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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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    • CommentAuthordiscotroll
    • CommentTimeOct 24th 2016
    Hi everyone -

    I'm just working on the build-up of the proposed insulation and internal structure of a conversion project but I don't seem to be able to find anywhere a clear and definitive guide on what u-values I need to be hitting re floor, walls and roof. :sad:

    I've read that as a conversion I need to adhere to Doc L1b (something around 0.18w/m2k for the flat roof, etc) then I see that if it's a new roof I need to adhere to new-build targets. I've seen info saying that the u-value can end up worse than a specified target figure and I've read other stuff saying that the overall property needs to come in at a crazy 0.13w/m2k.. :shocked:

    I've got my calculator for working out the build-up and thickness of insulation and other materials, but I could do with just knowing that the u-value figure I'm aiming for is 'X' and that is what I must hit to get building regs approval.

    If anyone could shed any light on this and how I can move forwards that would be great.


    • CommentAuthortony
    • CommentTimeOct 24th 2016
    Why aim at the minimum ones set by building regulations, in twenty years time we will be wondering why they weren't set better.

    How about aiming at 0.1 and 0.7 for windows.
    • CommentTimeOct 25th 2016 edited
    That's right. The cost of insulation is mainly in the fixed associated labour and accessory products - is relatively insensitive to insulation thickness per se. So you might as well not skimp on thickness.

    So how thick - as you say, what U value? Well, that's what the Passive House approach is about - to define the optimum U value that's sufficient but no more (diminishing returns). That depends on factors particular to each building - local climate, and building shape (i.e. ratio of heat-losing external surface area to useful floor area) mainly. But it'll certainly be higher than 'Bldg Regs', up in the 'crazy' 0.13 bracket.

    The optimum is effectively the threshold, the point at which suddenly you don't need to spend money on a heating system, the point at which the building pretty much heats itself unaided, thro solar gain, body and cooking heat etc.

    Of course there's more to it than that - airtightness as well as insulation, and 24/7 heat recovery ventilation.

    On a conversion (rather than new build) you may not be able to quite get to that optimum overall - the Passive House Enerphit standard shifts its emphases to acknowledge that.
    • CommentAuthorringi
    • CommentTimeOct 25th 2016
    Remember that 25mm of insulation fixed with no gaps and very good air tightness is better than 75mm fitted to normal “builder’s standards”
    • CommentAuthorDarylP
    • CommentTimeOct 25th 2016
    A 'conversion - change of use' dwelling should meet the minimum stds in AD L1B
    However as said above, these are minimum stds... in a few years you will be kicking yourself for not doing it better!
    Also the 'Regs' are different in England and Wales.....:wink:
    Aim for 0.1 U-values across the board, and 1.0 for windows/doors....:cool:
    • CommentAuthorJSC
    • CommentTimeOct 25th 2016
    Best summary I have come across is here...


    What ringi said is very pertinent.
    • CommentAuthorneilu
    • CommentTimeOct 25th 2016
    I would suggest downloading AD L1B and having a look through it, its free.

    Starting on page 12, paragraph 4.11 will confirm that a conversion is classed as a material change of use.
    The following sections then guide you as to the U-values you will need to achieve.

    Any existing retained thermal elements (floors, walls, roofs) need to comply with paragraph 5.12 and table 3. This basically says that if an existing element doesn't achieve a certain U-value then it has to be upgraded. For example an un-insulated cavity wall needs to be upgraded to achieve a U-value of 0.55 which is roughly equivalent to filling an existing 50mm cavity with good insulation.

    Any new thermal elements you build will need to achieve the slightly better U-values stated in table 2 on page 16.

    The possible grey area comes with if the current building isn't heated then is an existing wall a retained thermal element? Or is it a new thermal element bearing because it wasn't previously preventing heat loss.

    Most architects just treat all elements as if they were new and so specify the U-values in table 2. If you run with this then you won't go wrong.

    I'd like to say hope this has helped but I'm sure its just added to the confusion.
    • CommentAuthorneilu
    • CommentTimeOct 25th 2016
    The other thing you could do is simply ring up the technical departments of the big insulation manufacturers.
    Tell them about your project and they will advise you on what you need to do. Obviously they will specify their own products but they do give very good advice.
    I'd start with Kingspan or Celotex for rigid insulations and Knauf for mineral wool type insulations.
    • CommentAuthordiscotroll
    • CommentTimeOct 25th 2016
    Hi everyone -

    Just back from work and had a chance to read through all of the helpful comments.

    I should have made it clear that I wasn't just looking to squeeze in under the threshold on u-values but was more genuinely confused by the array of information out there.

    I agree that both kingspan and celotex sites are really good for helping get the best solution (don't mind so much them specifying their own product as it's part of the exchange, as it were).

    I'll be doing the insulation/energy side of the job myself so it'll be done properly with absolutely no air gaps in the insulation materials and as close to a continuous layer between me and the outside world as possible.

    right - I'm off to follow up the links supplied and do some further reading...:bigsmile:
    Are you internally or externally insulating the walls? External is much more risk-free. If you have to go internal, the whole process becomes much more of a risk management exercise.
    • CommentTimeOct 25th 2016
    Posted By: neilua conversion is classed as a material change of use
    Ang on, are we sure what's meant by 'conversion' in this case? if simply upgrade/alteration/extension it's not 'material'.
    • CommentAuthordiscotroll
    • CommentTimeOct 25th 2016
    We're conversion - as in converting a non-residential to residential. All is internal so, as you say, there's a lot more to think about than simply laying on the insulation....
    • CommentAuthorringi
    • CommentTimeOct 25th 2016
    Look at putting up a stud walls close to but not touching your current outside walls, then getting someone in to spray insulation. Good airtightness and a LOT quicker than cutting insulation boards to fit.
    (I wish I had done this!)
    • CommentTimeOct 26th 2016
    The only safe way to do that - and it's the v best way to do IWI if you can spare the space - is to make everything 100% breatheable (but airtight) and COPIOUSLY ventilate bottom to top the min 50mm clear space between stud and existing wall. That space must be effectively outside air, the outside wall effectively a freestanding garden wall, or a rainscreen (though it can take structural stability from the internal stud frame if necessary).
    • CommentAuthordiscotroll
    • CommentTimeOct 26th 2016
    Thanks Ringi and Fostertom -

    That is the way I'm planning on working - like a completely separate sealed-off inner structure where the insulation takes place and we live, with the outer original structure just supporting the roof etc and being cared for through ventilation etc between the two.

    Good to know that I'm thinking along the right lines....
    • CommentAuthorcjard
    • CommentTimeOct 30th 2016
    I did this, built my conversion as though it were a timber frame where the outer was erected first. It's certainly been a challenge. The only part of the existing structure I attached to was the roof, turning it into a warm deck. A big part of me wishes I hadn't bothered, as it's taken months and months. Using spray insulation would have been fast, instead I cut and foamed kingspan in every part and then covered it with a vapour control layer that was meticulously taped and glued at every seam. Shaping it round iron roof trusses was slow work and hard to get the excellent result I wanted.
    I've probably spent fifteen thousand quid on kingspan and membranes/or on/Tescon tapes.. the icynene quote was 25k I think.. for the extra ten grand I'd have got an awful lot of my life back, probably 5 months of it.. it's hard to weigh up sometimes, because I only get 2 days a week to build, the rest is taken up with full time job so while in raw days it would have saved about 40, its one of those things where.. is it 10k for 5 months hence 25 ish grand a year, I.e. Small fry, or is it 10k for 40 days hence more like 85k a year I.e. A reasonable saving on the mortgage..

    Ask my little lad, who once upon a time said "see you later, daddy going to the building site" but increasingly these days is pulling a face/crying as I leave, what he thinks the time is worth!
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