Hi I am building a two storey extension. My builder has concreted the foundations and is about to lay bricks. They intend to use concrete blocks on both inner and outer skin of the external walls. My question is can you use concrete blocks on the inner skin assuming no further insulation such as insulation backed plasterboard is being used. Or should they use Thermalite? 300mm cavity walls. Engineering blocks underground Outer skin 7.3N Concrete blocks Inner skin ?
I'm sure it's a national requirement, but here in Oxford, the building inspectors would not pass it if it didn't have at least 75mm of approved insulation in the cavity PLUS thermalite or equivalent as the inside skin. Building control approval is to confirm that the minimum acceptable insulation values are respected; so they only check what's the mandatory minimum. However, with every house sale or rental now requiring an EPC certificate, you'd be well advised to go above and beyond the minimum requirements. C or B rated houses rent for more and sell for more than D and E rated houses. It's very easy now to go above and beyond ... it's much harder to do it later. And then bear in mind that these will become more and more stringent as time goes on, and as materials improve; so do you want your house to fall into the undesirable category in years to come?
When using full fill cavity batts it's best to make the cavity 5mm or so wider to allow for this. A lot of builders in this area have stopped using AAC blocks on the inner skin because of the shrinkage cracking. It's OK as long the walls meet the required U values.
Not using too strong a mix can help. Some people put reinforcing in every 3rd course as well. Also make sure they are really dry before laying. Ones straight from the works are the worst.
Thanks for everyone's replies. I still however feel somewhat undecided as to the best blocks to buy. I need to order the blocks as they will be ready to lay early next week. the builder (who isn't the bricklayer) has asked me to buy 7.3N Concretes for everything above the block and beams, including in the concrete beams. A couple of friends in the building game have said I can only use Thermalites unless adding additional insulation on top of the blocks, such as insulation backed plasterboard (Which I am against). One of these friends asked his (not my) local Building Control Officer who said if using concrete blocks 100mm Celotex must be used instead of slab Rockwool. I have been ringing my local building control dept since posting this but have not been able to contact anybody (they are pretty useless) Any more views would be gratefully received.
You would never use a 100mm celotex in a 100mm cavity, it need an air gap. If using a firm board insulation it would be 50mm pinned to the inside face of the inner skin. Your best bet is to ask your local BC officer what they expect to see based on the fact you dont want to use insulated PB.
The builder was intending to use 100mm Slab Rockwool in the 100mm cavity. What would be the norm in a standard 300m cavity wall?
Normally I would use 75mm rockwool in a 100 cavity purely for ease of working. This though may not govenyou the required U Values. As i said above ask your BC officer.