More IWI questions...

[jsmac]

I broach this topic with some trepidation as a read through of previous threads suggests that I think I know what the answers will be.

Some background:
In one of the ground floor rooms that will soon be a kitchen the external wall is only a single course brick wall and much of it has been rendered internally in a hard cement. Removal of the cement render wasn't really an option since the force needed to knock it off would have almost certainly taken the wall down with it. The wall is a real hodge podge of different bricks and repairs over time. The internal face of the original bricks are visible in a small section of the wall but not enough (I think) to allow the wall to breath effectively. If we could do the work again we would likely knock down the whole wall (it is only about 9 sqm) and rebuild it properly rather than trying to keep it and increase the size of the window aperture. But we are where we are...

I have repointed the worst areas and levelled out and consolidated the external face of the wall with NHL. The plan is to insulate externally with 40mm of cork insulation and Warmcote/Limecote render.

And now onto the contentious IWI question...

It was suggested to us during the installation of the new larger window to install an internal stud wall as it would allow for insulation and the ability to properly fix shelves, cabinets, etc to the wall if we so wished (the window is installed in the brick wall, not the stud wall). So at the moment we have an exposed 3x2 stud wall on the external wall and party wall. I am loath to pull the stud wall down but at this juncture it is an option, I suppose. Of course if doing it all again I would fix cork/wood fibre to the wall internally...

Keeping in mind that there will be 40mm cork EWI I have come up with the following insulation buildup for the external wall. Your thoughts and advice would be most welcome. I appreciate that the answer to this is largely a known unknown - it might work, it might create problems - but I am hoping that a solution can be found that will allow me to work with what we have at the moment rather than taking a step backwards and starting again.

• OUTSIDE
• 20mm Warmcote/Limecote render
• 40mm cork insulation
• Single course brick wall
• 80-100mm Thermafleece between internal studs
• Airguard Smart Vapour Control Layer - fixed to stud wall with battens and corners and gaps taped
• 25mm flexible wood fibre insulation between battens
• 11mm OSB board
• 15mm wood wool board
• Lime plaster
• INSIDE

 

[aockenden]

Sry but that entire setup looks seriously ridiculous.

If you’re trying to keep your stud wall and it currently has no boarding fixed to it (aka its a skeleton of studs), then just cut sections of wood fibre insulation and fix them in between the studs (with a lime mortar buttered on the side in contact with the wall). Then fix 15 mm wood wool ontop of that (again with backside buttered with lime in direct contact with the wood fibre sitting slightly proud of the studs) and then lime skim plaster on top of that. No vapour membrane whatevers, no OSB.

Having both internal and external insulation is already a strange proposition and probably totally overkill anyways.

 

[jsmac]

Thank you for your feedback. I think that 'seriously ridiculous' is perhaps a bit harsh, but let's go through it.

The buildup I described gives a u value of about 0.22 whereas it is about 0.70 with only the cork EWI. It may well be possible to fix wood fibre and mortar directly to the wall and this is something I have considered but my concern was that it is not possible to get any insulation behind the studs thereby creating potential cold spots.

My understanding is that the use of a VCL to control the amount of moisture getting into the fabric of the wall is what they are designed to do. The technical folks at Thermafleece and Pavatex suggested it as an option (they suggest their own VCL, of course, but the principle is the same).

And the buildup for using wood fibre insulation in timber frames suggests OSB board internally as both a racking board and VCL (it is what we were advised to do by Mike Wye in the new timber frame extension we've built). In this case of the kitchen wall the OSB will provide rigidity for the stud wall and the ability to install fixings for lighter loads.

What am I missing here? The internal cementitious plaster internally will largely prevent the movement of vapour to the outside so I thought that it would be important to control the amount of moisture getting into the wall?

 

[fernicarry]

I don't much see the point of sandwiching thermafleece, a breathable insulation, and just about the most expensive one at that, in between a non-breathable cement render and a VCL. If you really can't get the wall breathing you might as well go with PIR which will allow you to reduce the depth of the build-up for the same U-value.

Regarding the OSB you're not building a structural timber frame so the requirement for racking isn't there. If you envisage fixings things to the wall then fair enough, but I'd want something a bit more substantial than 11mm OSB and you have the studs to fix to anyway.

 

[jsmac]

Thanks, Fernicarry.

I have a pack of Cosywool slab left over from insulating the original first storey timber frame so thought I'd put it to use and get it out of the way. I was advised that a hygroscopic insulation would be preferable in the event that moisture does make its way into the wall. And that an adaptive air and vapour control layer on the warm internal side of the wall would be a good idea.

I suppose that I can try cutting some strips of the internal cement render with a grinder and knocking it off now that the wall is more solid. It sounds like it would be beneficial.

It is certainly possible to eliminate the OSB from the buildup. Again, its installation is what was advised in conversations with the technical teams at various suppliers and manufacturers.

I've not done a job like this before and sometimes the advice from suppliers can be contradictory. And certainly the advice from any builders in our part of the southeast is at best thought about very, very carefully.

 

[aockenden]

my concern was that it is not possible to get any insulation behind the studs thereby creating potential cold spots.

In what I suggested the studs are probably 3.5 inches depth of wood, which is already a way better insulator than masonry, with another 15 mm of insulating wood wool board on top, all of which sits against a wall which is ALREADY insulated from the exterior by 60 mm of exterior insulation. Come on.

I stand by my statement. You’ve included 5 different unique insulation materials and 8 total layers. You’re overthinking it and over designing it. Good luck.

 

[jsmac]

Thank you for your input. I have learned an enormous amount from reading through the history of the helpful threads on this forum. That is how many of us have to learn and we have to start somewhere. Not everyone has the same level of experience and expertise so it is helpful to lean on those who are prepared to offer their help. I asked my original question in good faith and with the best intentions.

Normally the feedback received in this forum is offered without bias and patronisation. In my experience it has always been positive and constructive. If you think something is over-engineered or ridiculous then may I suggest that it is a lot more helpful to explain why. And if you feel the need to be condescending then maybe just don't.

 

[fernicarry]

sometimes the advice from suppliers can be contradictory

I think supplier guidance tends to be aimed at new construction rather than retro-fitting so it needs a bit of intepretation. And its all set against current regs that demand quite high standards of thermal performance and air tightness (although whether your typical commodity builder gets anywhere near meeting those is a different question...) so some of the details will reflect that.

Avoiding void spaces is good and valid, but you can do that with offcuts squeezed in behind the studs. The VCL could be stapled rather than battened. My friend did an extension to current regs and all his membranes were stapled in place so far as I recall. Battens are usually added on the inside of the build up to create a service void for cables or pipes but the space between wouldn't be insulated. Running services through solid insulation isn't fun as you'll see on other threads here.

I'm still a bit sceptical about VCL's, even the so-called intelligent ones. Those assume that any water that gets in during the winter will be balanced by what gets out in the summer (roughly speaking, its about relative vapour pressure either side not temperature). But you can imagine an external defect letting in a lot more water than can get out. And you now have one wall that's super vapour tight but still the same amount of vapour production in the building that has to find its way out through the remaining walls unless you improve the ventilation.

I suppose it all comes back to the wall not being terribly breathable so you thats forcing you to a compromise solution. I agree with @aockenden that you have more insulation than you need but that might be a function of the current position of the stud wall. Moving or removing it might allow you to simplify the whole endeavour.

 

[jsmac]

I think supplier guidance tends to be aimed at new construction rather than retro-fitting so it needs a bit of intepretation.

This makes a lot of sense. I hadn't thought about it in that way.

Battens are usually added on the inside of the build up to create a service void for cables or pipes but the space between wouldn't be insulated

This is what I was planning. There are no services in this room yet so I thought that it would be a good idea to create a void for them. Electrics could run between the battens and the switches and sockets could be fixed to the stud walls and/or OSB board. That was the thinking anyway.

I figured that I would just fill the spaces that don't have any pipes or cables with insulation. It sounds like this may well be unnecessary.

If I remember correctly part of the reasoning given for the stud wall is that it would allow for space to run drainage pipes for the kitchen sink and dishwasher. Of course some of the work undertaken by those contractors was questionable at best, which is why I've come here to try to find a solution.

I suppose it all comes back to the wall not being terribly breathable so you thats forcing you to a compromise solution.

I think that I will have to seriously look into removing as much of the internal non-breathable plaster as possible.

The buildup of insulation is indeed partly because of the location of the stud wall - maybe I need to have a re-think about the viability of removing it from the external masonry wall at a minimum. This will potentially simplify matters in the long run.