what size new cavity wall on top of existing cavity wall

[APJones]

I have an existing extension built in the 1980s with what I term a standard 11 inch wall i.e 100mm brick outer, 50mm cavity (rockwool filled ) and 100mm celcon inner layer with 12mm plaster scim .

I now want to build above this extension ( and carport ) and need to know what dimensions the new first storey cavity wall dimensions need to be .
I 'd prefer the same construction i.e matching external bricks but I have heard that the cavity has to be beteween 75 100mm and with the 100mm celcon inner leaf I am a tad concerned how I go about this .

Note as part of this new extension I plan to build over the existing carport so there will be long 4200mm lintels , do I will need to have the same cavity wal construction above this as the first floor extension will essentially be a single room so a change in wall width will be unnacceptable
OK experts have you any advice , tIA

 

[big-all]

heeelllooo and welcome apjones

are the foundations deep enough to modern standards for a second story

 

[APJones]

GOOD POINT - THEY ARE ONE METRE DEEP 18 OR 24" WIDE CONCRETE STRIP ON SANDSTONE !
WAS TOLD BY THE BUILDING INSPECTOR WHEN I QUERIED WHY IT HAD TO BE THAT DEEP (AS I DIDN'T LIKE CHOPPING THE SANDSTONE THAT STARTED AT ABOUT 300mm DEEP)
- HE EXPLAINED THAT IT MEANT IF A SUBSEQUENT OWNER WANTED TO BUILD ABOVE ALL 'THEY' WOULD NEED ME TO DO IS TO DIG AN INSPECTION HOLE AT A COUPLE OF PLACES ALONG THE EXISTING WALL TO MAKE SURE THAT THE FOOTINGS WERE AS ON THE PREVIOUS PLANS AND OK

THE FOOTINGS FOR THE EXISTING SINGLE STORY EXTENSION ARE THE SAME AS THE REST OF THE 2 STOREY HOUSE SO i SHOULD BE OK TO BUILD OVER

 

[APJones]

Whoops I am CAD mode so text was in capitals sorry

 

[noseall]

You can either build up as per thickness as existing and then fit insulated plasterboard to the internal face of the inner leaf...

....or engineer a corbel arrangement with galvanized 10mm plate steel whereby you can build wider above than below.

You may still not achieve full reg's wall thickness even with steel corbelling as you can only overhang a certain percentage of the wall, which may not be enough.

 

[APJones]

Thanks for the info above guys

could I acheive the same effect by incorporating a lintel ? or by a layer of horizontal / cut celcon blocks on the inner wall ?

 

[joe-90]

Whoops I am CAD mode so text was in capitals sorry

You are a CAD and a scoundrel, sir!.

 

[theoldun]

Your U vale of existing wall, allowing for cold bridging is approx 0.42W/m2K and resistance of 2.39m2K/W. Current U values are 0.28W/m2K, therefore, allowing for cold bridge you need to increase resistance by 1.60m2K/W.
2.39 + 1.60 = 3.99 - 10% cold bridge = 3.59m2/KW = 0.28W/m2K.
There are a number of different ways to achieve this figure, but, would suggest that you go the route suggested by Nose (Lord of Staffs) and insulate inside internal skin.
Here’s how to do it.
102 outer skin brick plus internal and ext pressure = 0.30
50mm cavity fully filled with Dritherm 32 ultimate = 1.56
Internal 100mm blockwork. Either Celcon Solar,
Durox Superblock, Thermalite Turbo, Topblock
Superblock, all these blocks have a lambda of 0.11
and resistance = 0.99.
Dot and dab with either 60.5mmThermoline Plus, or 25mm Celotex PL4000. Either of these will give a resistance of 1.10. Add Resistance up and comes to 3.95 less cold bridge = 3.59 = U value 0.28W/m2K.
There are numerous other ways to achieve this U value, but this is about the easiest way.
Anything you do not understand, then come back.
Regards oldun.

 

[APJones]

A good answer - I mentioned my problem to a builder and said about internal insulation - he came up with same answer
BUT
gave me another problem I have a Leylandii hedge 3m tall about 6m from where I intend to build - He said I would need to have 2m deep footings - the existing house has only 1m footings .
I don't want to involve the council planning / building regs people just yet - I could reduce the hedge to 2m before they do an inspection. it hasn't caused any problems as far as I know , it is 300mm from tarmac pavement and not caused any problems - The hedge is about 25 yers old and is pruned twice a year

 

[theoldun]

Your builder is not correct and does not know any thing about working with in the zone of influence of trees.
A Leylandii is a coniferous tree with a mature height of 20metres. With out going into a lot of detail with regard to root system, due to he fact that they throw down a deep tap root the lateral and feeder roots only go out to 4 to 5 metres with a diameter at the thickest part of possibly 150mm, and they are in the top 600mm of ground
Any way all that is irreverent as you are not forming a new foundation, but building on top of existing single storey extension.
Once again the above is also irreverent, as even if you were forming new foundations 6 metres away from the trees 1 metre depth is with in the limits on low shrinkable soil which you are if in sandstone.
Hope that is all clear to you and don’t forget to come back and say Ta.
Regards oldun.

 

[APJones]

ta oldun I didn't realise that i had mentioned the trees here I thought I had lost the email on the pc so I started again - I must have posted it.
- only just seen it, as the comment about internal insulation had answered my original question. thanks

I am slowly finding my way around this site - how to I post a thankyou ?
thankyou

PS The weeping willow is in the bathroom ! nasty roots and thirsty too !