IWI for 1930s solid brick wall

[Chloe]

Hi everyone,

I’m new to this forum and are looking for advice on an internal wall insulation (IWI) project for a 1930s semi-detached chalet. It has original solid brick walls (no cavity), but a complex mix of exterior claddings on our freezing North-facing elevation.

 

The Setup (North-Facing):

Ground Floor (Lounge): The exterior has an exposed original brick plinth at the bottom, and cement pebbledash on the upper half. Inside, there is a large original-style bay window with integrated shutter blinds.

 

First Floor Right (Master Bedroom): The exterior here is entirely covered in cement pebbledash. Also has a large bay window.

 

First Floor Left (Ensuite): This has a unique diagonal split exterior—the top half is vertical tile hanging, and the bottom half is cement pebbledash. Inside, we currently have severe black mould in the shower area on this external wall.

 

My Dilemma:

My builder has recommended Kingspan K118 (PIR) drylining for its high U-value and thin profile. However, I’m highly concerned about trapping moisture behind a non-breathable foil-backed board, especially where the exterior is wrapped in waterproof cement pebbledash.

I'd love your thoughts on these three specific issues:

 

1. Breathability vs. PIR: Given the cement pebbledash, would a vapour-permeable system (Wood Fibre + Lime Plaster) be definitively safer for the ground floor Lounge and the Master Bedroom to prevent interstitial condensation?

 

2. The Bay Window "Shutter Trap": I have integrated shutter blinds in the bays. If I use a breathable wood fibre system (typically 60mm+), the shutters will hit the new thicker walls and won't fold back. Has anyone safely used a thinner PIR board here by applying a breathable hydrophobic cream (like Stormdry/ProPerla) to the exterior pebbledash? Or is Aerogel the only safe thin option?

 

3. The Mixed Ensuite Wall (Jackoboard): My builder suggested stripping the mouldy ensuite wall back to brick and using Jackoboard (XPS) to create a waterproof, insulated "warm box" for the shower. Since the exterior of this specific wall is half breathable (tile hanging) and half non-breathable (pebbledash), is sealing the inside with vapour-impermeable XPS safe for the brickwork across both of those exterior finishes?

 

Any advice would be hugely appreciated! Thanks so much!!

[Redbeard]

Welcome!

 

Showing that an impermeable insulant works is generally done with the aid of a 'Glaser Method' condensation risk assessment (henceforth CRA so I don't have to type that again). Glaser method is the British Standard method, still, AFAIK. It is not a 'dynamic' tool and is a bit of a blunt instrument. Again, AFAIK, it always did assume, and I think still assumes that all moisture in a wall comes from inside (in the form of water vapour). Therefore a sheet of foil will stop all moisture and everything will be hunky dory. Each of your composite boards has a VCL. Where is the VCL at the joint? I have favoured wood-fibre (and a request for a waiver on the U value) for a long time, but if I was doing PIR for clients I would use 'raw' PIR at 50mm, taped at all joints and perimeters, battens to fix it to the wall and a further 25mm PIR before plasterboard. You have to be really picky to get the VCL right. I prefer WF which has no VCL. No VCL = no VCL to get wrong.

 

Just a pedantic point -sorry! - you say 'high U value'. High insulation value, which is a low U value.

 

Also, to get 0.3W/m2K on a solid brick wall you will need a board of about 70mm (60 insulation and 10ish plasterboard) so your shutters won't work anyway. What thickness is your builder proposing? Who is doing the Building Control application. (In case it has not been mentioned, you need one.) You could claculate the area-weighted U value so that you could have 'fat' insulation away from your shutter boxes and thin at the boxes, but you would need to be sure that the thin insulation would not give you a dew-point.

4 hours ago, Chloe said:

Has anyone safely used a thinner PIR board here by applying a breathable hydrophobic cream (like Stormdry/ProPerla) to the exterior pebbledash? Or is Aerogel the only safe thin option?

 

I am not sure how that would help? Do you think your bricks leak water? If they do, they'll leak with a thin thing or a thicker thing attached to them.

 

Re the bathroom XPS will give you a VCL (the XPS is its own VCL) which you need, unless you use Wood Fibre (which is not recommended for wet rooms). But thin board won't achieve the target 0.3W/m2K U value.

 

I hope this sounds positive, not negative! Queries welcomed.

[Redbeard]

1 hour ago, Redbeard said:

but you would need to be sure that the thin insulation would not give you a dew-point.

 

Sorry, that's drivel!! Reaching the dew-point on the inside of the external wall is more likely with thicker insulation, not thinner. I know that! I think my brain must have gone on holiday for a few minutes...

 

Oops!

 

Try this:

 

I would not recommend having no insulation at all in the shutter-boxes.  If you can use a high-grade insulant (such as the aerogel you referred to) to ensure that the surface is warmed up sufficiently in the shutter-boxes for condensation not to occur then (though the insulation value may still be cr*p compared to the rest of the newly-insulated wall, you may avoid a local mould problem inside the shutter-boxes.

[Chloe]

Hi Redbeard,

 

Thanks so much for the detailed replies—and for the correction on the dew point! That makes complete sense. By adding thick internal insulation, the original brick gets much colder, increasing the interstitial condensation risk if moisture gets through.

Your point about the shutter boxes is exactly my fear (creating a massive cold bridge). I am definitely planning to use a high-grade insulant there to stop condensation. I'm looking at something like 10mm Spacetherm WL (Aerogel) for the reveals and shutter boxes. As you mentioned, it won't hit the 0.30 U-value (Thanks for the correction on the U-values!) but hopefully, it warms the surface enough to prevent local mould.

 

This brings me to the Building Control aspect you rightly pointed out. Because I physically cannot hit 0.30 W/m²K in the bay windows without destroying the functionality of the integrated shutters, do you find that Building Control is usually open to granting a 'waiver' (or accepting an area-weighted U-value calculation) for these kinds of original/fixed features?

 

Regarding my original question about the exterior pebbledash: my main concern with using Kingspan/PIR on the flat walls isn't just that the bricks are leaky, but that the existing cement pebbledash is highly impermeable. If any moisture gets into the brick (or gets trapped during construction), it’s sandwiched between PIR foil on the inside and cement render on the outside.

Does treating the pebbledash with a breathable hydrophobic cream (like Stormdry) mitigate this risk enough to make PIR a sensible choice? Or, in your experience, is Wood Fibre (with no VCL to get wrong) really the only 'fail-safe' option for a cement-rendered solid wall?

 

My main concern is that my builder has no experience with wood fibre or lime plaster, and I'm worried that a poorly-installed breathable system might be more dangerous than a standard PIR system he knows how to install correctly.

 

Thanks again for your time—it's incredibly helpful!

[Redbeard]

If you go with Wood-fibre you could go to a merchant which offers (dynamic, more accurate, condensation risk assessment) WUFI for free.

 

Building Control depts are very variable. Mine used to be quite hard to get a conversation with, so if you were doing anything with any element of doubt you just had to go with your gut and hope the BCO agreed. Since they may only do 2 (or even one) visit(s) you don't, ideally, want to wait till you have done something (based on your best researches) to get a BCO opinion on whether you've done right. They should have no problem with an area-weighted calc, and should also be OK with you not quite achieving 0.3W/m2K (0.3 with WF may be an IC risk). I used to use the WUFI calc to back me up on that. On the other hand some merchants do a cost/benefit trade-off and suggest even less than I'd use. Not sure always how you get that past BCO.

 

My feeling re WF is that if it functions without a VCL then you don't have a VCL to get wrong. Just go tight as a tight thing and stuff any gaps with 'fluff' (cutting 'swarf' mixed with water). Always use a (lime, in my view) parge coat. Views about gypsum as an under-layer vary.

 

As I say, I have used PIR when the client cannot pay for WF, but I'd much rather use WF.

[Chloe]

Hi Redbeard,

 

Thanks again for the advice on the wood fibre vs. PIR. I’ve drafted the following and see if you have any comments. Thanks!  

 

North Walls (Lounge/Master Bed): 40mm Wood Fibre + 15mm Lime Plaster (~0.65 U value). This is the max I can go while keeping the shutter hinges functional.

 

North Window Reveals: 10mm Aerogel + thin-coat lime (~0.8 U value). Essential for a thermal break without blocking the shutters.

 

East Wall (Lounge): 60mm Wood Fibre (~0.48 U value). Since this is a flat wall without any window, I'm pushing for better performance here.

 

Finish: Parge coat for airtightness, mesh-reinforced lime plaster, and Claypaint/Silicate paint throughout the insulated zones.

 

Ensuite: 20mm Jackoboard (XPS) (~0.95U value) Primarily for waterproofing and as a space-saving tile-backer.

 

Does this setup look reasonable to you, or am I over-complicating it with the different

thicknesses?

 

My current builder is great but has zero experience with wood fibre or lime. Is this a "learn on the job" kind of task if they follow the manufacturer's guides, or is the risk of interstitial condensation/poor airtightness high enough that I should insist on a specialist for the insulation phase?

 

Do you have any recommendations for reliable wood fibre merchants that deliver to the Greater London / SE area?

 

Thanks for your advice again! 

 

Edited 6 hours ago by Chloe

[sgt_woulds]

Just to pull up @Redbeard slightly, although I agree with his preference for woodfibre (I work for a woodfibre manufacturer so that's a given 🙂), this is a situation where a membrane may still be required.  

 

The best advice here is for you to have your external walls assessed via hygrothermal software -  such as WUFI - which will take all of the site variables into account.  Internal wall insulation is more complicated than external insulation due to the way it moves the dew point within the construction.  Standard U-value calculations (Glaser) will not correctly account for the sorption properties of wood fibres nor their ability to pass on liquid water through capillary action. WUFI purely considers moisture issues and how the various elements of the building fabric will deal with the volumes based on site-specific conditions.  

 

More specifically, where you have an external finish that restricts 'breathability' you need to be really careful to reduce the amount of internal moisture that can get behind the insulation layer.  Woodfibre will handle this better than Unnatural insulations, but it needs careful assessment.

 

With woodfibre, it may be more appropriate to incorporate a framed system (with flexible insulation batts between) as this allows the addition of a moisture-vapour variable (VVCL) membrane such as STEICOmulti renova or SIGA Majrex® or PRO CLIMA intello plus .  These membranes will limit the amount of water vapour that enters the fabric, but allow breathability back to the inside during warmer periods. 

 

Correct installation of the membrane and connections to all surrounding elements is the critical factor to get right with this approach, but it is less risky than it would be with unnatural insulations such as PIR.  Woodfibre, actively transports moisture due to the sorbative fibres (I've seen studies that have shown the addition of woodfibre drawing moisture out of wet structures) whereas somthing like the Kingspan boards would trap it behind the insulation.  It is nearly imposible to assure that any VCL (or taped PIR boards) are 100% perfect.  That is without building trades or future homeowners unknowingly driving screws or causing other penetrations through the membranes after completion..

 

It is important to note that we do not generally recommend achieving high U-values with IWI due to the condensation risk to the structure.  There is, generally, a sweet spot between 40-100mm of woodfibre that balances the energy savings, cost, and condensation risk.  

 

Part L of the Building Regulations for England offers flexibility when retrofitting existing walls, roofs and floors; an improved U-value of 0.30 W/(m2·K) is the target but a ‘threshold’ level of up to 0.70 W/(m2·K) is sufficient, as long as the approach can achieve a payback not exceeding 15 years and is ‘technically and functionally feasible’.  

 

It is worth discussing with your local authority / BCO; for an older building, most BCOs are now sympathetic to the argument that breathability is more important than the insulation value.
 

 

Back to Earth offers WUFI, and I think Mike Wye & EWI pro / EWI store do too if you ask.

 

 

[sgt_woulds]

If your builder hasn't used woodfibre before (and it's great that he is open to trying)  make sure he has the correct tools for cutting the boards as they are much denser than the materials he will be used to.

 

 

Assuming a preference for power tools:

 

For flexible woodfibre batts - a jig-saw or sabre-saw with fibre insulation blades  (you can also use an insulation hand saw but this tends to rip the back of the insulation batt)

 

 

For dense boards - a table saw or circular saw with a large blade and 'bastard teeth' (few teeth with large gaps between) and the best extraction they can get as woodfibre is very good at gumming up the works, in particular the safety guards.  (You can get circular saw blades specifically for woodfibre, but these tend to be only available in Europe and for eyewatering prices)

 

 

At home I have an ancient (1970's) open frame circular saw (no safety of any kind!) that works brilliantly, but only has a 50mm cut, so for deeper boards I have to turn them over and cut twice...

 

[Redbeard]

2 hours ago, sgt_woulds said:

Just to pull up @Redbeard slightly, although I agree with his preference for woodfibre (I work for a woodfibre manufacturer so that's a given 🙂), this is a situation where a membrane may still be required.  

 

@sgt_woulds I appreciate the 'pulling up', and I usually recommend WUFI to my clients anyway, as each case is different, but my suggestion not to have a membrane was not without (WUFI'd) precedent, from a refurb we did on a cement-based pebble-dashed 19th C house 8 or 9 years ago in Yorkshire (micro-climates vary!!). Nevertheless you are right that WUFI is advisable in the vast majority of IWI cases (in my vie whatever material you are using), especially if you use a merchant who offers it free anyway. My own preference, when I was contracting, was to aim for 100mm WF (c0.35W/m2K on a 225 solid brick wall - no so far from the Part L 0.3 target) and then use WUFI to tell me it's safe or not.

 

A general note - If you don't find  merchant offering WUFI then independent 'WUFI-wranglers' are a bit few and far-between. I have been told of one, though I am not sure on Buildhub's advertising rules. *Mods,( @Nickfromwales, are you a mod - I think so. Sorry if I am wrong) can you confirm it's OK to give the name*? I can't see why not.

 

Edit: I see I did give the advice to use WUFI in my post on Monday at 20.35. I read @sgt_woulds's post as if I hadn't, and was surprised at myself!

 

If framing I agree with your use of Intello and have used a great deal of it, not least in my own house.

Edited 2 hours ago by Redbeard
Insert space, and conf. re WUFI

[ADLIan]

3 hours ago, sgt_woulds said:

It is important to note that we do not generally recommend achieving high U-values with IWI due to the condensation risk to the structure.  There is, generally, a sweet spot between 40-100mm of woodfibre that balances the energy savings, cost, and condensation risk.

 

It's not a very good advert for an insulation manufacturer when they don't know the difference between a high and a low U-value!

[Iceverge]

Good airtightness is massively important here. The prettiest U value on paper compliant system doesn't matter much when gaps at joist ends or skirting boards allow damp air to simply blow into the middle of the wall.

 

To do this properly you'll need to ensure all penetrations are sealed, joist ends etc so will need taking down ceilings and lifting floor boards etc. Even so, it is wise to assume the wall will contain some residual water and will need to dry so a more vapour permeable material, even mineral wool and plasterboard is better than foil faced sheets. 

 

Managing internal humidity, ideally through continuous mechanical ventilation is important too. 

[sgt_woulds]

54 minutes ago, ADLIan said:

 

It's not a very good advert for an insulation manufacturer when they don't know the difference between a high and a low U-value!

 

Ah darnit, I've done this before as well!  Copying and pasting from a standard e-mail response which says 'a high level of insulation' then changed it to U-value and forget to change it to say low U-value!   It's an age thing sonny... 🙂 

[sgt_woulds]

Useful advice can also be found in the following links:

 

Rethinking IWI with Natural Fibre Insulation

 

Insulation and retrofit - Finding the sweet spot - The Alliance for Sustainable Building Products (asbp.org.uk)

 

The-use-of-natural-insulation-materials-in-retrofit.pdf (stbauk.org)