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.

 

 

[Mike]

Out of curiosity, I've just been checking the requirements in France.

 

A quick look suggests that there may be no requirements for individual & semi-detached homes, which isn't going to help much. For terraced housing & larger buildings the solution seems to be to use a 15mm fire-resistant plywood cladding to the outside face of the stud (rated B-s3 d0), followed by the external membrane & cladding (to which you could add cavity barriers - not required in France, it seems). Of course, that means keeping the insulation within the studs, with supplementary insulation on the inside face of the studs, not the outside.

 

Edited Friday at 08:52 by Mike

[ADLIan]

The cavity barrier would normally be from the main stud (structural) wall through the 'external' woodfibre insulation to the back of the cladding - in this respect I would imagine there are quite a few manufacturers. There are some cavity barriers that can sit external to a non-combustible 'cavity' insulation but in this instance the cavity barrier must be tested in conjunction with the cavity insulation. I note the ply/OSB sheathing is internal in this instance, perhaps this is where some of the problem lies. In dwellings the cavity barriers are normally around openings and at each storey height and at the top of the cavity. I doubt the company linked to above will have the technical resource to comment and advise on something as specific as the English Regs & Approved Docs.

 

NHBC and/or LABC technical manuals may help. Perhaps ask your BCO for guidance, citing specific requirement and sections in Appr Doc B.

[WisteriaMews]

@Mike we think it is possibly a UK requirement (post Grenfell) and not required in Europe. This is part of the problem, it becomes a bit niche when it is just the UK so there is no approved details.

 

@ADLIan we're finding BCO won't give advice any more as they cannot act as designers, you need to present them with an option that they'll either approve or reject. 

[Iceverge]

Could you swap out the woodfiber for dense EWI mineral wool batts and then add Intumescent fire cavity strips on the battens. 

 

This would nicely compartmentalise any fire and the mineral wool has a class A1 fire rating. 

 

What U value have you planned for the wall? Is it an on-site or factory built frame as you may need to alter the position of the OSB too.

[WisteriaMews]

@Iceverge we're planning a factory built frame by All Timber Frames from near Exeter. Aiming for a u-value of 0.1.

 

Rockwool seem to have a intumescent cavity barrier certified for use with their rainscreen duo rainscreen product.

 

If we can't find an intumescent cavity barrier certified for us with woodfibre then we may need to explore whether switching over will be possible.

 

 

[SteamyTea]

5 minutes ago, WisteriaMews said:

intumescent cavity barrier certified

Would coating with intumescent paint get around the problems.

Not sure if that could be done in the factory, but you cannot be the first person to have this crop up.

[ADLIan]

The section on cavities and cavity barriers in dwellings in Approved Doc B is relatively short and straightforward. Follow this and if BCO raises an objection ask exactly what their issue. Rockwool will insist that you use their insulation with their cavity barriers - if so try another manufacturer such as Promat FSI or Siderise. The type of 'cavity' insulation is not relevant.

[gavztheouch]

My building control officer has told me I can use an open rain screen and therefore have no cavity barriers. 
 

 

[WisteriaMews]

@SteamyTea from what I'm learning in the post Grenfell world you can't have a good idea like covering the wood fibre in intumescent paint. It needs to be actually tested and certified to be accepted. The tests cost £30k and there is apparently quite a waiting list, so you're basically dependent on manufacturers certifying their products and it doesn't look like anyone has certied wood fibre insulation yet.

 

@gavztheouch we have bedrooms above the height where you have to start considering the fire regs. I wish we didn't but planning restrictions (we're in a conservation area) have forced us into a certain design. 

[SteamyTea]

8 hours ago, WisteriaMews said:

you can't have a good idea like covering the wood fibre in intumescent paint.

Goes back to what I have been saying about the UK for years, 'for every law that says we must do something, we have another rule that says we cannot do it'.