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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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    Hi, I'm looking for some advice on IWI options and the potential for extension works to cause rising damp issues for a house built in c1902. The walls are typically 9" brick.

    The front and the back are on different levels, so that we currently effectively have basement (c 1.5m head height), ground and first at the front and ground, 1st and 2nd floors at the rear. Raised timber floors with air vents at ground level, from what I can see at basement level there is no existing insulation under the floor and just 100mm or so of fibreglass in the loft.

    We are in London and the front of the building is south facing, no signs of existing damp. The house in in a conservation area and adjacent to a locally listed building so EWI is unlikely to be an option.

    It is in a terrace but has an an access arch through and a rear outrigger, so most of the ground floor also has an external ground floor wall to the side and even the small suspended timber floor over the top of the access arch has 4 large air vents underneath.

    We are planning to dig down and underpin the 4m x 4m basement so this has sufficient headroom to be usable and install a delta membrane or similar system and c50% of the rear elevation will also be extended, for which I assume a new concrete slab/dpc and c80mm of insulated PIR would be the usual spec and perhaps a new concrete slab may be suggested for the existing rear ground floor. We may also potentially add a dormer to the loft.

    They are not all in great condition, but all the existing windows are double glazed UPVC.

    It is also currently split into flats, so we will need to do significant work to the electric/gas supplies and plumbing, so given all the work that we plan to do in any event to extend and covert back to a single dwelling, moving sockets and possibly radiators off the external walls shouldn't be a major issue if this is recommended.

    My queries are:

    1) how do we mitigate the extension changes to the basement and ground floor leading to rising damp in the existing structure?;

    2) would any IWI options be recommended, again bearing in mind the milder weather in London and the south facing front external wall?; and

    3) what are the things to consider for chimney breasts and the chimney with regard to insulation/ventilation?

    4) should we also be thinking about benchmarking most of the works against a likely need to achieve EPC C in the future when selling the property? I would also add that I expect to want to sell the property in 10 years to move back to the area of London that we live in now with regard to EPC and the cost of IWI vs payback in heating cost savings.

    Ideally the IWI would be a basic insulated plasterboard DIY option that I could just mechanically fix, without the added work of needing to add a coat of anything to the rear.

    Having had real issues with finding a plasterer that could competently use lime plaster in the past I would much rather use an alternative plaster if possible, accepting that some breathability may be lost.

    Sorry for the long post, any advice will be gratefully received!

    • CommentAuthorEd Davies
    • CommentTimeJan 3rd 2022
    Why would the extension have a concrete slab rather than an (insulated, of course) timber (or other suspended) floor? I've no idea what the increase in rising damp risk for the existing house would be but imagine it would be a lot less with a suspended floor rather than a slab.
    • CommentAuthortony
    • CommentTimeJan 3rd 2022
    For me 400mm of quilt in the loft
    Prefer EWI to IWI in both cases thicker the better, I like 200mm
    I am unconvinced by rising damp,
    Chimney breasts on wholly internal walls don’t need to be insulated but the will such heated air out of a building. On external walls, I fill to level,of the top of the loft insulation with eps beads and trove a brick for ventilation and cap the pot.

    I would aim high, go for A or B , C is very poor and it is easy and cheap to double the insulation while you are doing it compared to doing it again at some future point.
    Hi Ed,

    Sorry, you're probably right. Looking at the structural plans for other works in the same road these look to have a concrete slab in the basement and suspended timber in the ground floor extension.


    • CommentTimeJan 3rd 2022
    Hi Nick, it might be worth looking at https://www.leti.london/retrofit if you haven't done so already.
    Thanks Tony,

    Given the conservation area and the access passage to the side I don't think EWI will be acceptable to the Council here and so it would need to be IWI as the option for insulating the external wall elements that aren't being extended.

    In many areas particularly on the side external wall to allow for access through doorways etc I'm unlikely to find it easy to go anywhere near 200mm, so only c50mm may be readily achievable in some areas.

    Thanks Dave, I wasn't aware of this resource.

    Have to be very honest here and say it's a poor start when you have so much detail about house usage plans - basement, remodelling from flats, extension, dormer in the loft - and yet 'green building' aspects involve no EWI (have you actually asked about the conservation area status?), DIY insulated plasterboard and EPC C rating.

    The point here is that for a whole house refurb it's a wasted opportunity - hardly 'green building' - unless you adopt, for example, the LETI strategy. As an investment, too, it makes sense to invest in an EnerPhit standard refurb - imagine how attractive that will make the house in 10 years (when you plan to sell).

    Just one example from LETI - p162 of the main guide, 'C.1 High level insulation strategies in a conservation area' could be a start. Maybe lots of people on here will 'know better' about the LETI guidance but it looks to offer a great start to properly planning a green building project - probably involving help from an EnerPhit person (LETI's 'about page' talks about members 'volunteering their time and expertise' so it may not cost to get help?).
    EWI has significant disadvantages in a terraced house (as well as the visual ones) that you end up with cold bridges all around the front and rear wall insulation, where it meets the foundations, both neighbours, and the roof. Whereas IWI can be joined easily onto the underfloor and loft insulation and can be returned down the inside of party walls, so fixing those cold bridges. If there are internal partition walls that meet the front/rear wall, IWI can be continued through them if they are lath/plaster, but needs to be returned down them if they are masonry.

    Set against that, IWI reduces internal space, in my experience this isn't noticeable unless in (say) a narrow stairwell. IWI will be messy and disruptive, especially if there are finishes you want to keep (cornices etc).

    So it's a judgement call which one is appropriate for each building, one size doesn't fit all. We went for IWI in our previous old house and are tending to EWI in our current one.

    On the plus side, terraces intrinsically lose less heat through external walls than detached houses do, and so floor and roof insulation and airtightness are relatively more important.

    Search for the Historic Scotland Refurbishment Case Study series, they found a lot of insulation was possible in old buildings without compromise on damp etc.

    Also consider the embodied carbon/resources in the materials you use - timber and mineral wool are better than cement, lime, polystyrene, etc.

    Finally you didn't mention how you will heat the building and provide hot water - a U=0.3 building with a heatpump uses less primary energy and carbon than a U=0.1 building with electric or gas heating, so that needs to be factored in your design.

    Good luck!
    Thanks John and Will,

    Looking at the LETI strategy I note that for the examples that include costings the minimum is c£100,000 per residential unit, up to many multiples of this, which may be affordable for Councils and housing associations to implement as case studies to help them review options for their housing stock but are not viable for most householders and my own financial realities, hence the need for a DIY compromise solution involving IWI that I can install room by room.
    It's about the proportion of cost assigned to energy efficiency and the need for a whole house strategy.

    The latter is a problem on here in that GBF has failed to move on from well meaning individuals as the basis for knowledge of green building. If we had developed our thinking we'd be experts in published whole house methodologies, instead of being experts in individual aspects of green building.

    If that had happened, we would probably point you to low-cost whole house refurb design methodologies which might enable you to incorporate far more and save more energy than might be thought through 'common sense'.
    Historic Scotland ran a long research programme into energy saving refurbs of typical pre-war buildings (IE not precious listed monuments), there are a lot of academic papers and dozens of case study buildings, this guide book is a good intro. They did a lot of work on different kinds of IWI - they also discuss EWI but only recommended for certain situations.

    It is very much possible to save the majority of the energy losses from these buildings without damaging them in the process, but needs different approaches from those that may be appropriate for newer buildings.

    "Fabric Improvments for Energy Efficiency in Traditional Buildings"

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