How 'thin' can i make an external cavity wall?

[sleeper341]

How 'thin' can i make an external cavity wall and still meet the expected U rating of .28?

The reason I ask is that the current side wall of my house is only 23cm thick so when the extension goes up the new wall will be about 5cm wider meaning that it will not run flush internally (I presume my neighbour won't want me taking 5cms of their garden so that my internal wall stays flush with the extension walls).

So can I make a cavity wall that is 103mm brick, fully fill a 50mm cavity with a mineral/glass/rockwool solution and have something like 100mm celcon solar blocks. I might get away with sneaking 2cms over the boundary and keep my walls internally flush?

Many thanks

 

[TheHandyAndy]

I might get away with sneaking 2cms over the boundary and keep my walls internally flush?

Many thanks

Glad you're not my neighbour. I wouldn't advise sneaking over the boundary even if it's only 2cms, could be a legal minefield.

 

[sleeper341]

ok, I won't sneak out over the boundary, but a 2cm overhang internally is more acceptable than a 5cm overhang on what should have been a flush surface.

Still, if anyone can give me any advice on making the cavity wall as thin as possible that would be great.

 

[Pallatrax]

If your house if old enough it would appear no one actually knows where the damn boundary is these days - by that I mean the 'line' not the physical one...

Case in point, our land registry docs show our boundary as straight and is clearly kinks due to aerial pictures and boundaries beyond ours - plus the obvious kinky fence in place now.

But you're right, I'd steer clear of any boundary issues - it is simply not worth it!!

Tom

 

[tony1851]

Why can't you have the overhang on your side, and fill in the recessed part with plasterboard or similar to get a flush finish internally?

 

[RonnyRaygun]

Why can't you have the overhang on your side, and fill in the recessed part with plasterboard or similar to get a flush finish internally?

Yes, and you could build a solid 215mm thick wall and insulate the new wall and the existing one internally.

Not a party wall is it, as there would be little point insulating then...

 

[sleeper341]

Why can't you have the overhang on your side, and fill in the recessed part with plasterboard or similar to get a flush finish internally?

Yes, and you could build a solid 215mm thick wall and insulate the new wall and the existing one internally.

Not a party wall is it, as there would be little point insulating then...

Yes, the wall I'm joining to is a party wall. I like the idea of a 215mm wall and using insulating board all the way down, but I'd need pretty thick board and I'm trying to limit losing any internal space as most of the existing wall is the side of a 900mm wide corridor.

Am I correct that the new wall has to have a U rating of .28?

I guess I need to just accept that I'm going to lose a couple of cm down the corridor.