External cavity wall - Building Control Approval Query

[Can_Do]

Hi All

 

We have just started our side and rear extension with foundations poured yesterday. The BC Person while approving foundation depths mentioned that if the external walls are going to be with 100mm cavity then Kingspan K106 90mm interlocking cavity wall insulation (or similar) with 10mm air gap has to be used. Or else the cavity needs to be increased to 150mm and then wool batts type insulation can be used. I checked with the Builder later and he says that it would be impractical to use rigid insulation with only 10mm gap considering the mortar dropping, wall ties etc. There are few 90 degree bends in the walls as well.  He also mentioned that rigid wall insulation is very costly and it will cost me dear considering the total combined area of external walls is around 75 sqm.

 

I am planning and designing the build myself and don't have an Architect or SE engaged. My layout plan is based on 300mm thick external walls with facing bricks + 100mm cavity+ concrete blocks. I would like to keep the cavity to 100mm as the layout design is tight in some areas and also to maximise my internal usable area. So even if I decide to pay a lot more

for rigid insulation, it may still not deliver the required thermal performance in real life based on Builder's view on practicality. So wondering if there are other economic and practical options with 100mm cavity which I can take back to BC to consider.

 

Any advice on this will be greatly appreciated.

 

  

 

 

 

[Big Jimbo]

100mm cavity with full fill cav32 probably won't get you to building regs. I expect you would need insulated plasterboard on the inside. That will reduce your internal usable area. 

[kandgmitchell]

I agree - you're going to struggle with Dri-Therm 32 or similar in a 100mm cavity - using lightweight aircrete blocks as the inner leaf you will still need a 35mm plasterboard/insulation laminate. There are many U value calculators on manufacturers websites. Try out other combinations but rigid in the cavity is usually the way now but I agree 10mm air space is a big ask.

[Mr Punter]

The way the manufacturers show the rigid full-fill insulation is a world apart from how it goes in on site.  DriTherm or blown poly bead with a wider cavity may be better.

[DevilDamo]

If you’re not going to use Kingspan or an equivalent product within a 100mm cavity, then your only other options are to add additional insulation externally or internally. Both have their pro’s and con’s and by the time you weigh up those options, it may take you back to wanting to use Kingspan.

 

This is one of the downsides in not using an architectural designer from the outset. Any reason why you chose to bypass an AD as well as an engineer?

[Can_Do]

Thanks all for your quick responses and suggestions. I  have tried couple of proprietary U value calculators. Another middle option between 100mm and 150mm cavity  could be 'Partial Fill Insulation'. As per Celotex Calculator, a combination of 100mm facing brick + 50mm air gap + 85mm CW4000 + 100mm light weight blocks with wet plaster gives U value of 0.18 as required. This option reduces the cavity to 135mm and the cost of 85mm CW4000 appears to be similar to 150mm DriTherm 32 insulation and much cheaper than Kingspan K106 or Celotex Thermaclass.  

 

The Calculator doesn't give me the option of reducing the airgap from 50mm to see if I can try other combinations of insulation thickness to reduce the overall cavity bit more. Also, the Lambda values for both Celotex CW series and GA series is showing same as 0.022 on their website. Would this mean that I can possibly use GA4090 instead and achieve the same U value. GA series is further cheaper as well but would require some extra cutting on site. Not sure if the partial fill cavity is a practical option though and if BC would agree to it. I might have to settle for Celotex Thermaclass 90mm with overall cavity of 100mm in the end. 

 

Looking forward to further suggestions and thoughts. Thanks.

 

 

[Can_Do]

21 hours ago, DevilDamo said:

If you’re not going to use Kingspan or an equivalent product within a 100mm cavity, then your only other options are to add additional insulation externally or internally. Both have their pro’s and con’s and by the time you weigh up those options, it may take you back to wanting to use Kingspan.

 

This is one of the downsides in not using an architectural designer from the outset. Any reason why you chose to bypass an AD as well as an engineer?

The main reason for not using an AD was feedback from couple of friends who didn't had great experiences. The SE my builder had used in past turned out to be similar when i met him so decided to not use any. I needed flexibility as the internal layout and details have been changing few times as myself and family making our mind gradually. This wouldn't be possible with fixed fee professionals.

 

The other reason was I felt confident to do it myself based on great support available online from a Forum like this. Will ever be grateful for all the Forum members sparing their time to help others.     

[Canski]

You would have to increase your cavity by 25 mm but I am using Dritherm 32 with a 125 mm cavity and Ytong internal blocks and faced brickwork. That passed BC and my SAP calculations. Thermalise Shield passed as well but I dislike them.

[Can_Do]

2 hours ago, Canski said:

You would have to increase your cavity by 25 mm but I am using Dritherm 32 with a 125 mm cavity and Ytong internal blocks and faced brickwork. That passed BC and my SAP calculations. Thermalise Shield passed as well but I dislike them.

Thanks for sharing your wall detailing. I just spoke to the BC person he says that if we can show him the calculation with 125mm cavity then it would be okay. Can you please advise which Website/Calculator you used and details of the Ytong blocks as these seems to have too many types. I can then run the calculations and send to BC.  

[Canski]

I’ll have a look tomorrow. All of my calcs were done by the SAP company.

If it helps the Ytong blocks are 100 mm standard I think. Deffo 3.6Kn and supplied by Zella through MKM ( not many others stock them) they are a great block to work with. True / square and solid. I used the 100 mm 610 mm long x 215 high version and they are far more stable than thermalites or Celcon. Also NO WASTE ! After building 2 houses to joist I have less than a quarter of  a pallet of offcuts that WILL be used. We cut them in seconds with a circular saw. A true clean cut which equals no waste if you can drum that in to the minds of brickies. Or ask them to contribute to a) £ 1.75 for each block wasted and a contribution to the costs of skip and acceptance of the disruption to them  due to lack of turning space for the telehandler.
 

I don’t know what others have experienced down the line though. I’d like to hear of anything to watch out for.

[goodbyegti]

We used 90mm k106 with a 100mm cavity. The builder had no issue keeping the cavity clean. I think he hosed it on the odd occasion.

 

The negative was the breathable tape for the joints. It cost loads and didn't always stick.

 

 

Edited October 30, 2023 by goodbyegti