# hollow block wall - how much inside insulation?



## drunat (11 Apr 2013)

Hi. We are doing kitchen extension at the moment - had agreed with the builder that he'll be doing hollow walls, he has now changed his mind and said because the house is built of hollow block he has to use hollow blocks so the width of the walls matches and there is no "step in" i.e. the new and old wall joins rather than sticks out. He said he would put 100mm high density insulation on the inside and it will be higher than regulation requires. I heard hollow block walls are not great - do you think they could be made good with sufficient insulation as he suggests?


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## quentingargan (12 Apr 2013)

I'm no architect, but I would look for another builder personally. There are other ways of ensuring that the wall is the same width. How about timber frame for the extension for example?


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## itsallwrong (12 Apr 2013)

The proper way to build exterior walls is twin leaf cavity block.
Make your own mind up by reading the links below.
One thing... Get it right now - cost a lot more in the future to retro fit.

http://www.kingspaninsulation.ie/applications.aspx?cid=walls 
[broken link removed]

The best Kingspan is Kooltherm based products.

'has to use hollow blocks'?
I think you should prioritise doing the job 100% over looks.


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## Leo (12 Apr 2013)

Change builder or get them to come up with a better solution! Hollow blocks are a disaster, ask them what producxt they intend to use for the internal insulation, and what U-value that will achieve.

You also need to know the insulation details around the floor slab and roof interface points. 

Do you have an architect or engineer representing your interests on this project? Who is going to sign-off that the work is compliant with the building regs?


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## drunat (12 Apr 2013)

the engineer OKed it. It's only two walls only, they'll be putting on 100mm high density kingspan foam (on batons so there will be a gap between  the block and insulation), ensure no dampness comes through the insulation (tape?) and then plaster board I believe. They told me the U value will be higher then required. Not sure about outside - they mentioned some waterproof finish or something...


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## SPC100 (12 Apr 2013)

Due to the possible health hazards from mould, and the risk of mould growth due to interstitial condensation I would not put 100mm of insulation inside a hollow block wall, especially if I had another choice.

Most builders have not even heard of the risk.

If you google you will find articles about it. i think one was called breaking the mould.

The reality is that we have not been putting 100mm of insulation on the inside face of hollow block walls for long enough, to know how effectively mould will grow, and how much the inhabitants will suffer.

The wall behind the insulation will be cooler, so it will be below the dew point more frequently, so more water will condense on it and mould is more likely to grow.

It will not be able to breathe as much internally into the house, so it will give up less moisture during the warmer summer months. over several months or years, the wall will become wet.

If you have to go this route, dont put much insulation on it.

I'm not a professional in this area, but I did some research for a project I was looking at.


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## lowCO2design (13 Apr 2013)

drunat said:


> Hi. We are doing kitchen extension at the moment - had agreed with the builder that he'll be doing hollow walls, he has now changed his mind and said because the house is built of hollow block he has to use hollow blocks so the width of the walls matches and there is no "step in" i.e. the new and old wall joins rather than sticks out. He said he would put *100mm high density insulation on the inside and it will be higher than regulation requires.* I heard hollow block walls are not great - do you think they could be made good with sufficient insulation as he suggests?


I'd recommend he puts the insulation on the OUTSIDE and goes for block on flat over hollow block. dry-lining with this depth of insulation is inappropriate. if the contractor or your engineer disagree ask your engineer to carry out a due point calculation and show him this   http://www.josephlittlearchitects.c...e_Mould_1_Construct_Ireland_Issue_6_Vol_4.pdf


> the engineer OKed it. It's only two walls only, they'll be putting on  100mm high density kingspan foam *(on batons so there will be a gap  between  the block and insulation), ensure no dampness comes through the  insulation (tape?) and then plaster board I b*elieve. They told me the U  value will be higher then required. Not sure about outside - they  mentioned some waterproof finish or something...


so where DOES the dampness go? it just sits there trapped in a cavity?


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