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Cavity wall or cavity block

  • 08-08-2022 4:26pm
    Registered Users Posts: 8,894 ✭✭✭

    We have a cavity block house built 2012. Builder is asking do we want cavity wall or cavity block in extension.

    Cavity wall is better but then inconsistent with rest of house so thinking cavity block.

    Any advice appreciated.



  • Registered Users Posts: 751 ✭✭✭C. Eastwood

    If your in Dublin the builder will probably want to use a cavity block wall.

    I’m not in Dublin and I would only specify a Cavity Constructed Wall in the Extensions.

    I haven’t Specified a Cavity Block Wall in many years.

    It is necessary along with fixing insulation in the cavity to also fix an Insulated slab to the inner face of all the existing external walls and all of the the new extension external walls, to prevent cold bridges.

    See photos below

  • Moderators, Home & Garden Moderators Posts: 10,130 Mod ✭✭✭✭BryanF

    In photo 2 where is the air-tightness wall ‘line’?

    re insulation and cold bridges: all floor & internal walls are thermal bridges with the dry-lining approach, the op may be better factoring in ewi into the extension costs, for the whole property, and just go with block on flat for extension with ewi.

  • Registered Users Posts: 8,894 ✭✭✭Tim Robbins

    Thanks a million. The builder was proposing cavity wall. My question was is it ok to do extension in cavity wall if the rest of house is cavity block?

  • Registered Users Posts: 3,202 ✭✭✭cruizer101

    Were they still building with cavity block in 2012?

    I thought any houses built this century would be cavity wall, cavity block was only for sheds/garages.

  • Registered Users Posts: 751 ✭✭✭C. Eastwood

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  • Registered Users Posts: 762 ✭✭✭keno-daytrader

    Those photos don't show good air tightness practices?? Is this a modern build, which I assume it is as the hollow core seems to be wrapped with an airtight membrane.

    The insulated plasterboard is fixed to the block with no airtight layer?

  • Moderators, Regional North West Moderators Posts: 45,350 Mod ✭✭✭✭muffler

    You should never allow builders to come up with proposals such as the type of wall that will be built. You need to engage a professional (who doesn't have to be chartered btw) who will design, specify, inspect and certify the correct materials and methods to ensure compliance with statutory building regulations.

  • Moderators, Home & Garden Moderators Posts: 10,130 Mod ✭✭✭✭BryanF

    Noting wrong with block/cavity/block construction, junction with existing house will need to be detailed to avoid thermal bridging etc. all doable. But depending on budgets etc I’d be leaning towards ewi (external wall insulation)

  • Registered Users Posts: 30,975 ✭✭✭✭Lumen

    Insulation on either side is a PITA to fix into/through and EWI is more fragile than rendered block, so I would have thought that a cavity wall offers the most robust and flexible option in a new build extension.

    Are there other arguments for EWI over an appropriate insulated (pumped?) wide cavity in an extension?

  • Registered Users Posts: 751 ✭✭✭C. Eastwood

    I would prefer to construct the new cavity wall Extension with insulation and a residual cavity between the inner and outer concrete leaves. This wall does not require EWI, and applying EWI to it is a complete waste of money.

    Connecting the new Cavity constructed wall on to an existing Cavity Block wall as seen in the photo I posted above on 8 Aug at 6.55, the existing Cavity Block wall will be a cold bridge.

    Even if EWI is fixed to the external face of the Cavity Block walls, the cold bridge at the junction of the new walls and the existing walls must be eliminated. There are many solutions to do this.

    One of many solution to this is to construct the cavity wall without any insulation and fix EWI to the new and existing walls if soffit and services etc allows.

    EWI will be more expensive for the extension than using cavity wall insulation as seen in the photo.

    I would not allow a New Cavity Block wall to be constructed for the proposed extension.

    Furthermore, a builder will give biased advice - and will utter the words- “the best thing to do”……

    the builder will not finish the sentence which would be as follows- “the best thing to do is ….blah blah... for me to take as much money from you as possible an do as little as possible…”

    Get professionals advice - retain a Construction Professional.

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  • Registered Users Posts: 8,894 ✭✭✭Tim Robbins

    What type of professional do you need for advice? Thanks

  • Registered Users Posts: 751 ✭✭✭C. Eastwood

    You can retain any Private Practice with one of the following Professionals:-

    1. Chartered Building Surveyor. Look up the website of the Society of Chartered Surveyors of Ireland.(SCSI)
    2. Registered Building Surveyor. Look up SCSI.
    3. Chartered Building Engineer. Look up -
    4. Chartered Civil Engineer. Look up Engineers Ireland.
    5. Architect. Look up RIAI

  • Moderators, Regional North West Moderators Posts: 45,350 Mod ✭✭✭✭muffler

    If you're looking for advice then any of the professions mentioned by Clint but they don't have to be chartered. He's been pushing this "chartered" crap for ages.

    Add architectural technicians to that list of professionals.

  • Registered Users Posts: 5,677 ✭✭✭10-10-20

    Photo 3 appears to be missing the physical separation cut on the existing wall within where the DPM would be installed to separate the external and internal portions of the vertical wall. Either that or how is moisture prevented from tracking in via the block and remaining render?

  • Registered Users Posts: 751 ✭✭✭C. Eastwood

    As I indicated by you, If there was a vertical cut in the existing cavity block wall to facilitate the insertion of a vertical DPC, in location of the new cavity, the 150 mm remaining cavity block to the existing window reveal at the RHS, would be compromised,

    Furthermore if the slot for the vertical DPC was cut in 100 mm in to the existing Cavity Blocks at this junction, the remaining short 150 mm pieces of cavity blocks may result in a vertical crack on the inner face of the wall.

    One size does not fit all in building works to Existing buildings.

    Remember the Technical Guidance Documents - inter alia for Existing Buildings - the magic words, “as far as practicable “

    Your comment:- “Either that or how is moisture prevented from tracking in via the block and remaining render?”

    My Answer:- To prevent the ingress of rainwater through the remaining 50 mm of cavity block between the external face of the new wall and the reveal of the existing window, the wall was plastered properly as specified by me.

    Further education for you, Cavity Block Constructed external walls are being constructed in Ireland since the 50’s, and are still used in Dublin, and when rendered properly and correctly on externally for their location it prevents the ingress of rainwater.

    Heretofore there was no ingress of rainwater through the externally rendered Cavity Block wall which was constructed in 1957.

    I put these photos of real examples of construction to help the OP. As usual there are many wonderful praising comments by critics which I have and will ignore.

    I did not see any photographic examples put up by you and any the of the clique.

    I will not be replying to you or any other wonderful comments here, no matter how ridiculous.

    Note to Tim Robbins- I will reply and answer any queries that you have.

  • Registered Users Posts: 751 ✭✭✭C. Eastwood

    Chartered Crap:-

    Not all of the above listed are Chartered. I do not know where you got that idea.

    Architectural Technician???????????????? thought these were replaced with the invention of ‘cut and paste’

    OP, add a Chartered Architectural Technologist to the list above.

    O P, I should have added Chartered Architectural Technologists, who are excellent and very well qualified to look after your proposed refurbishment and extension.

    Look up CIAT for the list of Chartered Members (MCIAT) at

    A recent Graduate with an Honours Degree in Architectural Technology will be an Architectural Technologist and will have the necessary education.

    Moreover, a Chartered Architectural Technologist (MCIAT) will have the necessary Education and Experience and Training and Knowledge and Employment.

    Furthermore all Members of CIAT must comply with the Code of Conduct of CIAT.

    OP, only retain a Construction Professional who is a Member of one of the Professional Institutions listed by me above.

    If a Member of any of those Professional Institutions, does not preform their Professional Services and Duties in accordance with the Institutions Code of Conduct, the Member may be Sanctioned, Fined and/ or Expelled.

  • Registered Users Posts: 5,677 ✭✭✭10-10-20

    Get you, thanks for the response. I was wondering if there was a type of tie available for stitching such or would the window ope need to be modified, but it seems not.

  • Registered Users Posts: 8,894 ✭✭✭Tim Robbins

    I think is what we will do. Thanks for your help