Cavity wall thickness for new portion of wall

[ashdene]

Just trying to get a heads up before I speak to the BCO about my new extension.

I have a 7 meter long wall which has a 50mm cavity, I will be extending the wall by another 1.2meters and joining the cavities up, however the new regs specify a 100mm cavity which means im going to have a stupid looking 50mm step visible on the inner wall over the last 1.2meters. Can this be avoided, I could pack out the old wall to match but it seems ridiculous to loose 50mm of room space over a 7meter lenght just to maintain a 100m cavity.

 

[jeds]

The regs don't say you must have a 100mm cavity, what they say is you must reach a u-value of 0.28 and a common way to achieve that is with a 100mm cavity. You could make the cavity 50mm and then upgrade the value of the whole wall (existing and new) by adding a layer of internal insulation. As long as the overall result is the equivalent of the new wall being 0.28 that should be acceptable.

 

[ashdene]

The regs don't say you must have a 100mm cavity, what they say is you must reach a u-value of 0.28 and a common way to achieve that is with a 100mm cavity. You could make the cavity 50mm and then upgrade the value of the whole wall (existing and new) by adding a layer of internal insulation. As long as the overall result is the equivalent of the new wall being 0.28 that should be acceptable.

Ohh OK Thanks, Whats the cheapest way to achieve a .28 value then?
Thanks

 

[jeds]

50mm cavity with Superwall full-fill cavity batts and inner leaf of Celcon Standard blocks with 40mm Lafarge thermal laminate k board overlaid gives you 0.28 for the new wall. Using 30mm laminate over the whole lot drops your new wall to about 0.30 but upgrades the overall wall. If you put that to BC I don't think they'd query it.

 

[freddiemercurystwin]

I'd ask Mr Building Control if he'd accept 25mm Celotex + 25mm air gap and increase the roof insulation to compensate. Worth a try anyhoo.

Or what about 75mm Celcon standard blocks, 50mm Celotex, 25mm air gap, 100 brick = 0.27 total thickness 263mm? We don't know if this is single or double storey or what if anything the inner skin is supporting so that would need to be considered.

Brickwork or rendered on the outside?

 

[ashdene]

Many thanks guys, its a bungalow with a renderred exterior wall

 

[freddiemercurystwin]

Maybe change the outer skin to 75mm Celcon standard + render the overall thickness will be near enough the same (thinner).

Often Mr building Control is reasonable and will see sense and a little negotiation will work. Other inspectors, on the other hand though .....