Cavity wall alternatives

[ban-all-sheds]

I have a house with solid walls, about 230mm/9" thick.

I want to build a single-storey full-width extension across the rear of the house, which will be open to the house, i.e. not separate rooms with doorways. I want to minimise the "step" difference between the existing inside surface of the walls, and the inner skin of the new walls. Are there any alternatives to a "standard" cavity wall, which would probably end up around 100mm thicker than my existing walls?

Am I right in assuming that a plain solid brick wall will no longer meet the regs in terms of insulation value? And are the requirements due to get more onerous in April?

 

[Softus]

b-a-s, two comments on this...

When I added a similar extension in 1986, the existing house (c. 1860) had 9" solid walls, but I was obliged to build cavity walls for the extension. Although, I'm a long way from expert, on the basis of that experience the only reason I can imagine for yours being different would be if it were listed.

Regarding the potentially ugly "step" at the join in the walls, I surmise that you're opening up an outside wall of the house where the extention joins on. If so, then there will be a significant lintel to replace the wall that gets removed. This, again, is similar to something I did, and I was obliged (by the LABC, and quite rightly so) to provide support for the lintel (it was an I frame RSJ) at each end.

My architectural engineer suggested steel uprights, which were a damned site neater than brick piers, and this was accepted by the LABC, albeit with surprisingly deep footings for the base of the steel supports. The uprights were then boxed in, and didn't appear intrusive.

Anyway, my point is, unless I've guessed wrong, you'll be having a meaty lintel, so the piers that support it will far outshadow the "step".

 

[ban-all-sheds]

Your point about the piers is true, but one of the rooms being extended is the kitchen, so I want to be able to have a run of worktop & cupboards down the wall, even if I need a cutout and/or gaps where the pier is....

 

[anobium]

Just a thought.The difference between the thickness of the new and old walls will only be 50mm. I don't know what areas are involved but have you considered making good the difference on the old wall with battens and plasterboard. I did it many years ago without any problem, in fact it helped with the rewiring in the kitchen.

 

[masona]

Another thought unless I'm missing something here,

Is it possible to have the step on the external wall so the internal wall is flush?

 

[ban-all-sheds]

Just a thought.The difference between the thickness of the new and old walls will only be 50mm. I don't know what areas are involved but have you considered making good the difference on the old wall with battens and plasterboard. I did it many years ago without any problem, in fact it helped with the rewiring in the kitchen.

I did think of that, and it would make wiring easier, and it would conceal some pipes, and it will probably be what I do. It's just that the kitchen is already a narrow galley-style, and I would have liked not to make it narrower....

Another thought unless I'm missing something here,

Is it possible to have the step on the external wall so the internal wall is flush?

I'm not sure - I could ask the planners.

 

[Nige F]

would planners allow solid celcon blocks rendered on outside, to correct u values, could even insulate the outside under the render if you had to

 

[peca27]

ordinarily u would have to have a 300mm cavity wall, but there is this new foam product on the market, i saw it on that channel 4 prog.

It's a foam block that is stacked onto each other, via an interlocking system, + then u pour concrete into them. U then render outside.........works a treat i've heard & meets building regs approval as has been used here before.

no idea of costs or suppliers tho

 

[ban-all-sheds]

Sounds interesting - the outside would need rendering anyway, as it's a pebbledash finish...

 

[Thermo]

you could do it with bales of straw, rendered etc. Youd need a frame for the structural elements of the building, but fantastic insulation properties!

 

[ban-all-sheds]

Yup - straw bales, lime plaster....

Umm - how thick would the walls be?

 

[Thermo]

about two foot!

 

[Nige F]

ordinarily u would have to have a 300mm cavity wall, but there is this new foam product on the market, i saw it on that channel 4 prog.

It's a foam block that is stacked onto each other, via an interlocking system, + then u pour concrete into them. U then render outside.........works a treat i've heard & meets building regs approval as has been used here before.

no idea of costs or suppliers tho

Grand Designs.....just don`t pour too much `crete @ a time

 

[masona]

Another one I have seen in my area is the 50mm insulation slabs on the outside wall with wire mesh then rendering or pebbledash, mostly done on council houses. I can't remember the name of it now