Wrap Around Extension - What Insulation should I use

[Ambaz79]

Hi All

 

I am doing a wrap around extension and the external walls on the current drawing have a 100mm Cavity for insulation however I am looking to achieve a U Value of around 0.15.  What would be the best option to achieve this at the lowest cost? I have the option to increase the cavity to 150mm if needed as we have not yet started the build.

 

Its going to be a Block construction using Thermalite and dense blocks.  Renders and cladded on the outside.

 

Currently looking at Xtratherms Cavitytherm CT-PIR 150mm insulation boards.  but the cost is really working out to be very high. 

 

Attached a few plans.   

 

Thanks in advance.

02C_2061BR.pdf 03C_2061BR.pdf 04B_2061BR.pdf 05B_2061BR.pdf 06B_2061BR.pdf

Edited January 5, 2022 by Ambaz79

[PeterW]

Go for 150mm cavity; EPS blown graphite beads and then line internally with 37.5mm insulated plasterboard, ideally fixed tight to the blocks for air tightness. 

[Ambaz79]

Hi Peter

 

I don't really want insulated plasterboard inside.  So really only looking for the best option for the Cavity.

[MikeGrahamT21]

I'm also planning on CT-PIR for an upcoming build, 100mm for me though.

 

There are a few other options, all equally expensive...

 

https://www.travisperkins.co.uk/insulation-board/ecotherm-eco-cavity-full-fill-insulation-board-1200-x-450-x-90mm/p/670253

https://www.buildingmaterials.co.uk/celotex-cf5000-premium-full-fill-cavity-boards-1200mm-x-1200mm-x-97mm-pack-of-96-138-24m2

https://build4less.co.uk/products/thermaclass-cavity-wall-21?variant=40086149398721

https://www.kingspan.com/gb/en-gb/products/insulation-boards/insulation-boards/kooltherm/kooltherm-k106-cavity-board

[Russell griffiths]

1 hour ago, Ambaz79 said:

Hi Peter

 

I don't really want insulated plasterboard inside.  So really only looking for the best option for the Cavity.

You won’t get anywhere near that u value unless you increase the insulation somewhere, 150 isn’t enough. 

Despite what any manufacturer says. 

You need to also factor in the poor fitting of the product, so it might add up in a laboratory, but it won’t be up to scratch on site, the eps beads take out the poor bricklayers fitting by fully filling all gaps, add in the insulated plasterboard, and you have got a good outcome. 

[A_L]

5 hours ago, Ambaz79 said:

I am looking to achieve a U Value of around 0.15.

 

If you must have 0.15 then take a look at Walltite, and injectable polyurethane foam from BASF. A filled 150mm cavity will  be around 0.14 but it will not be cheap.