Cavity Barrier Exemptions

[Revol]

Hi,

Maybe it's just my brain needing to warm up on a Monday morning but I'm trying to understand what AD-B clause 9.10 a. is getting at, i.e. cavity barriers as prescribed in Table 13 in 'a wall that should be fire-resisting only because it is loadbearing;' This could be read to imply that if a building has two stories the external walls that support the first floor require fire resistance as they are loadbearing elements of structures, the walls on the first floor only support the roof and therefore do not require fire resistance. If you apply 9.10 a. you would not need to install cavity barriers to the ground floor wall but you would to the first???

 

[Phoenix]

If your first floor walls are not fire resisting then what is the point of putting cavity barriers in them?  A fire could just go around any barriers you put in the cavities.

Remember 9.10 is under the heading "Extensive Cavities".  You still need all the fire stopping and cavity barriers as shown in Diagram 33.

I think the point is, if you don't need a barrier to fire inside the building then why do you need one inside the cavities? I see what you're saying though and it seems to fall short of what is reallly required because the current guidance could allow extensive undetected travel of fire through the building (via the cavities) in a way that a fire inside rooms in the building would not.

[col10]

The load bearing  wall is required to be fire resistant , under test, when exposed  each side separately.  Maybe, there is an assumption that there will be a resistance to fire entering the cavity and some inherent resistance to fire breaking out from the cavity, due to the fire resistant sides. 

[wee brian]

Dont forget that the exemptions in para 9.10 only relate to limiting extensive cavities.

The provisions of 9.2 a are unaffected.

[GB]

So taking Jon's question and putting it in application - if I have a timber frame building with insulation board attached directly to the internal timbers, the caviting being formed by timber battens which a weather board is fixed to, whilst I may have a cavity barrier as ADB Diagram 33 at the junction of floors and windows etc, the fire can spread up between apartments either via the insulation board (which is open at the bottom) and this would still comply with ADB?

[wee brian]

No

[GB]

Is that because it would not comply with ADB 8.6 Junctions?

[wee brian]

9.3

[GB]

Thanks Wee Brian,
I have a very well known engineering company saying that a sprinkler system within the property is a suitable compensatory feature for the lack of fire stopping and an insufficient cavity barrier.
A Class 1 insulation is within the cavity directly behind a timber stud and 2 sheets of plasterboard and they say the fire won't reach the cavity due to the sprinklers.
The cavity has a non fire rated vent at the bottom of the detail on the external render board - just about where the folks store their bins!!

[Revol]

Thanks for the replies ... still don't think I've reached a conclusion on this. Ignore 9.3 for a moment as this is clearly about compartmention ... although I'm sure there is another debate to be had over the fact that you could have a 90 min compartment wall with only a 30 min cavity barrier! If the purpose of sub-dividing extensive cavities is to reduce the potential for unseen spread of smoke and fire which could either hinder means of escape or block the retreat of firefighters does the fact that the wall is fire resistant or not affect the need to install a cavity barrier? I understand the logic of omitting cavity barriers at 20 meters centers for example within a masonry wall (Diag 34) as there is an expectation that inner leaf is robust enough to prevent fire entering the cavity. However, in a lot of MMC (modern methods of construction) there is little or no fire resistance from the inside, power sockets etc. also provide ready routes for smoke and fire to enter the cavity. It's always been my view that in this case there is a need to sub-divide the cavity within the wall in order to achieve 'functional requirement' of B3. I'd welcome any other views ...

[wee brian]

tbh, I'm not sure I understand the logic behind the exemption. have you put any comments in on the DCLG survey?

[Fishy]

Thanks Wee Brian,
I have a very well known engineering company saying that a sprinkler system within the property is a suitable compensatory feature for the lack of fire stopping and an insufficient cavity barrier.
A Class 1 insulation is within the cavity directly behind a timber stud and 2 sheets of plasterboard and they say the fire won't reach the cavity due to the sprinklers.
The cavity has a non fire rated vent at the bottom of the detail on the external render board - just about where the folks store their bins!!

Whilst it's always risky (and potentially disrespectful) to comment on someone else's fire strategy without having seen it, or without even knowing what the premises looks like, I have an overwhelming urge to shout "B*****KS"!  The sprinklers might make the premises more safe overall, but I really can't see how they help address the lack of cavity barriers at all.  If a small fire gets into the wall (which, as you say, could quite foreseeably come from outside) then the sprinklers won't touch it & by the time it breaks back into the building a) it'll be too late and b) it might be too big for them to handle?

I was once called into a brand-new university student accommodation site where multiple blocks had been built and had been partially occupied.  Someone had been trying to set fire to the timber cladding at multiple locations on different occasions (students were the main suspects but site security wasn't brilliant so it could have been anyone) and this had revealed that the cavity barriers were missing. Smoke from even small fires was turning up in very strange places (we were told it had been spotted leaking out of plug sockets for example)! 

[wee brian]

http://www.nbcnewyork.com/news/local/Edgewater-Fire-Avalon-Hudson-Apartments-911-Calls-Proposal-Stop-Building-290385281.html

might help

[Revol]

tbh, I'm not sure I understand the logic behind the exemption. have you put any comments in on the DCLG survey?

WB- isn't the survey about the usability of the AD's rather than technical content?

[wee brian]

Yes, but maybe flaging that a particular paragraph makes no sense is worth raising