penetration or ground moisture, which yours seems to be.

 

6 hours ago, Annker said:

I have read elsewhere alright that Ubakus is not that comprehensive.

I played around with the outside temp & humidity values to somewhat compensate for that

The Glaser method that Ubakus uses just isn't up to predicting problems with internal wall insulation.

 

6 hours ago, Annker said:

It was interesting to see that with the outside temp set to -10 degrees the model with 60mm PIR remined condensate free.

However condensation was indicated in the model with 60mm wood fibre (screen grab below)

Not sure if that observation has any validity/relevance.

None at all - the results aren't valid for the purpose. There are many factors it fails to take into account.

 

Edited April 21 by Mike

[Annker]

2 hours ago, Mike said:

Sounds good! You'll probably need to apply a parge coat over the brickwork to ensure all the joints are airtight.

I have considered a parge coat. Given I've ~130 sqm of walling to cover, it would be a significant addition cost so I am debating if it is required

 

Firstly, the internal face of the walls are plumb floor to ceiling but there are discrepancies in flatness. 

So a parge coat would fill out those discrepancies and hollows that otherwise may exist and I imagine could provide space for moisture to gather.

 

Secondly, as you say a parge coat would fill any brickwork joints, and there are a few joints that aren't fully filled.

However is the parge coat required to provide an airtight barrier?

Going down the vapour closed route the PIR layers, and their perimeter, will need to be meticulously sealed, that is critical for the system to work. I need to ensure that provides an airtight layer.

I wonder if I can consider a parge coat would optional second line of airtightness defence that I dont need to spend money on, or perhaps I should consider it as a critical component.      

 

[Mike]

1 hour ago, Annker said:

However is the parge coat required to provide an airtight barrier?

Going down the vapour closed route the PIR layers, and their perimeter, will need to be meticulously sealed, that is critical for the system to work. I need to ensure that provides an airtight layer.

One or the other is enough - it just needs to be very airtight.

 

1 hour ago, Annker said:

I have considered a parge coat. Given I've ~130 sqm of walling to cover, it would be a significant addition cost

Not that much of a cost - you only need to brush or trowel on a couple of mm thick - the cost is mainly labour. You have a cost whether you choose parge or PIR joint-sealing; probably not much difference.