Approved Document B3- provision of cavity barriers

[kurnal]

The 2000 edition of ADB table 13 made provision for cavity barriers within the void behind the external face of rainscreen cladding at every floor level and compartment wall of buildings  in purpose groups 1a and 2 with a floor 18m above floor level.

The 2006 edition makes no mention of rainscreen cladding. The BCO is telling me that because rainscreen cladding is no longer spefically mentioned in the ADB or excluded in para 9.9 and 9.10 this means that all rainscreen cladding in all purpose groups now have to be firestopped.

I think that the reverse is true. I think that as it is no longer mentioned at all in ADB it is no longer a consideration as its just a bit of additional cladding on the outside of a building that is already adequately protected in accordance with diagram 33.

Am I reading this wrong?

(The rainscreen cladding is generally a narrow perforated or ventilated space,  it fits over the structural wall, the structural wall being fire stopped at all relevant locations as per diagram 33. I believe because the gap behind rainscreen cladding has to be ventilated to do its job it is not a  concealed space  )

[PhilB]

Hi Kurnal

Doesn't  9.10 d. cover this? "The provisions in Table 13 do not apply to any cavity described below:formed behind the external skin of an external cladding system with a masonry or concrete inner leaf at least 75mm thick, or by overcladding an existing masonry (or concrete) external wall, or an existing concrete roof, provided that the cavity does not contain combustible insulation and the building is not put to a residential or institutional use."

But note also 12.9 In the case of a an external wall construction, of a building which, by virtue of paragraph 9.1 0d (external cladding system with a masonry or
concrete inner leaf), is not subject to the provisions of Table 13 Maximum dimensions of cavities in non-domestic buildings, the surfaces which face into cavities should also meet the provisions of Diagram 40.

So as long as the cavity contains no combustibles and the internal leaf is masonary or brick there is no need to limit the size of the cavity so no barriers would be required. But it would need to satisfy the surface spread of flame requirements in diagram 40.

But 9.10 d. does not apply to residential or institutional, so in the case you describe barriers would be required, in my opinion. I don't agree that ventilation at the top of the cladding stops this being a concealed space.

[wee brian]

They got rid of the term rainscreen cladding because it had a dual meaning.

The cladding you describe, as Phil so eloquently explained, needs cavity barriers. It always did but people were confused as to what consitutes "rainscreen cladding".

[Midland Retty]

I concur with PhilB in terms of his interpretation from ADB

As you point out Kurnal the ventilated void acts to equalise external wind pressure and prevent the ingress of water through the joints of the rainscreen cladding.

The void should not contain any combustible insulation and therefore should not present any risk a far as Im concerned.

Whilst I interpret ADB in the same way Phil does I dont quite agree with or understand why residential / instiutional buildings would require the fire stopping in the cavities

I can't see the benefit of providing fire stopping within the voids between cladding which is fitted on top of a structural brick wall which as you say Kurnal is fire stopped anyway.

Perhaps someone can enlighten me as to why that provisio is included

[kurnal]

Thanks for your help, I see where you are coming from and will go back to the BCO for further discussion.

I suppose something else I hadn't thought through was the treatment of windows and the risk of fire spread in the event of a window venting a fire, clearly these is a good case for firestopping above and below the window penetration of the rainscreen.

[kurnal]

Just to bring this topic to a close, and because it may be useful to others, the outcome of my original posting is this. The changes to the requirements for firestopping of cladding applied to the exterior of a building have changed very signiificantly between ADB 2000 and 2006.
All such cladding- including rainscreen cladding now need to be firestopped in all residential and institutonal buildings without exception. There are some relaxations for other purpose groups provided that the wall is of masonry- see para 9.10 for the details.

To meet this change, products like that described in the following link will allow these cavities to be ventilated but will provide the required firestopping  in a fire situation.

http://www.rainscreenworks.com/rw_html/companies/promat/index.html

Hope this may be of some help to others

[Midland Retty]

Hi Prof

Thanks for the info.

I still can't see the benefit of fire stopping in this situation.

I asked my colleagues for their thoughts - alas they too were a tad confused. I even contacted my local BCO who was also somewhat baffled by this requirement.

Even with a fire venting out of an open window I cant imagine it being able to spread up the cladding. Has anyone clarified further to you why fire stopping is required?

I could understand such a requirement if combustibles were present in the void such as electrical cabling or other services etc, but if the cladding material is fire resisting and the voids are sterile why is there a need to fire stop?

[wee brian]

Kurnal - you need to read the second edition of BRE report BR135. all is explained.

[CivvyFSO]

In the purpose groups it applies to we are generally looking at small compartment sizes, this should negate the risk of vertical fire spread due to limited fire loading, cooling of the fire/smoke due to entrainment etc. (Often small ledges are used so that the plume does not adhere to the wall) The void under the cladding can give a 'sheltered' path for fire/smoke to travel up, where it is not going to entrain much air, and it is not going to be cooled much, and what is more it is going to be delivered right to the most vulnerable point of the floor above.