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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

PLEASE NOTE: A download link for Volume 1 will be sent to you by email and Volume 2 will be sent to you by post as a book.

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    • CommentAuthorsmudger
    • CommentTimeJul 30th 2018
    I'd appreciate help on understanding fire resistance of various natural and petrochem insualation materials.

    I'm looking at options to tidy up the internal walls during renovations. We already have external wall insulation and we are at 0.30 U value. Its likely to be 30-50mm insulation internally, which would not increase interstitial condensation (if permeable open) and be accommodated within the loss of room space.

    There is a lack of info on websites on the fire resistance / flammability of the materials. Apart from personal safety (noxious fumes) we live rurally so 30 mins for brigade response likely.

    Refs to class O (spread of flame) and EN13501 (without referencing classification).

    Options being considered are Celotex or XPS (Marmox for bathrooms/block walls) or wood fibre / cork for breathable sections. Some reference class E which 'could lead to flashover'. Only Rockwool promotes its A1 rating which makes me suspicious of the others.
    • CommentTimeJul 30th 2018
    The way the regulations are written provides a loophole that most insulation manufacturers exploit. The regulations care about the performance of installed systems, so if it is assumed that all the insulation is behind a sheet of plasterboard, then the sheet of plasterboard is what determines the fire resistance to a very great degree. So insulation suppliers tend to rely on that, and minimise discussion of the products they sell.

    Polystyrene (EPS & XPS) melts and burns at quite a low temperature, so building products usually have fire retardant added. Never use recycled packaging or old ceiling tiles since they don't have fire retardant.

    Polyurethane (PUR & PIR) doesn't melt and is quite difficult to make burn, but in a hot fire they produce hydrogen cyanide.

    Phenolic is also quite difficult to make burn and produces less toxic products when it does. If I were going to use a plastic product somewhere it might burn, this is what I would choose.

    Natural products tend to burn with smoke, but hopefully reasonably slowly. Some have fire retardant treatment added.

    Mineral wool is fundamentally safer than all the above.

    Having said all that, I do have XPS boards (Bushboard Nuance) exposed in my bathrooms. :devil:

    There's a good article at
    Yes, the key thing is that the insulation is protected by plaster/board. If the fire gets so intense it can breach the plasterboard, then the radiant heat will be sufficient to ignite anything combustible in the room, including wooden furniture, doors and floors, and (sadly) people, this radiant ignition is called 'flashover'. The fire performance of the insulation in this circumstance will not be much concern to those affected.

    I tried burning offcuts of T&G floorboards and skirting - the resinous pine burns like mad - I see the wood in our house as much more a threat than the PIR insulation, which is all behind plasterboard.

    Wood produces both carbon monoxide and hydrogen cyanide in a house fire, as well as 1000s of other compounds. The carbon monoxide is more often the cause of death, but has less 'fear factor' in the news than the cyanide. Smoke and CO detectors have been a big improvement in the last decades.
    • CommentAuthorSilky
    • CommentTimeJul 30th 2018
    there are some well written documents from fermacell that give example partition wall build-ups and tell you what you should achieve in terms of fire protection and sound performance, I found those useful.

    my renovation is full of OSB, a huge amount, that stuff also burns like crazy once it gets going, I put some off-cuts in my wood burner and it set fire to the chimney.. the flames went roaring up, had never happened before.. even when loaded to the top with ash logs, I presume because it was giving off some gas which then ignited, quite terrifying. In the end I will have a floorrock type product + dry-screed board on the tops of all floors and all the ceilings have fermacell on the underside, and there are no cavities for fire to burn through.. belt and braces!
    • CommentAuthorsmudger
    • CommentTimeJul 30th 2018
    Many thanks. I'll restrict Marmox to bathroom (with tiled finish) and look at cork through out. I want to avoid plasterboard finish - and strictly it would only offer 30 mins FR at most without upping spec (and I'm trying to avoid battening out). I took a financially pragmatic view with the external eps insulation but it just doesnt seem right to increase risk internally.

    there was another post about fire resistance of lime plaster which consensus was yes - this includes the specialist finishing coats that come with wood fibre and cork?
    • CommentTimeJul 30th 2018 edited
    Posted By: smudgerthere was another post about fire resistance of lime plaster which consensus was yes - this includes the specialist finishing coats that come with wood fibre and cork?

    I would think so, but do check the individual product documentation, or ask the manufacturer in writing.
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