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EWI vs IWI for solid wall house

  • 18-01-2022 4:21pm
    Registered Users Posts: 26,046 ✭✭✭✭ GreeBo

    Hey all

    I have a 1930s detached house which appears to be rendered, solid brick with no cavity.

    We are in the process of speccing out a major refurb which will involve new windows & doors, replacing suspended floor with insulated concrete pad, heat pump, solar panels etc.

    It will also involve re-rendering the exterior (currently pebbledash), drylining internal walls (which are wet plastered) and new internal floors throughout.

    The architect is pushing for all IWI whereas all my reading is telling me that EWI would be more efficient and a better use of the thermal mass. I also don't want to lose 3-4 inches on external walls, especially where the stairs is.

    His argument is around what it will do to the external look of the house around window reveals & cills etc. and that part of the planning reqs are to be sympathetic with existing/surrounding houses. However many of them have been refurbed over the last year or so and are a more modern, smooth rendered look.

    What are anyones thoughts? Cost seems to be around 10-15K more for EWI over IWI. My only concerns with EWI are below the DPC and how to avoid a thermal bridge from the foundations as I can't see how it will be possible to avoid this, only minimise? Also how to join with roof insulation. Warm roof vs cold roof for example?

    Help! :)


  • Registered Users Posts: 8,029 ✭✭✭ Markcheese

    Well he's the professional .. but youre already going to be changing the doors and windows and replastering outside , so I don't see why you'd go with internal - it's by far the poorer job - the sills can be altered to suit while fitting the new windows , and you'll prob be bringing out the new windows as well , so it won't look much different -

    There's going to be a level of cold bridging with ewi - it's not a new build but it'll be far less than with internal ... And you're probably able to insulate well below the dpc inside and out which should take care of a lot of the old bridging ..

    Slava ukraini 🇺🇦

  • Registered Users Posts: 2,716 ✭✭✭ MicktheMan

    External all day long and twice on a Sunday.

    For many reasons that have been debated to death on here over the years. I would also not dry-line the internal face of the external walls, instead take advantage of the high thermal mass of the external walls matched to your low grade heat source & distribution system.

    Tb's etc can be easily eliminated with proper detailing including the continuity with the roof insulation.

  • Moderators, Home & Garden Moderators Posts: 10,110 Mod ✭✭✭✭ BryanF

    Extend insulation externally below ground level to top of foundation (or 600mm below internal floor level). Extend internal insulation when digging out, down the equivalent distance on the inside. This will address the Thermal bridge.

    re roof / wall junction, is this a terrace/semi-d? If not you could lift the last few rows of tiles and insulate. If it’s connected to other dwellings then you’ll need to look at the details - I’d probably use aerogel to overlap ewi and attic insulation, twill be slow install around joists etc, Considering ventilation as you go.

  • Registered Users Posts: 4,258 ✭✭✭ 10-10-20

    BrianF, do you have any detail on how the aerogel could be used as such? Would it be applied as a flat blanket installed horizontally, overlapping the EWI and covering the wall-plates?

    Also, is aerogel now easily available as the last time I looked it was bonkers-money to buy and the sources appeared questionable/grey-market.

    Anyone got any off-cuts? 😉

  • Moderators, Home & Garden Moderators Posts: 10,110 Mod ✭✭✭✭ BryanF

    Bonkers moneys, but serves a purpose in tight spots. If space allows use mineral wool, problem is older buildings don’t generally have great ‘height’ to insulate within.

    Post edited by BryanF on

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  • Registered Users Posts: 4,258 ✭✭✭ 10-10-20

    Like look at this for crazy: Radionics - never the cheapest on the best of days - have an 100cm x 70cm x 10mm Aerogel blanket at 192 EUR ex-VAT - and they don't even mention the R-value in the datasheet!

    Anyway GreeBo - sorry to get off track.

  • Registered Users Posts: 26,046 ✭✭✭✭ GreeBo

    Its detached so (other than budget!) we are free to do what we want.

    I was thinking that converting to a warm attic would help to continue the EWI to the roof insulation without any bridging, but not sure what that will mean regarding the roof.

    It's a terracotta clay tiled roof (which doesn't seem to be original but I would guess is still 40+ years old) but there is felt underneath so Im guessing its not from 1930s. At some stage its going to need to be redone as many of the tiles are blown and there are a few tears etc in the felt.

  • Registered Users Posts: 26,046 ✭✭✭✭ GreeBo

    On the insulation part, what you are suggesting ties in with what I have seen recommended in other places, but I guess there is still a bridge since the thermal envelope is not continuous?

    Assuming its best that can be done(I certainly can't think of a viable alternative!), is it acceptable since the foundations are below grade and such won't be as exposed/cold as an external wall would be, so its not acting as a heat sink? Or are there still risks of condensation in the corners?

  • Registered Users Posts: 26,046 ✭✭✭✭ GreeBo

    The internal drylining would just be to create a services cavity and "fix-up" the walls aesthetically.

    Would plain plasterboard (9mm?) with a services cavity impact the ability of the walls to be a thermal mass?

    /edit it would also need a VCL on these walls anyway, right?

  • Registered Users Posts: 2,716 ✭✭✭ MicktheMan

    Not sure exactly of what you're asking here.

    A service cavity should be on the warm side of a vcl or air tight barrier

    Vcl or air tight barrier should be on warm side of the insulation layer

    Any thermally light material (PB) to the warm side of a block walls will reduce the thermal mass of the wall (i.e, the thermal storage capacity of the wall)

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  • Registered Users Posts: 4,432 ✭✭✭ Gusser09

    Have EWI myself and have to say I dont think the performance of IWI comes anything close to it. You wont look back. The level of comfort from EWI is too class.

  • Registered Users Posts: 26,046 ✭✭✭✭ GreeBo

    Yeah exactly, the architect wants IWI with a services cavity on the warm side, I want EWI with a services cavity internally behind new plasterboard wall.

    I take your point about blocking the thermal mass and actually its 'worse' since my internal walls are also solid.

    I guess that would force us down the chasing and wet plaster (lime for breathability?) finish and make it much harder to make any additions etc in future, especially with solid floors. Well have to plan that out well in advance! (I'd be used to just adding a socket or rad/tap as required :( )

    Do you not need a VCL with this approach? The external wall "dries" into the room if needed?

  • Registered Users Posts: 946 ✭✭✭ iColdFusion

    My gut would have said internal insulation is the way to go unless you would be losing too much internal area, you are already planning on dry lining it so it would be pretty easy to add an insulation layer to that and focus on your internal air tightness.

    Why do you need the thermal mass of the external walls? Surely you will have underfloor heating with your new heat pump and the floor is your thermal mass that you heat over night?

    For me all I see is those old external walls sucking the heat from your rooms and transferring it down to their foundations, sure they are thermal mass but only once you have heated them up!

  • Registered Users Posts: 4,432 ✭✭✭ Gusser09

    Just for reference I'm sitting in my home office now.House is EWI. Heating has been on for 1 hour today at about 10. Room is currently 26.1 degrees. Never drops below 21 regardless of heating being on during the day or not.

  • Registered Users Posts: 26,046 ✭✭✭✭ GreeBo

    Other than the stairs I'm not too worried about losing internal space, but Im terrified that we will end up with a house full of individual thermal/VCL rooms and that either then builder or some time in the future, someone is going to puncture the VCL hanging a tv or cabinet and expose a cold wall that will result in damp & mould.

    The exterior is being totally redone anyway (windows, cills, fascia/soffit/gutter, re-rendered, chimney removal) so I'm hoping this would offset much of the additional cost of EWI over IWI.

    The inner walls need something done to them, be that drylining or lime plaster etc, I'm assuming they are already lime, the majority arent boarded anyway.

    We will have UFH both upstairs and down (down will be poured concrete floor, upstairs will be trays or a thin screed)

    We get huge solar gain in the house, it was 26* over the weekend in the south facing rooms with no heat on and <5* outside.

    I partly want the mass to keep the place cool in summer!

  • Registered Users Posts: 1,423 ✭✭✭ hesker

    In very similar position to yourself OP. Old single wall house. Engaging with companies hoping to avail of SEAI grants. Waiting on an announcement on this that was promised in Oct to come out before the end of the year.

    Incredibly frustrating process. The most engaged and promising company has no interest in addressing air tightness for example.

    You could always apply for a section 5 declaration on the EWI to answer your architect’s concerns.