9 Inch Cavity Block Construction

Novice Self-Build · 15-10-2022 6:56pm #1

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.

Thanks

mike_2009 · 16-10-2022 3:33pm

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:

https://www.ubakus.com/en/r-value-calculator/

Novice Self-Build · 16-10-2022 6:15pm

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.

Mellor · 17-10-2022 12:25pm

https://www.boards.ie/discussion/2058267640/9-inch-cavity-block-construction

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

chooseusername · 17-10-2022 7:06pm

https://www.boards.ie/discussion/comment/119755845#Comment_119755845

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?

Mellor · 18-10-2022 10:20am

https://www.boards.ie/discussion/comment/119757658#Comment_119757658

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.

chooseusername · 18-10-2022 11:10am

https://www.boards.ie/discussion/2058267640/9-inch-cavity-block-construction

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.

Ush1 · 18-10-2022 11:33am

What's wrong with cavity blocks?

Furze99 · 18-10-2022 11:39am

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.

Novice Self-Build · 19-10-2022 5:42am

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.

Mellor · 20-10-2022 8:32am

https://www.boards.ie/discussion/comment/119763512#Comment_119763512

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

sydthebeat · 20-10-2022 10:09am

https://www.boards.ie/discussion/comment/119763512#Comment_119763512

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.

https://passivehouseplus.ie/articles/insulation/breaking-the-mould-part-i

https://passivehouseplus.ie/articles/insulation/breaking-the-mould-part-ii

Novice Self-Build · 20-10-2022 1:00pm

https://www.boards.ie/discussion/comment/119769773#Comment_119769773

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.

ballinea · 26-02-2024 7:14am

Just wondering if you went with the cavity block in the end? Doing similar here and thinking of using a cavity block with a damp proof membrane on inside.. and closed cell insulation spray fill in a 3" stud..

Novice Self-Build · 26-02-2024 7:51am

In the end I went for two 4" blocks with 150mm full fill insulation, blowerproof liquid paint in inside of inner lear and an insulated slab on top of that, with every joint taped with foil tape. I took my time and spent a bit of money but very happy I did so.

I also got a quote of 9K for the blockwork for a 200 sqm house so good value overall.

9 Inch Cavity Block Construction

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