Cavity closure timber frame

[Selfbuildsarah]

Advice please re fire rated mineral wool cavity closure. We will be having part timber clad and part render on our MBC timber frame airtight house. Do we need different types of cavity closure according to surface. What about ventilation? A possible builder is asking what we want!

Edited May 10 by Selfbuildsarah
Omission

[G and J]

We are timber frame, part rendered on block,  part timber clad on cement board

 

We closed the cavities in the block with mineral wall batts and have used envirogaf intumescent strips where we are clad.

[Mr Punter]

With timber frame and timber cladding you will not get decent fire rating without further protection such as boards or coatings.

[Nickfromwales]

5 minutes ago, Mr Punter said:

With timber frame and timber cladding you will not get decent fire rating without further protection such as boards or coatings.

Current 3 projects (2 x MBC PH TF & 1 x stick built PH) have needed no further protection; mixes of brick / render / cedar / composite / brick slips and more.
 

I think it would only jump if it’s a 3 story dwelling.

 

I’m a fan of keeping the cavities clear, so intumescent strips work well here. A few ££ more but tbh this is not somewhere where you should have any kind of compromise.  

[Mr Punter]

4 minutes ago, Nickfromwales said:

Current 3 projects (2 x MBC PH TF & 1 x stick built PH) have needed no further protection; mixes of brick / render / cedar / composite / brick slips and more.

 

Only the timber cladding would be a concern, as well as the "composite" if it is flammable, especially if the breather paper and sheathing are also combustible.

[Owain1602]

You will need to provide ventilation behind both cladding types, MBC insist on it.

 

I worked through all the details for our MBC house a couple of months ago, and put a case together for both building control and structural warranty inspectors to agree.

It’s quite tricky is you want to provide proper ventilation, guard against insects, allow the cavity to drain any moisture that gets in there, conform to BR cavity barrier requirements and of course….look nice. 
Most examples I see just close off the heads of the window reveals, and moisture then just sit against the building.

 

Do you have any section drawings to share what you have so far?

[Selfbuildsarah]

Apologies, forgot to post reply to this. Do we use the Rockwool type or the flexible pvc wrapped cavity barrier? 
Thank you

[Selfbuildsarah]

Our QS says he can’t find a supplier for cavity barriers for timber frame! 🤔🤦‍♀️

[Mr Punter]

Cavity barriers and firestopping is often left to "others".  They often get left out as nobody really knows what to specify or order or fit.  Were they specified and detailed on the architects drawings?  Rockwool would be a decent choice for non-vented and Envirograf for intumescent, vented rainscreen applications.  I would want detailed spec and drawings, so nothing gets missed on your order or on site.

[Selfbuildsarah]

Details from architect above