Fire Rated Cavity Closers

[Jammy5]

The SAP calc is asking for 63mm insulated plasterboard between garage and dwelling walls.

 

Then for the ceiling it's 200mm R40 between joists and under drawn with 63mm insulated plasterboard and a layer of fire line board.

 

[ETC]

24 minutes ago, Jammy5 said:

The SAP calc is asking for 63mm insulated plasterboard between garage and dwelling walls.

 

Then for the ceiling it's 200mm R40 between joists and under drawn with 63mm insulated plasterboard and a layer of fire line board.

 

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

[Nickfromwales]

28 minutes ago, ETC said:

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

If it's 9mm PB then yes, but if it's 12.5mm with plaster skim coat then the 30 mins regs are satisfied?

[ETC]

1 hour ago, Nickfromwales said:

If it's 9mm PB then yes, but if it's 12.5mm with plaster skim coat then the 30 mins regs are satisfied?

I’d like to see a test certificate for a 30 minute FR floor using insulated plasterboard below the floor joists.

[ADLIan]

On 31/01/2026 at 22:21, ETC said:

You need to close the cavity to the perimeter of ALL external doors and windows with an 1/2HRFR cavity barrier irrespective of what insulation is in your cavity. The only “get out” for not installing a cavity closer is at the eaves and verge ONLY and that is on condition that the cavity is fully filled with insulation (pumped bonded bead being one example).

However, until I see conclusive evidence that the insulation in a cavity is non-combustible I will always ask for the cavity to be closed at the eaves and verge.

If this is the case why are there so many products on the market for use around door and window openings that are based on polystyrene or PUR insulation that have no fire resistance properties quoted.

[ETC]

1 hour ago, ADLIan said:

If this is the case why are there so many products on the market for use around door and window openings that are based on polystyrene or PUR insulation that have no fire resistance properties quoted.

I suppose where these are used the FR is provided by some other means. Don’t forget that timber windows do not need a cavity barrier as long as the frame covers the cavity.

Edited 6 hours ago by ETC

[Mr Punter]

@ETC and @ADLIan are both experienced professionals and do not see eye-to-eye on this, so what hope do the rest of us have?  Cavity closer, fire stopping and cavity barrier seem to get used interchangeably.

 

I like the idea of @ETC's calcium silicate board cavity closer but Google does not come up with any.  Mostly they are plastic with polystyrene or FR ones that are plastic with mineral wool.  I can't imagine many bricklayers cutting up calcium silicate board on site.

 

The lack of easy practical and cost effective solutions often leads to shortcuts and poor implementation.

[ETC]

6 minutes ago, Mr Punter said:

@ETC and @ADLIan are both experienced professionals and do not see eye-to-eye on this, so what hope do the rest of us have?  Cavity closer, fire stopping and cavity barrier seem to get used interchangeably.

 

I like the idea of @ETC's calcium silicate board cavity closer but Google does not come up with any.  Mostly they are plastic with polystyrene or FR ones that are plastic with mineral wool.  I can't imagine many bricklayers cutting up calcium silicate board on site.

 

The lack of easy practical and cost effective solutions often leads to shortcuts and poor implementation.

The Approved Document is quite clear.

You need to install FR cavity barriers to the perimeter of all external doors and windows.

There are a number of ways of doing this and are listed in the AD - including the installation of calcium silicate board - https://www.promat.com/en-gb/construction/products-systems/products/fire-resistant-boards/supalux/

And yes - builders do cut calcium silicate board on site - I see it everyday. Some builders will line the complete jamb, head and cill with the board - others will cut a 50mm strip and close the cavity.

[Nickfromwales]

14 minutes ago, ETC said:

I suppose where these are used the FR is provided by some other means. Don’t forget that timber windows do not need a cavity barrier as long as the frame covers the cavity.

My point exactly. If fires not getting to it, it doesn’t need to be considered in the FR makeup. 
 

After 30 mins of flames licking something, in a domestic fire that’s fully involved, either everyone’s already dead or they’re outside watching the fire brigade do their thing. 
 

Steels only need FR PB over them if they’re exposed, for eg.

[Nickfromwales]

16 minutes ago, Mr Punter said:

@ETC and @ADLIan are both experienced professionals and do not see eye-to-eye on this, so what hope do the rest of us have?  Cavity closer, fire stopping and cavity barrier seem to get used interchangeably.

 

I like the idea of @ETC's calcium silicate board cavity closer but Google does not come up with any.  Mostly they are plastic with polystyrene or FR ones that are plastic with mineral wool.  I can't imagine many bricklayers cutting up calcium silicate board on site.

 

The lack of easy practical and cost effective solutions often leads to shortcuts and poor implementation.

FR PB mechanically fixed over standard closers should be suffice, maybe if Dias that was bolstered with 15mm FR PB being the minimum acceptable material to form these barriers.

 

Intumescent foam is flammable, go figure!?!?!

[ETC]

The other thing that comes to mind is the idea/notion/concept that where there isn’t a cavity (where the cavity is fully pumped with insulation) why on earth would you need a cavity closer? No cavity - no closer.🤷🏼‍♂️

[Nickfromwales]

7 minutes ago, ETC said:

The other thing that comes to mind is the idea/notion/concept that where there isn’t a cavity (where the cavity is fully pumped with insulation) why on earth would you need a cavity closer? No cavity - no closer.🤷🏼‍♂️

Maybe lack of (known) FR of the infill material. 
 

But obvs should all be ironed out at the outset……just amazes me how much of this gets left to ambiguity and misadventure. See it all the time and it’s just crazy. 

[saveasteading]

20 minutes ago, Mr Punter said:

both experienced professionals and do not see eye-to-eye

Because it depends on the circumstances and the product.

 

Eg timber is combustible but may satisfy the requirement.. it chars and needs lots of heat and air to burn.

Some firestopping is plastic wrapping to an intumescent core. The plastic melts and the core expands to fill the gap (it is like cigarette end ash)

If you don't need the ventilation then timber or rockwool may suffice to fill the gap.

But you also have to convince the bco And they need proof, eg a test certificate matching the proposal.

 

[ETC]

Just now, Nickfromwales said:

Maybe lack of FR of the infill material. 
 

But obvs should all be ironed out at the outset……just amazes me how much of this gets left to ambiguity and misadventure. See it all the time and it’s just crazy. 

Exactly - there is the idea that cavity insulation may be combustible and until this can be proved/disproved FR cavity closers will be required.

[ETC]

1 minute ago, saveasteading said:

Because it depends on the circumstances and the product.

 

Eg timber is combustible but may satisfy the requirement.. it chars and needs lots of heat and air to burn.

Some firestopping is plastic wrapping to an intumescent core. The plastic melts and the core expands to fill the gap (it is like cigarette end ash)

If you don't need the ventilation then timber or rockwool may suffice to fill the gap.

But you also have to convince the bco And they need proof, eg a test certificate matching the proposal.

 

It still comes down to needing a FR cavity closer to the perimeter of all external doors and windows. The proprietary closers can be FR or non FR rated - as you say a piece of plastic with an insulation core. The AD even suggests 0.5mm thick steel - hence no requirement (at head level) where steel lintels are used.

 

However, I will always advocate calcium silicate board - it is relatively cheap to buy and relatively easy to install - unlike many of the proprietary products which need to be installed in accordance with the manufacturer’s instructions and plasterers do not like them because the plaster can crack.

 

A BCO will accept any product in the AD and those proprietary products that are certified to provide FR. The thermal requirement is different and if you use “standard” details these will need to be factored in.

[ADLIan]

AD B gives definitions of 'cavity barrier' and 'fire stopping' but not 'cavity closer'!

 

Diag 5.3, Note 1, in the current AD B is quite specific about there being no particular fire performance requirement for the materials used to close the cavity. This note was included only in recent versions of AD B (post 2019?) and is not in the technical guidance for Wales, Scotland & Ireland. I have own my thoughts on why this was changed!!

 

There is also no restriction on the fire performance of materials used in the cavity - mineral wool will be non-combustible, polystyrene and PUR/PIR will be classed as combustible.

[Nickfromwales]

27 minutes ago, ETC said:

It still comes down to needing a FR cavity closer to the perimeter of all external doors and windows. The proprietary closers can be FR or non FR rated - as you say a piece of plastic with an insulation core. The AD even suggests 0.5mm thick steel - hence no requirement (at head level) where steel lintels are used.

 

However, I will always advocate calcium silicate board - it is relatively cheap to buy and relatively easy to install - unlike many of the proprietary products which need to be installed in accordance with the manufacturer’s instructions and plasterers do not like them because the plaster can crack.

 

A BCO will accept any product in the AD and those proprietary products that are certified to provide FR. The thermal requirement is different and if you use “standard” details these will need to be factored in.

Should we add that this typically pertains to 2 or 3 storey dwellings, and is relaxed for single storey? 

[Mr Punter]

1 hour ago, ETC said:

The Approved Document is quite clear.

You need to install FR cavity barriers to the perimeter of all external doors and windows.

 

This does not apply if the wall consists of two leaves of brick or concrete, each at least 75mm thick.  See "Diagram 5.3 Cavity walls excluded from provisions for cavity barriers" ADB V1 2019.

[ETC]

54 minutes ago, Mr Punter said:

 

This does not apply if the wall consists of two leaves of brick or concrete, each at least 75mm thick.  See "Diagram 5.3 Cavity walls excluded from provisions for cavity barriers" ADB V1 2019.

You’ve lost me. Are you saying that if you have a standard masonry cavity wall you do not need a cavity barrier around an opening?

[Mr Punter]

3 minutes ago, ETC said:

You’ve lost me. Are you saying that if you have a standard masonry cavity wall you do not need a cavity barrier around an opening?

 

Have a read:

 

[ETC]

I have. I think it’s confusing - why would the diagram show cavity barriers around a window in a masonry cavity wall if they are not needed?

[ETC]

If this is the case I think the industry needs to realise this and to stop promoting FR cavity barriers for openings in masonry built dwellings. I genuinely think that they should be installed but if this is the case I stand corrected although I would question why the diagrams indicates cavity barriers if they are not needed.

[Mr Punter]

44 minutes ago, ETC said:

I have. I think it’s confusing - why would the diagram show cavity barriers around a window in a masonry cavity wall if they are not needed?

 

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

[kandgmitchell]

1 hour ago, ETC said:

why would the diagram show cavity barriers around a window in a masonry cavity wall if they are not needed?

I think this needs a step back. Cavities are concealed spaces within a building, a cavity within an external wall is simply one example of a cavity, there are many other types.

 

So, the regulations say that you do not need to provide cavity barriers with the specific fire resistance stated within para 5.20 within an external wall if it meets the criteria set out in the diagram 5.3, i.e that the cavity is closed around openings in the wall and at the head if the cavity is not filled completely with insulation. There is no requirement for the closers around the window to be fire resisting. 

 

However, I do agree that the guidance is confusing, if the space between the two masonry leaves is filled with insulation then there is no "cavity" and thus the perimeter to openings does not need to be closed with any additional material at all.

 

Furthermore I believe there was a mention above of fitting self closers to the internal fire doors leading onto the protected stairway. They are not required. The only door in a normal three storey dwelling house that needs a closer is a door between the dwelling and an integral garage.

[ETC]

I’m confused why cavity barriers are even shown in the diagram if this section relates to fire safety - it has always been my understanding that a cavity barrier should have FR but I am open to being corrected. I agree that insulation needs to be provided at a head, jamb and chill.

 

I stand corrected in relation to self-closing fire doors. They are however a requirement in TBE NI Building Regulations - where alternative final exits are provided.

 

Could I direct you to https://www.buildingcontrol-ni.com/assets/pdf/BCNI_CONSTRUCTION_DETAILS_-_November_2023.pdf where FR cavity barriers are required here in NI.