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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.

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    • CommentAuthorBlakey
    • CommentTimeJan 28th 2020
    Hi everyone,
    I have an 1897 mid-terrace and I'm about to embark on IWI of 2 external walls in a bedroom, the original (probable lime) plaster already chopped back to a red brick solid leaf (225mm-ish).

    I'm much impressed by Pavatex Pavadentro wood fibre (discontinued but still available, poss. discounted!), [other brand available] in view of it's breathable and vapour-open system which would go well up-against my equally breathable walls (with lime mortar). I'm reluctant to choose a modern insulation design in light their potential for thermal by-pass problems and sealing failures in any vapour barriers (=mould!), especially in a refurb. project like mine.

    In initial discussions with the local BCO he immediatly expects me to meet the 0.30 U-value for solid walls. This seems nigh impossible here as Pavadentro @ 60mm thick offers a 0.52 W/m^2K u-value, the 80mm is 0.42 -The 60mm is my preference as I have a sash window close to the wall corner, with it's box edge only 88mm from corner edge (for these batts remember I add 20-25mm of lime plaster). But I'd be happy with the thicker one if necessary.

    If anyone has ideas for my pleading mitigating circumstances (re the Regs) or any technical input at-all, I'd be very pleased to hear.

    One possibility I see is, as under the Regs, Part L1B, especially para 3.8, which says - "There are three further classes of buildings where special considerations in making reasonable provision for the conservation of fuel or power may apply:" . . and item C. states: "buildings of traditional construction with permeable fabric that both absorbs and readily allows the evaporation of moisture." - & THIS DESCRIBES MY PROPERTY!

    Also Pavatex themselves say that "reducing U-values beyond 0.4 inadvisible as would have too significant effect on rate of drying of (exterior) wall, hence frost damage".

    Any ideas? Many thanks
    Firstly, I'd recommend the 'Breaking the Mould' series of articles by Joe Little, available here: http://www.buildinglifeconsultancy.com/articles

    There's plenty to read there but you can arm yourself with the necessary statements and warnings about going too low with internal insulation (which backs up Pavatex's statement).

    I'd then talk to Natural Building Technologies who should be able to advise further (they do retrofit systems designed for different brick types) and should even provide you with Pavatex-specific u-value calcs. NOTE: they were recently taken over by a rigid insulation supplier and most of the people I know that worked there seem to have left, so I can't vouch for how helpful they are nowadays but they certainly used to know this stuff inside out and a few years ago were applying for BBA certificates etc.
    • CommentAuthorMike1
    • CommentTimeJan 28th 2020
    The Building Regs refer to the 'detailed technical guidance' produced by English Heritage.

    The part of English Heritage dealing with such matters rebranded as Historic England and their guidance can be found at https://historicengland.org.uk/advice/technical-advice/energy-efficiency-and-historic-buildings/

    It does include advise on insulating solid masonry walls; if you make specific references to their recommendations I would expect your BCO to take notice.
    Is your home listed? If it is or it's in a conservation area then it's exempt from meeting building regs:


    Another alternative is to fit better performing breathable insulation:


    Spacetherm Multi consists of Spacetherm Aerogel insulation blanket bonded to a 6mm Magnesium Oxide Board. Spacetherm Aerogel blanket is available in various thickness from 5mm to 40mm (in multiples of 5).

    Expensive, but as you're in a mid terrace, it's just the front and rear walls that need to be insulated for building regs (party walls don't)
    Welcome to the Forum.

    Pavadentro discontinued? Oh tish! Hadn't heard that. I've just quoted for another Pavadentro job. Best get onto NBT!

    I have used 100mm on a number of occasions which, with a 'base case' U value of 1.7 (SAP assumption - used to be 2.1 for many years), gives about 0.34. In a way I am delighted to hear BCOs questioning it, but you are quite right that the "buildings of traditional construction with permeable fabric that both absorbs and readily allows the evaporation of moisture." clause should work for you. The issue is going to be the BCO's opinion about what is a reasonable 'allowance'. He/she could argue that the max 100mm Pavadentro, giving 0.34, should be used.

    It's a bit late to advise you to leave the existing lime plaster on the walls as a parge coat. I think you'd have to be pretty keen with the trowel to use 20-25mm. I usually do a parge coat of around 10mm, which can be 'toothed' as the adhesive coat. Base and top coats need be no more than 6mm max.

    It's interesting that 'Also Pavatex themselves say that "reducing U-values beyond 0.4 inadvisible as would have too significant effect on rate of drying of (exterior) wall, hence frost damage".' NBT have done WUFI calcs for me on a number of occasions to check the wisdom of using 100mm.

    I am a big fan of this (ex?) product, and don't often do IWI with anything else, but am interested to hear why you feel that modern insulants should be any more prone to thermal by-pass than anything else.
    • CommentAuthorvord
    • CommentTimeJan 28th 2020 edited
    My building regs guy would have preferred vapour barriers and impermeable insulation, and I found it difficult to argue the potential damp issues on solid walls as he didn't have experience of older construction. No matter that the wall he wanted me to insulate was green with mould (before I started to fix the water ingress and let it breathe).

    I used a flat coat of lime, 60mm woodfibre, lime plaster. Seemed fairly safe when the outside of the wall is lime pointed and not painted, and I've not had problems 4 years later. I'm in a conservation area. It's not an exemption as such, but it's clear I wouldn't be allowed to replace things like gothic windows with nice new uPVC so had to secondary glaze instead under Part L1b.

    For a plan of action I would suggest you decide what insulation you are prepared to add, work out thicknesses and U values etc, and white a short summary about why that is appropriate and why the modern alternative would not be appropriate. L1b does have an exemption for stuff that would harm the building (or has more than 15 years payback on energy saving costs. My guy wanted me to go for 1000 year payback so maybe use as an argument to do what you want rather than not do anything).

    If it's a private building regs guy arguing you put modern stuff on an old building then maybe run away and go council. I have some mental health issues after not doing that 6 years ago when I first had the problem with mine.
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