Go to Forum Home Building Refurbishment and Retrofit EWI general advice and Victorian cavities

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    • #31644
      Gervase Mangwana


      I am running an energy assessment project in Manchester. We are doing blower door tests and thermal imaging along with a SAP based calc and occupancy/bill data analysis.
      The report for each property includes recommendations.
      The area in question contains a large proportion of early 20th century brick semis and terraces.
      Obviously these are contenders for EWI but I have a few questions before I feel able to give firm advice.

      There seems to be two types of EWI:
      1) Cladding with slabs of phenolic foam, mineral wool or wood fibre and then rendering
      2) Using an insulated render with some sort of poly beads in.

      I am pretty familiar with the concepts and U-values for 1) but know very little about 2). I understand it is used in thinner layers which means it won't perform as well but also that alterations to roof lines and windows might be avoided. Google has failed to show any useful info. Can anyone shed any light on this type of product. I'm expecting it to be alot cheaper /m2. Anyone got any rough costs for either type?

      One issue we have been surprised to come across is that every house we've assessed has a 30mm cavity on it's front wall. The fronts tend to be done in Accrington brick. There are no header courses and the walls are 260mm thick so I'm assuming a cavity of 30mm TBH. Some research shows this was pretty common. Have seen semis and end terraces with these cavities down the side. I spoke to an insulation installer who said they would not be able to fill them due to the limited width and probable presence of crud in the cavity.

      My question is can I recommend that EWI is used on these cavity walls? They are presumably ventilated at the top and through airbricks at the bottom.

      Thanks and Kind regards


    • #38472
      Mark Siddall

      Unless you can close the cavity you may have thermal bypass issues to deal with which could compromise the performance. I've retrofitted a pair of cavity wall semi-detached homes using EWI but we had to come up with some pretty demanding details to avoid bypass (ended up removing the roof tiles and taking an air vapour barrier over the top of the existing trusses, and then insulated over the top of that NOTE: we removed any inferior old insulation in the loft to avoid moisture problems.)


    • #38473

      I can't imagine the K value of render with EXP beads mixed in will be very good, and if in thin layers as you suggest (25mm?) it's not going to improve matters very much, so bang:buck ratio will be poor and people will be left wondering why they spent their money, maybe giving you hassle for recommending it.

      There really is no substitute for a thick (and continuous) layer of something fluffy when it comes to conductive losses! I forsee this being a bone of contention in the not too distant future, when we are forced to decide between keeping our pretty, maybe listed facades and (proper) EWI.

      Got a project on site right now where they bit the bullet and built a wooden frame around the house (solid wall) to create a 200mm void which will be filled with mineral wool, finished with wood wool boards and render. Not trivial or cheap, but will transform the thermal performance of the house.

    • #38474
      Tom Foster

      Was conventional EWI considered?

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