# concrete block wall insulation & air tightness



## Birds (23 Feb 2012)

Hi,
I am in the middle of a new build and I am just about ready for first fix. Construction is standard block wall with 150mm blown cavity and I am currently considering putting a 37mm + 12 insulated slab on the inner leaf, I am building a high A3/low A2 standard with full wind and air tightness. My deliberation is around the detailling on sealing the block work with sand/cement skim followed by insulated slad with final skim coat over this. Was wondering if anyone could offer some advise on the best way of completing this. Thanks


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## threebedsemi (23 Feb 2012)

I'm generally not a fan of the insulated cavity + drylinings solution, but that aside it's good to see people concerned anout airtightness. To comment on your question:

1. You will get a pretty good airtightness rating from the wall itself if its wet plastered with two coats.
2. The problems occur where there is a break in the wall (windows, services etc.).  You cannot depend on wet plaster to provide a long term seal here, as the plaster will dry out, constrict, and leave gaps.
3 The use of specialised tapes (proclima or siga being the most common brands) around openings will give excellent results if fitted properly and at the right time (in your case that would be pretty soon).

Google the above brands for more info, the suppliers are happy to advise. Also, there were 'passivehaus' type videos on youtube a while back, some of which outlined best practice in airtightness. I dont have the link to hand but it should be easy enough to find.

Also, dont forget your roof construction, as this is where a lot of problems manifest themselves.

www.studioplustwo.com


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## about2build (28 Mar 2012)

So you are samd and cement plastering and then skim coating and then drylining and then skimming again....am i reading this correctly??


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## Patrick2008 (28 Mar 2012)

threebedsemi. can I ask what you would recommend in lieu of the external block cavity wall, cavity insulation sand cement internal face of external wall and 37.5mm insulated board? Are you suggesting timber frame?


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## lowCO2design (29 Mar 2012)

Patrick2008 said:


> ulation sand cement internal face of external wall and 37.5mm insulated board? Are you suggesting timber frame?


NO - why make a cavity only to place dry lining inside it? makes no sense!


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## lowCO2design (29 Mar 2012)

Birds said:


> Hi,
> My deliberation is around the detailling on *sealing the block work with sand/cement skim* followed by insulated slad with final skim coat over this. Was wondering if anyone could offer some advise on the best way of completing this. Thanks


highlighted is the important bit. in this instant I cant suggest strongly enough, that your air-tightness layer is considered before the drylining happens otherwise it will potentially not be fixed/sealed to windows, ceiling, floor, perforations etc.


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