What's the best wall structure to balance performance and cost in a new stick frame extension, and is continuous insulation worth it?

[ADLIan]

The failure mode with this form of construction (render/insulation/sheathing(?)/timber frame) was well documented following failures many years ago in Canada. It was the same form of failure identified in the 2 houses I had dealings with last year. The information that I have to hand including Building Reg Approved Documents, warranty providers requirements, BRE and TRADA all show a render board (or other cladding) with ventilated airspace behind.

 

@ProDave had an issue with similar construction but I'm not sure he ever got to the bottom of it. He chose the system as it had BBA Certification from the wood fibre manufacturer (not sure which). Strangely this Certificate was withdrawn shortly after he completed the work - I suspect because the 'system' was inherently unsafe! Without some form of 3rd party Certification/Assessment I doubt any BCO would accept the system.

 

7 hours ago, sgt_woulds said:

Most EWI systems use direct render onto the insulation.  Why should this be different for timber frame?

Sorry but this shows a lack of understanding of this particular problem.

 

2 hours ago, sgt_woulds said:

 Seek the advice of the woodfibre and render manufacturers and build specific to the local microclimate.

If your company is supplying product for this render system into the UK it must be suitable (robust, inherently safe etc) in all locations, from the bottom of Cornwall to the norther tip of Scotland and from east to west. Does your company undertake some form of 'assessment' to assess suitability for each project in its particular microclimate?

 

7 hours ago, sgt_woulds said:

STEICO (and other) woodfibre insulations use a hydrophobic coating on external sheathing boards.  In addition to the render it is more than adequate to keep external driven rain away from the structure.  Unlike unatural insulations, the woodfibre actively wickes moisture away when conditions allow

Can you please explain how the product can be hydrophobic yet can still wick moisture away?

[Mr Punter]

Yes.  Look up "Leaky Condo" and see the consequences of omitting a drained vented cavity on timber frame.

[Roger440]

5 hours ago, Iceverge said:

 Sadly @Gone West it was @ProDave who had the issues. Baumit silicone render over wood fiber I think I remember. 

 

That's assuming it's vapour driven moisture that's the issue. It's largely isn't in reality but rather the gaping holes left by the building process in the wall. 

 

I would be shy of rendering onto insulation in anything but a dry and stable climate.  Ireland certainly isn't.  I've seen another render failure over EPS near me recently. Constructed about 5 years ago. 

 

Meanwhile I've seen plenty of westerly facing sand and cement plaster over 60 years old on our farm with zero paint or maintenance and it seems to be holding on fine. 

 

For Timber frame especially I would like a cavity. Mineral wool or wood fiber being my preference for insulation external to the studs as they won't trap moisture. 

 

In any case I think for @Jess Shannon  you'll make very little difference to the performance of an old building by getting caught in this rabbit hole.  

 

My suggestion of a 170mm wall, airtight layer and 50mm service cavity stands. 

 

 

 

 

 

 

 

 

Good summary.

 

Theres not a chance in the world i would put a render over woodfibre.

 

Whilst the principle of vapour open is fine of itself, the reality of our climate especially here in wales, means that it will be getting wet, much more than it gets a chance to dry.

 

Water will always find a way in. The idea just doest pass the common sense test.

 

It was earlier this year i watched a brief storm batter my house from the west with horizontal rain, causing a rooster tail of the ridge tiles.  Shame i didnt have my phone. But a handy reminder why we invented cavities.