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3 bed self build dwelling - wall section design

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First time posting on the forum. I'm currently going through the self-build process. So far, I have received planning permission for the dwelling and I am hoping to start digging the foundations in March 24, providing the self build mortgage, warranty, insurance and conveyancing all go to plan.


I am in the process of trying to hire a SAP engineer. This is so I can obtain an 'as designed' EPC and subsequently a Predicted Energy Assessment for the lenders. I decided to obtain accurate U-values from Saint-Gobain as it would save me a few quid as its a free service, plus they are very useful for advice. When in contact with them they raised an issue regarding the wall section. They said that they would expect to have a cavity between any non-engineered stone and their insulation, purely to avoid any piercing or damage to the insulation. 


They recommended Surecav as an alternative between the stone and insulation. However this will require amendments to the drawings and adds additional cost.


My architect and I opted for a blue lias stone outer leaf, 140mm Celotex thermaclass full fill cavity insulation. With 100mm Thermalite Turbo block inner skin. We expected to have a 10mm residual cavity between the outer leaf stone and insulation.


Has anyone else had this dilemma when building with a natural stone outer leaf? Is there a clever way of maintaining a 10mm residual cavity whilst not damaging the insulation? I'm thinking I could wield them with a thin piece of something that they could lay against and then remove the next day?


If push comes to shove then I will look to reduce the 140mm insulation, and then make up the gap with 25mm Surecav (something my architect does need seem too keen to do).


Any advise would be great. As you may be able to see, this has stopped progression with the SAP engineer and needs a resolution to avoid problems later down the line.


thanks, Harry




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If it's this I would suggest it's equivalent to engineered stone. 


I am against PIR boards in cavity walls. 


Preferable in my opinion to use a wider cavity with 2 x layers of mineral wool batts or EPS blown beads. AHA Denby Dale Passivhaus. 


The Golcar Passivhaus: Denby Dale mark 2


I would just use standard blocks internally too. Less cracking and cheaper. They add almost nothing to the thermal performance compared to including slightly more insulation. 






What I'd suggest. 





Plenty of free U-Value calculators online or you can do it by hand. No need to feel compelled to use a manufacturers detail. 









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Thanks for the informative response. Its funny, I have visited Purnell's and received quotes for approx 70 bags. Small world...


I will investigate the difference in thermalites vs concrete blocks. I wasn't aware of the little difference in U-value.


Thanks again,

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