Fire barriers required behind cladding.

[Big Jimbo]

Just a heads up. I'm planning on building in Block and Block rendered on the external bottom half, and clad in Hardi plank on the top half.  Now the medium density blocks are not flamable, and neither is the cladding. However, the cladding will be mounted on timber battens and will therefore have a ventilated cavity behind it.  My understanding of the current building regs is that i am going to have to have a fire stop in this cavity.  Because the cavity has to remain open, i am going to have to get something like Tenmat FF102/50.  For me to put a band around the bottom of the cladding, and a band around the top, i'm  going to need about 150m of the stuff It's blinking £25 a meter ! That's about £3750.......Yikes, that is going to be painfull on the budget.

Can anybody tell me i dont need it ?

As a second question. Do i now need to close off the top of my cavity ? I'm having a cold roof, and have read that moist air in the cavity can rise into the roof, and cause damp/condensation.Is this the case even though i am having a full fill cavity ?

Thanks for all your help as usual.

[Temp]

Google found..

 

https://www.labcwarranty.co.uk/technical-blog/cavity-barriers-behind-decorative-cladding

 

 

Quote

 

Therefore, to satisfy Building Regulations there should be cavity barriers installed in the cavity formed behind a ‘decorative cladding’ (such as timber boarding, hung slates or tiles etc. or any other finish) which is constructed onto the outside of an external wall masonry wall.  

 

This will include concealed spaces behind cladding on detached houses as well as semi-detached and apartments/flats.

 

 

I have a small area of vertical hung tiles on battens so my house built in 2007 would no longer comply.

 

[Temp]

1 hour ago, Big Jimbo said:

i am going to have to get something like Tenmat FF102/50.  For me to put a band around the bottom of the cladding, and a band around the top

 

And something similar where the cladding has a window in it?

[eandg]

Treated 2*2s wrapped in DPC around your windows. 

 

Envirograf does the same but for about two-thirds of the cost of Tenmat. 

[Big Jimbo]

Thanks @eandg i will check that out.

[Big Jimbo]

Anybody got any idea if i will have to close off the top of my masonary, full fill cavity wall, before it enters the cold roof ? I would have liked to have ran the insulation through to join up with the loft insulation.

[markharro]

I have just managed to persuade our BCO that cavity barriers pointless and not needed behind our open timber cladding for what it is worth. We are in Scotland.

[TonyT]

Have a look at the technical standards for cavity closers and speak to your local building control.

 

I would see more cold bridging if the cavity insulation didn’t extend into the roof void

[gavztheouch]

On 31/05/2023 at 11:32, markharro said:

I have just managed to persuade our BCO that cavity barriers pointless and not needed behind our open timber cladding for what it is worth. We are in Scotland.

Hi I’m in Scotland and would love to not put horizontal cavity barriers behind my open rain screen. Mainly because wind driven rain will get past the cladding and then I have the worry of horizontal cavity barriers (held on with screws) blocking the water escaping easily and potentially seeping in behind the cavity barrier fixings. And then there is added cost and complexity. Probably 5k in materials and time. 
 

what was your main argument?

 

 

 

[gavztheouch]

On 31/05/2023 at 11:32, markharro said:

 

My thoughts were a cavity barrier won’t stop the supply of air but they might physically stop the movement of a flame progressing upwards. However as my boards are not very deep it would only be a 90mm jump for the fire to bypass the cavity barrier and jump into the next cavity. 
 

one positive I can think of with open rain screen is there is less of a chimney effect cause it’s not closed off like a chimney. 
 

Did they not try and make you use a closed cladding system?

[markharro]

Hi I cant remember the detail but I think my argument was simply that open vertical cladding wouldnt mean that there was a cavity.

[gavztheouch]

5 hours ago, markharro said:

Hi I cant remember the detail but I think my argument was simply that open vertical cladding wouldnt mean that there was a cavity.

Thanks Mark, I spoke to my warrant officer and he said I wouldn’t need fire battens as just like you said he would not consider it a cavity. My architect is arguing that he is wrong and I will need them. 

Edited August 4 by gavztheouch