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.