2 storey extension advice - building on skin with no cavity

[JonHeath]

Hi all,

I'm in the planning stages of a 2 storey rear extension. It will be rectangular in size, extending the kitchen on the ground floor and adding a bedroom on the 1st floor.

The property was built in 1938. The external walls are 2 (both brick) but there's no real cavity.

The extension will partly build a second storey on top of existing single storey. (I might need to use RSJ's in places depending on the foundations with I've not yet explored).

I've drawn a crude diagram showing the back of the existing house with the orange and green showing the existing structure and the blue line showing the extension.

Untitled

• JonHeath
• 16 Jul 2013
• 1

Orange shows existing two storey structure whilst the green shows where the structure is one storey only. The extension will need to build on top of the existing single storey structure in two places.

Today's building standards would see a 2 skin wall obviously have an insulation filled cavity making the total thickness thicker than my existing wall. Would I be able to build on top of the existing walls as they are in those two places? If not then how would I achieve a compliant wall in those spots?

Thanks
Jon

 

[^woody^]

Today's building standards would see a 2 skin wall obviously have an insulation filled cavity

The standards are that the wall stops water getting to the inside, is thermally insulated, and can hold up the roof. It does not need to be cavity

 

[JonHeath]

Thanks for the reply. That sounds like good news. If the insulation isn't in the cavity between the skins then what other techniques class as "thermally insulated"? Ideally I don't want to add insulation on the internal wall as it would make the gap in the knock through upstairs narrower than I'd like...

 

[noseall]

Thanks for the reply. That sounds like good news. If the insulation isn't in the cavity between the skins then what other techniques class as "thermally insulated"? Ideally I don't want to add insulation on the internal wall as it would make the gap in the knock through upstairs narrower than I'd like...

Whilst Woodflops answer is correct it is a bit vague.

You can not avoid fitting insulation, it's just a case of what is the most practical solution.

For example a one brick thick (225mm) wall would have to be upgraded using say 40-70mm thick celotex insulation on either the outer face and rendered or the inner face and plasterboarded over. Either way, if you are not using cavity construction you are shifting the location of the insulation to a less desirable position.

No doubt some clever teccie could design you an all timber-stylie sips panel building or a timber/masonry hybrid that could both support the roof and be the same finished thickness as your existing walls, but would this be practical?

 

[JonHeath]

Thanks for the reply. That sounds like good news. If the insulation isn't in the cavity between the skins then what other techniques class as "thermally insulated"? Ideally I don't want to add insulation on the internal wall as it would make the gap in the knock through upstairs narrower than I'd like...

Whilst Woodflops answer is correct it is a bit vague.

You can not avoid fitting insulation, it's just a case of what is the most practical solution.

For example a one brick thick (225mm) wall would have to be upgraded using say 40-70mm thick celotex insulation on either the outer face and rendered or the inner face and plasterboarded over. Either way, if you are not using cavity construction you are shifting the location of the insulation to a less desirable position.

No doubt some clever teccie could design you an all timber-stylie sips panel building or a timber/masonry hybrid that could both support the roof and be the same finished thickness as your existing walls, but would this be practical?

Probably not practical or affordable!

Thanks for the answer. So basically looks like I'll end up studding the internal wall in these spots and filling with insulation. Resulting in a loss of around 100mm which narrows the walkway plans I had into the extension. Back to the drawing board with that bit!

 

[tony1851]

you could do a one-half brick skin for the outer leaf; a 1" gap: and a 3x2 studwork inner skin with OSB lining + tyvek, gaps between studs filled with kingspan; plasterboard + skim internally.

This would be barely thicker than your existing wall and should just about meet Part L.