Fire Rated Cavity Closers

[ADLIan]

1 hour ago, Mr Punter said:

 

I agree.  Do you think the guidance is aimed at timber and metal frame type walls?

The exception is for masonry walls only - 2 leaves at least 75mm thick. Note this is only the English Regs - there is no similar exemption in Scotland, Wales or Ireland that I am aware of. Timber or steel framed construction is NOT included. The issue with a fully filled cavity (hence no cavity!) is covered by not having to seal the top of the cavity. I'm sure this guidance is based on risk and the fire 'engineering' related to single houses and fire/smoke in masonry cavity walls.

 

I wonder if the above diagram may have been better if it called them 'cavity closers' with a nod to the thermal requirements of AD L. I note it does not use the phrase 'cavity barriers'.

[dpmiller]

2 hours ago, ADLIan said:

Scotland, Wales or Ireland

 

what has Ireland got to do with it?

[kandgmitchell]

19 hours ago, ETC said:

FR cavity barriers are required here in NI.

Ah, always a bit of an issue on BuildHub when discussing building standards as they do vary across the home countries. It would be useful if OP's state which country legislation they'd be subject to. I'd admit to know nothing about the regs in other parts of the UK. England alone is hard enough!

[ETC]

@kandgmitchell

I have spoken to a senior BCO today who confirmed that here in NI FR cavity closers are required to the perimeter of all openings in an external cavity wall and that the text relating to the cavity walls is illustrative only.

[Gus Potter]

On 01/02/2026 at 17:21, Jammy5 said:

These are all of the drawings and were the drawings used to obtain Building Control plan approval.

Hiya. 

 

Thanks for posting the drawings. I've had a quick first look.. need to digest more, but it's been along day so far. As others have said the level of drawing information required to get BC approval varies depending on where you are in the UK. In Scotland you would not get approval based on this level of drawing / design information. 

 

At a first glance, I would look at.. ok you have BC approval. What do we need to communicate to the builder to enable them to price and for you to tie them down without incurring extras.. or them going off an doing their own thing. 

 

If you paid for a set of drawings and the deal was for that alone then that bargin is concluded? If you want a set of drawings that a builder can use and give you a bit of price certainty then these are not enough.

[Gus Potter]

@Jammy5

Three story building, advised fire rated cavity closers on all openings. Are they really required or will normal insulated cavity closers suffice?

 

1 hour ago, ETC said:

I have spoken to a senior BCO today who confirmed that here in NI FR cavity closers are required to the perimeter of all openings in an external cavity wall and that the text relating to the cavity walls is illustrative only

I'm going to have a go at this at a basic SE/ practical level which might help folk, no matter where you live in the UK, to get their head round this stuff a bit. I'll start with masonry cavity wall construction. Timber frame is much more complex and I've not covered it here. Please excuse my spelling / grammer as it's at the end of the day.

 

But to start:

 

1/ One of the key concepts for fire design is that it can be split into two stages.. and that is where many folk on BH find it hard to get their head round things when looking at the regs. The following is just a rough summary, but for the newcomer I hope it gives you a bit of an insight.

 

Stage one:

 

(a) We give warning to the occupants of smoke, flame / heat..these are your smoke and heat detectors, this indicates you need to get out the house.

(b) On taller houses we need to recognise that there may be disabled people, so we need things like protected stairs / floors.

(c) All that happens, occupants are safe and the insurer's pick up the tab.

 

Stage two:

 

The fire service attend. They first ask.. is everyone out? Their next job is to not to protect your property, (as of course you are insured) it's to assess whether your burning house is going to set fire to the rest of the street and put other folks lives at risk. The regs are founded on the fire of London in 1666, I kid you not!

 

https://www.london-fire.gov.uk/museum/london-fire-brigade-history-and-stories/fires-and-incidents-that-changed-history/the-great-fire-of-london/

 

Now the fire service then need to assess how close they can get to your house without it suddenly collapsing on them. And this is where the regs have in interest. A 3 story masonry house ( 2 floors  + an attic often relies on the fact that the floors stabalise the higher walls for example. Now you may legitimately ask.. but there are many town houses like this.. why do I need to close my cavities..ok this is supposed to be fun so and humour is welcome!

 

The fundamental difference is that on Victorian or earlier houses the walls were often solid. In the latter day with cavity masonry the cavities were closed off by returning the inner masonry leaf with a DPC against the outer leaf. 

 

But now we are wanting to stop that thermal bridge so the cavity is open at the window reveals and over the head. @Jammy5, now you could ask.. why can't I do this.. leave the cavity exposed as I'm filling the wall with insulation. Well the insulation could not be that fire proof. If it is you are going to have to prove that it is equivalent to a closed cavity with a proper and tested material. You'll struggle to to this and you'll also need a fire engineer.. they will likely tell you.. don't waste your money!

 

You ask.. it seems ok to me. But to reinforce my point. Any cavity acts as a chimney and draws.. this excacerbates the development of fire. That fire can shoot up the cavity and compromise the floors etc that are providing lateral stability to the walls. If that happens the walls can fall on the fire brigade. Things like posi joists are designed to hold day to day working loads with some basic fire protection only, unless specifically detailed = money. Solid timbers are bit more forgiving.. provided the connections hold up.

 

Now in the old days folk had few furnishings, there was no plastic, kids toys and appliances say. In a modern house what we call the "fire load" is much increased.. that is the stuff that maintains not just the duration of the fire but also it's intensity at times.

 

In summary you can see why BC and designers are saying.. you need fire rated cavity closing.. and as a benchmark lets use / set that against the traditional way of closing off a cavity wall by returning the masonry.

 

So hopefully that has given you all and understanding about what we are trying to do and it will let you see behind the curtain when looking at the regs in your particular part of the UK.

 

 

 

 

 

 

 

 

 

Edited 7 hours ago by Gus Potter

[Gus Potter]

On 01/02/2026 at 19:21, ETC said:

I’m surprised that your BCO will allow you to put insulated PB to the underside of a FR floor.

This is a great point, I'm surprised also as insulated plasterboard is not tested for this, in fact many bodies have realised that this does not work!