How thick does a cavity wall need to be?

northender

We are hoping to build an extension to our pitch roof, single storey kitchen (just adding on 2m in length). Our house is a 1930s semi so apparently the walls now need to be thicker than the existing ones. My question is, how much thicker? This makes a fundamental difference as to whether we will fit the type of kitchen we want into the space.

From what I can make out from the plans the existing wall is 270mm and he has suggested 450mm for the extension. A builder suggested the difference would only be 50mm which is quite a discrepancy. Am I missing something obvious (quite likely), who is likely to be closer to the truth and do we have any viable alternatives?

Thanks

adr0ck

alternative = timber frame

150mm frame & insulation
50 mm cavity
100mm brickwork

= 300mm

northender

Thanks for that, it helps. What would you allow for plaster?

woodbutcher_2

Currently,the standard is 300mm.100mm each for outside skin,inside skin and insulation.You can drop to 70mm insulation if you use thermalite blocks on the inside leaf.Your current house will be 9 inch brickwork with no cavity.

adr0ck

12.5mm + 3mm skim = 15.5mm

You could get your frame thickness down to 100mm if you upped the insulation spec (this would cost a lot more)

stephen25uk

northender wrote: »

Thanks for that, it helps. What would you allow for plaster?

if you go timber fram you dont really need to plaster you can just dry line with plasterboard tape the joints and seal with plasterboard sealer and decorate over the top,with it being a kitchen though plaster is advisable and plaster over plaster board is just a skim cout so the thickness of the actual plaster would only be 2mm or so

stephen25uk

northender wrote: »

Thanks for that, it helps. What would you allow for plaster?

if you go timber frame you dont really need to plaster you can just dry line with plasterboard tape the joints and seal with plasterboard sealer and decorate over the top,with it being a kitchen though plaster is advisable and plaster over plaster board is just a skim cout so the thickness of the actual plaster would only be 2mm or so

sorry for the double post....hate my computer:mad:

adr0ck

for standard wall construction look at following link

could get down to approx 275mm (+ dot and dab and then plasterboard)

block 100mm
insulation 50mm
cavity 25mm
brick 100mm

http://www.insulation.kingspan.com/uk/pdf/k8.pdf

northender

sorry Stephen and adrock you've lost me with the timber frame stuff (building novice!).

northender

Thanks for all this so far it's really helpful. Am looking at that link now thanks adrock

m_13

Our extension had 70mm cavity to keep it in line with the remainder of the house but we did have to user thermalite blocks on the inner skin, insulate well and have foil backed plasterboard which played havoc with the wi-fi signal!

Our building control were really helpful and pragmatic. For example, the house was on top of a windy hill so they allowed us not to have trickle vents on the rear elevation so long as we had sufficient vents on the opposite side of the house.