Non-combustible cement boards as sheathing, racking resistance, & aluminium cladding

[AdTee]

I'm currently in the middle of timber framing my DIY extension. One long flank is right on the boundary, so non-combustible cement board was specified by the architect as sheathing, but this is causing me a few headaches.  Architect spec:

Insulated PB
Vapour Barrier
Timber Frame (insulation between studs)
Cement Board (direct to frame) - RCM Y Wall
Battens
Aluminium Cladding

Problem is that Y Wall and all other cement boards that I've found also specify a ventilated cavity between the board and the timber frame. I've spent ages trying to find cement boards with racking resistance comparable or better than OSB, but now I'm worried that it will be compromised if not planked directly on the studwork. Not to mention that I'm having to add a second cavity which means I'd probably need another housewrap layer (and possibly 9mm osb) between the battens and the frame. 

Of all the boards, STS is coming out the cheapest, and Klasse C-Board seems to have the best spec for racking.

Has anyone had any similar experiences?

Thanks
 

[sgt_woulds]

I'd ask the Architect on what basis they specified that build-up?   Did they consider moisture at any stage?  WUFI assessment?  

 

With a non-permeable sheathing board, your internal VCL and sealing would have to be immaculate to prevent damp issues with the frame - that is practically impossible to achieve on site with your average Great British Builder.  

 

For my own timber walls, I used a system that allows fully open moisture transport and easy sealing.  Uses fewer components, zero membranes, and is less critical on detailing if Bob the Builder is having a bad day or there is a football match they need to watch... 😉

 

 

Inside to out:

 

Plasterboard 

25mm battens (with woodfibre or hemp flex between to help moderate internal moisture levels)

15mm OSB3 such as Smartply or equivalent with air and moisture tightness.  Taped and sealed, this forms both the racking and airtightness in.  Easy sealing and hard to damage.

TF (I-joist studs for better u-value) with Woodfibre or hemp flexible insulation - mineral wools work, but I like to maximise the decrement delay for future heatwaves

T&G Woodfibre sheathing - provides additional airtightness and reduces cold bridging.  

Breathable render

 

Wood fibre used in a certified system behind a render or plaster system achieves a B-s1,d0 classification for both internal and external use. 

This is a classification of the render system; depending on which system is used, the manufacturer can provide the relevant data.

 

Approved document B asks for external cladding within 1m of the boundary to be Euroclass B, so rendered woodfibre is fine as long as the height of the building does not exceed 11m.

 

If the aluminium cladding is a must, this could be used over the woodfibre (without render, but with additional breathable membrane), but you'll need to speak to the woodfibre manufacturers to confirm the build-up for ventilation and fire rating.

 

 

 

 

 

 

[sgt_woulds]

I should add that I only used this construction for my garage.

 

The main house extension used SIPs (Never, ever, again!), again with direct rendered woodfibre externally to prove a fire-rated finish.

 

 

[AdTee]

On 23/07/2025 at 11:40, sgt_woulds said:

I'd ask the Architect on what basis they specified that build-up?   Did they consider moisture at any stage?  WUFI assessment?  

 

With a non-permeable sheathing board, your internal VCL and sealing would have to be immaculate to prevent damp issues with the frame - that is practically impossible to achieve on site with your average Great British Builder.  

 

For my own timber walls, I used a system that allows fully open moisture transport and easy sealing.  Uses fewer components, zero membranes, and is less critical on detailing if Bob the Builder is having a bad day or there is a football match they need to watch... 😉

 

 

Inside to out:

 

Plasterboard 

25mm battens (with woodfibre or hemp flex between to help moderate internal moisture levels)

15mm OSB3 such as Smartply or equivalent with air and moisture tightness.  Taped and sealed, this forms both the racking and airtightness in.  Easy sealing and hard to damage.

TF (I-joist studs for better u-value) with Woodfibre or hemp flexible insulation - mineral wools work, but I like to maximise the decrement delay for future heatwaves

T&G Woodfibre sheathing - provides additional airtightness and reduces cold bridging.  

Breathable render

 

Wood fibre used in a certified system behind a render or plaster system achieves a B-s1,d0 classification for both internal and external use. 

This is a classification of the render system; depending on which system is used, the manufacturer can provide the relevant data.

 

Approved document B asks for external cladding within 1m of the boundary to be Euroclass B, so rendered woodfibre is fine as long as the height of the building does not exceed 11m.

 

If the aluminium cladding is a must, this could be used over the woodfibre (without render, but with additional breathable membrane), but you'll need to speak to the woodfibre manufacturers to confirm the build-up for ventilation and fire rating.

 

 

 

 

 

 

Thanks @sgt_woulds, and apologies for the delay in replying. We've spoken to Klasse who say their board can be directly mounted to the timber frame, but as you mentioned preventing moisture build-up could be tricky. I'll look into woodfibre although with aluminium cladding it would need a ventilated cavity behind it, so I'm not sure if that would be possible.

[sgt_woulds]

Sorry, I missed your reply...

 

It depends on the specification of the woodfibre sheathing.  Some are available with a hydrophobic coating that allows any moisture that gets behind a properly ventilated rainscreen to run off and additional membranes would not be required in most parts of the UK.

 

If you are in an exposed location where rain could force its way into the cavity, or if the rainscreen has less ventilation (due to firestops, or behind a brick or stone skin) then it will require a waterproof breathable membrane fitted, before the battens and rainscreen go on.  Ideally, a nail-sealing tape should be used between the battens and the membrane to ensure that no water can run down the wall and find its way into the insulation zone.

 

At least one manufacturer makes a woodfibre board that is pre-bonded with membrane to make this task easier and quicker (although slightly more expensive in material terms, you save on labour costs and install time, and reach a weather-tight stage, sooner)

 

REI60 should be possible with the correct cavity closers and detailing.  The aluminium skin would reduce the spread of flame, and woodfibre - although a class E product - smoulders and self-extinguishes (unlike plastic insulations which melt and ignite, spreading fire), so you are unlikely to have a Grenfell situation.  

 

The woodfibre manufacturer should be able to advise on this, and it is better to have e-mail evidence from a manufacturer to show building control if needed.