cavity wall thickness

[nottsrob184]

Hi all.
I am building a rear extension (single storey). I have layed the footings (thanks to all from previous post), and have layed 1 row of foundation blocks on top. The blocks are 300mm wide so i have layed inner and outer wall (red eng' bricks) on the outer sides of the blocks.
After laying one row of the inner and outer bricks i measured my cavity and its 95mm.....is this a problem???....i read the cavity had to be 100mm!!
I measured the bricks and these are just over 100mm wide hence i have lost about 10mm of my cavity. Will the BC inspector be ok with that??
Thanks for any advice chaps

 

[freddiemercurystwin]

Bricks are 102.5mm wide as standard not 100mm. Well assuming you are doing partial fill insulation you can normally achieve the required U value with around 40mm insulation so this should leave a 50mm clear cavity although that really depends on the block/brick and insulation specifications. In any case (presumably you're using blocks for the inner skin and plastering or dry-lining?) you could just step the inner skin out a bit and make it up in the plaster or dry-lining. The exact required air gap depends on a few factors such as your location georgraphically and whether the joints are flush etc. BC are unlikely to get their tape measure out as long as it looks about right.

 

[theoldun]

Freddy is right in what he says, but nine times out of ten the cavity is fully filled with insulation.
If you are doing full fill cavity insulation, then set your cavity to 110mm, then you will have no problem with getting your cavity batts in.
Anything under 110mm, then you are up s**t creek without a paddle. Believe me. BC will be quite happy with this.
old un

 

[nottsrob184]

Anything under 110mm, then you are up s**t creek without a paddle. Believe me. BC will be quite happy with this.
old un[/quote]

Hi again...thanks for advice...just clarify, I can set my bricks over the edges of the foundation blocks to get my 100mm cavity back?.....and do you mean BI will be happy with that or the fact i am "up S^*t creek without a paddle"?.

 

[freddiemercurystwin]

oldun means if you are doing full fill cavity wall insulation you will need a cavity of 110mm.

 

[theoldun]

Just corbel over to get 110mm cavity.
Building control won’t be bothered, even if your playing ring a ring rosy with Robin Hood in Sherwood Forest.
old un.