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9 Inch Cavity Block Construction

  • 15-10-2022 6:56pm
    Registered Users Posts: 24

    Would appreciate your opinion on building a house with 9 inch concrete cavity block with internal 150mm insulated slab.

    I appreciate that this method might result in a lower BER and there is a risk of dampness but what are the other downsides. I have lived in a similar house for 10 years and have no complaints and it is easy to heat.

    I am interested in this method as I my foundation with be a raft and this would also reduce the weight of the overall building.



  • Registered Users Posts: 522 ✭✭✭mike_2009

    A raft foundation works best when integrated into the wall insulation to get a full wrap around with minimal thermal bridging. It tends to be a bit more expensive / specialized than a strip foundation most builders are familiar with.

    Nothing wrong with cavity block per se for the structure, especially if you can "join" the insulation of the raft and roof in one continuous line. The raft should have a structural outer beam to take the load of the cavity blocks. Dampness is about details - where is the heat/condensation line at different times of the year.

    You are sacrificing internal area to house the insulation. Although that more relevant with a retrofit, you can resize the rooms accordingly. I'd prefer to have external insulation so the home heating is stored in the walls and is released afterwards.

    You can use a site like the one below to "model" your wall makeup and determine the U value / condensation risk yourself but ideally an architect if onboard would advise:

  • Registered Users Posts: 24 Novice Self-Build

    Hi Mike

    Thanks for the advice. I will look at continuing or connecting the floor and wall insulation to avoid cold spots. My children are reared and I am not to worried about internal space. Before long there will be only two of us in the house.

  • Registered Users Posts: 38,486 ✭✭✭✭Mellor

    Why would it reduce the weight of the building? An insulated 9” block wall is heavier than a traditional 300mm cavity wall

  • Registered Users Posts: 2,981 ✭✭✭chooseusername

    For a cavity wall would you not need 2 standard 4" blocks (40 kg) per 1 standard 9" cavity block (27kg)?

    Or am I missing something here?

  • Registered Users Posts: 38,486 ✭✭✭✭Mellor

    I was assumed he was going with a 9" solid wall (as in a party wall) which weighs slightly more that a cavity block. But on a re-read I see he mentioned cavity block. Personal wouldn't go near cavity block walls for anything other than a shed.

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  • Registered Users Posts: 2,981 ✭✭✭chooseusername

    A cavity wall with standard 4" blocks would weigh about 50% more than a 9" cavity-block wall, and would be about 6" thicker

    But as said cavity-blocks are not ideal.

  • Registered Users Posts: 11,457 ✭✭✭✭Ush1

    What's wrong with cavity blocks?

  • Registered Users Posts: 8,180 ✭✭✭Furze99

    As a self builder, I've done both - 9" cavity blocks for shed and more conventional cavity construction with 4" blocks for dwelling. There may appear to be a bit more work with the latter but in many ways it's handier too as 4" blocks are more easily laid level & vertical etc and also handier to cut to size. Use reinforcing mesh in your slab and have a deeper ring around perimeter would be way to go I think. Check for radon ventilation advice as well.

  • Registered Users Posts: 24 Novice Self-Build

    Just catching up on this now. Thanks for the comments.

    A lot of houses built with a double leaf of 4 inch blocks with insulation in the cavity are still putting an insulated slad on the internal wall. Surely this eliminates any thermal mass benefit of the inner concrete leaf.

    I am wondering if a better investment is to build using a 9 inch single leaf block and a 100mm insulated lab.

    I have heard lots of negative theories regarding the 9 inch cavity but I am currently living in a house with this construction method and can keep the entire house warm with just a single pellet stove.

    I did oversee the fitting of the slabs and used competent people.

  • Registered Users Posts: 38,486 ✭✭✭✭Mellor

    Lining the inside of a insulated cavity wall is normally a retrofit to improve the performance of an existing wall.

    You wouldn’t necessarily detail a wall that way from the start

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  • Subscribers Posts: 39,905 ✭✭✭✭sydthebeat

    using insulated slabs on a new build is a bad idea and an indication of a lazy / unknowledgeable builder.

    building with 9" cavity block today, whilst still acceptable, is a very poor choice of building method.

  • Registered Users Posts: 24 Novice Self-Build

    I fully accept that theory and have read both of those articles many times. However 99.99% of these houses while may be cold due to a lack of insulation do not have any evidence of internal dampness. Also many house built with a 4" cavity wall are opting to put insulated slabs on the internal leaf and ICF also uses internal insulation. From personal experience I have no complaints of living in a house with cavity blocks with 100mm internal insulation.