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Green Building Bible, Fourth Edition
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General: Fire Cavity Barriers vs Cavity Ventilation

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    • CommentAuthorDanS
    • CommentTimeJan 29th 2026 edited
     
    Hello, I'm a long time lurker here and an avid admirer of a few regular contributors to this forum. I'd love to have your thoughts on the following conundrum (as I see it).

    I'm currently detailing my self-build home - Main wall fabric: Plasterboard+skim (breathable paint), timber frame studs + FrameTherm40, 9mm OSB, breather membrane, 50mm PIR, 50mm cavity, masonry skin. All standard (ish!** no VCL!).

    My issue is the 50mm cavity I believe offers a ventilated space to deal with water ingress through the masonry. BR AD-B says this cavity must be sealed by fire rated cavity barriers, top, sides, openings, compartment floors etc. So what's the best practice here?

    Intumescent cavity barriers obviously maintain the open cavity and solve this issue, but there are plenty of other compression mineral wool style barriers/socks that are approved for this use in timber frames with masonry skin. How is this acceptable? Plus dividing/sealing the cavities must be of some benefit thermally!? My gut tells me that a vapour-open sock would be the best of both worlds, but none of the manufacturers datasheets express this as a feature. And they appear to be wrapped in polythene...
    I don't really understand why the need for a 50mm cavity, if it is then going to be sealed shut for fire spread. A 25mm (or even 10mm! - as is now commonplace with T&G PIR cavity insulation) would suffice if circulation is not deemed necessary?

    Thanks in advance :)
    • CommentAuthorGreenPaddy
    • CommentTimeJan 30th 2026 edited
     
    A few thoughts, though not necessarily an answer...

    - you've mentioned the need to follow BRegs for the 50mm cavity, but I think you're suggesting not to follow the Regs on VCL?

    - I personally avoid blockwork as the outer skin, using renderboard if a rendered finish is required. Never understood the need to use 100mm of concrete to carry a 10mm render skin (high fire rating or harsh weather aside).

    - I happen to always use intumescent strips, over a 22mm timber batten, so the 50mm cavity is closed by 22mm timber and then intumescent expansion (keeping good ventilation which is important for timber kits) only restircted to 25mm at the intu strips. Remember to choose the line which is contiguous with the timber frame solid stud (top runner/headbinder/poleplate) (ie not where there is just insulation behind it internally).

    - using full cavity closure at say first floor, necesitates lots of weep vents above and below the closure, which can look a bit unsightly (IMO) and gives "vented" rather than ventilated (hence my previous comment about intu strips).

    - I choose to close cavities fully around openings (and vertically if the distances require it), but leave them open to ventilate horizontally at mid floor and eaves with intu strips.

    I'm not referencing BRegs, as I'm less aufait with Eng Regs, being based in Sctoland (often similar, but occasional differences)

    Hopes there's something above that helps.
    • CommentAuthorDanS
    • CommentTimeJan 31st 2026
     
    Thanks GreenPaddy, That is helpful, I'd not considered the additional weep vents etc that would be required if fully closing the cavity @ first floor level. And I'd also not considered your batten + intu strips idea, but it seems sensible and it would simplify the fixing against insulation board somewhat.

    I'm intending to use brick (Bekstone walling), rather than block+render, but otherwise agree with your blockwork comments.

    Reference no VCL... I can be persuaded around (particularly if Building Control are steadfast), but I've seen in the past how hard it is to maintain a decent and tight VCL against plumbers/sparks/installers, I was under the impression that a poorly fitted/damaged VCL is generally worse than just ensuring that the fabric is breathable and able to dry as moisture levels fluctuate through the seasons... Again I'm happy to be corrected on that!

    Thank you
    •  
      CommentAuthordjh
    • CommentTimeJan 31st 2026
     
    Posted By: DanSensuring that the fabric is breathable and able to dry as moisture levels fluctuate through the seasons
    But you're proposing to have PIR outside even the breather membrane, which means your construction isn't breathable at all, let alone defeating the purpose of a breather membrane.
    • CommentAuthorDanS
    • CommentTimeJan 31st 2026
     
    I accept that the fabric is closed to external, but I thought that having at least the internal side open would be worthwhile. Possibly not, and I'll stick to a conventional VCL + service cavity arrangement. To be honest I was simply gauging opinions on that.

    I agree that a breather membrane between the OSB and the PIR defies logic. But of the two products I can fine certified for this application, Unilin Thin-R (PIR) and Kingspan Kooltherm K1/K112 (phenolic), both have the breather membrane detailed in this location on their BBA certs and literature??
    •  
      CommentAuthorfostertom
    • CommentTimeFeb 2nd 2026 edited
     
    All a v gd reason to not have cavities in external walls, as is 100% possible and durable with timber frame construction, such as your inner skin. Is there a gd reason for the extremely expensive wallpaper (external brick skin) which yes necessitates that cavity?
    • CommentAuthorDanS
    • CommentTimeFeb 3rd 2026
     
    The reason for the brick skin is aesthetic only, continuity with surrounding buildings from a planning perspective + my wife's own preferences. I appreciate that this can also be achieved with slips on a render board etc, but in all honesty I'm unconvinced that this is a robust and time efficient method around opening reveals and other details.
    •  
      CommentAuthorfostertom
    • CommentTimeFeb 3rd 2026
     
    Shame. There are some attractive, traditional looking e.g. roughcast render finishes available for direct application to EWI. Personally, can't stand modern stretcher-bond brickwork, always a nasty caricature of beautiful old brickwork, if that's what's to be 'matched' for continuity.

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