Cavity barrier and wood fibre insulation

[WisteriaMews]

Our planned wall build up is a timber frame, wrapped on the outside with wood fibre insulation and then a ventilated cavity and timber cladding.

 

We'll need horizontal cavity barriers and haven't been able to find any manufacturers who say there product is approved for use with wood fibre. Consequently Building Control won't sign off our plans.

 

Has anyone recently built with a similar build up and found a cavity barrier approved for use with a timber frame covered in wood fibre?

 

Thank you.

[Redbeard]

No - sorry, but I have done a TF extension clad in WF EWI and rendered. BC (particularly under the 'new' (ish) regs) got a little concerned about 'surface spread of flame' and then decided that since (in my construction) there was no surface effectively exposed to flame - unless the whole sandwich' were already on fire - they had no concerns about surface spread of flame. In your set-up you do not mention a scratch-coat of render before the (cavity and) cladding, so you have, from outside, timber cladding, 50??mm gap, wood-fibre, timber frame. I assume the cavity is formed with 50mm sq battens. So it's all wood. In the past there used to be such details (the late NBT used to have one for a set-up like yours, but under the latest Regs/Bldg Safety Act/Post-Grenfell caution 'regime' I can understand BC asking 'for more'. By the way, what centres would your horizontal barriers go at, and do they not stop it being a ventilated cavity. I may have misunderstood. If I have, the cavity *is* ventilated, but somehow has barriers which would stop fire.

 

I am aware I may be being stupid! Can you dis-confuse me?

 

(Aside of that, how about you propose to BC that you do a coat or 3 of lime render on the WF EWI before affixing your battens (/?barriers?) and cladding, as a fire-stop?)

[SimonD]

That's a difficult one. Our WF has a reaction to fire classification of E but then becomes B-s1 Do when rendered. I guess the E risk factor is because WF contains paraffin. We're okay because it's rendered. It was at one point suggested that we batten and clad but obviously chose a different path. Haven't heard of cavity barriers for WF. As @Redbeard suggests, maybe a coating of thin coat render - you'd only need 6-8mm with mesh done in 2 coats and no need for the silicon top coat. Baumit or similar would be simple enough to get.

 

But with ours the buildup details show clad with ventilation gap but no mention of cavity closers https://www.schneider-holz.com/en/products/insulation/multitherm-wood-fibre-insulation-board/multitherm-140/

 

Have you contacted the manufacturer's technical department?

[WisteriaMews]

@Redbeard @SimonD

 

Our planned build up is very similar to the one Simon linked to (image below), except we're planning a batten and counter batten before the external cladding.

 

Our architect has taken the lead in trying to find a solution. They've been speaking to cladding suppliers and cavity barrier suppliers so far and drawn a blank.

 

I've contacted Ecological Building Systems who market a timber frame build up similar to the image and they haven't got back to me (despite a few chasers) so I suspect they don't have a solution.

 

We hadn't considered a render scratch coat. (Thanks for the suggestion.)

 

I'll also check if the architects have contacted the wood fibre manufacturer technical departments and if not get in touch.