2 storey side extension with only a narrow gap

[Professionally nosey]

Hi.

 

We're planning a self build 2 storey side extension to our semi. The extension is about 3.6m wide and so we don't really want to go any smaller but that only leaves 800mm gap to the boundary.

 

We're looking at a fully rendered wall so are planning on aircrete for the inner leaf and standard 7n blocks for the outer with a 350mm full full cavity to hit the required u value.

 

The problem is at only 800mm wide it isn't going to be easy to get scaffolding into that gap. We could probably ask the neighbour if we could put some scaffolding on their side but their gap is also small and I'm worried that would make their side passage impassable.

 

Other option would be overhand laying of the blocks from the inside, not ideal as they're heavy but pointing doesn't need to be perfect as we're rendering afterwards anyway.

 

Any other suggestions/thoughts

 

Thanks

[Mr Punter]

Do you really mean a 350mm cavity, or is it a 350mm wall with a 150mm cavity?

 

If space is tight it will be worse when you render as the poles will need to be pulled back off the building.

 

Have you ruled out timber frame?

[Professionally nosey]

Sorry yes I wasn't clear. 😀 350mm is total wall width (150mm cavity)

 

Is timber frame a narrower wall? It would have to match the rest of the house and the street which is why we were going render. Thanks

[Iceverge]

6 hours ago, Professionally nosey said:

Other option would be overhand laying of the blocks from the inside, not ideal as they're heavy but pointing doesn't need to be perfect as we're rendering afterwards anyway.

 

If you're rendering you'll need scaffolding anyway no? 

 

FWIW I regularly see block layers building the outer leaf from inside in up to 200mm cavities here . Their backs must be as strong as anvils. 

 

Could you not put a brick outer leaf. Lighter to Lean over and build. No need for render.

[DevilDamo]

You don’t need a 350mm wide cavity wall in order to meet the BR’s.

 

Is the potential hassle worth it over 200mm? What room(s) will the extension be accommodating?

 

Does your LPA have policies when building two storey side extensions towards the boundary? Most require a min. 1m gap for at least the first floor element.

[Mr Punter]

With timber frame you could have thin coat render onto carrier board, 15mm total, 25mm battens, breather membrane, 10mm sheathing board, 140mm timber frame with 120mm pir insulation, 12.5mm plasterboard and skim, so a total wall thickness of 202mm.  If you wanted better u values you could use an insulated plasterboard.

[DevilDamo]

@Mr Punter Do you have a u-value calculation for that build up?

[Mr Punter]

21 minutes ago, DevilDamo said:

@Mr Punter Do you have a u-value calculation for that build up?

 

0.25 W/m².K.  But you can get to 0.18 with 25mm internally, so 227mm total wall thickness.

[G and J]

With a wall like that would you be able to hear external sounds?

[Mr Punter]

On 28/05/2024 at 00:03, G and J said:

With a wall like that would you be able to hear external sounds?

 

I don't think it would be great, but should be acceptable in most cases.  No good if you are near a busy road.

 

There are lots of routes for sound into a building and more likely through windows, doors and roof.

[G and J]

5 minutes ago, Mr Punter said:

 

I don't think it would be great, but should be acceptable in most cases.  No good if you are near a busy road.

 

There are lots of routes for sound into a building and more likely through windows, doors and roof.

MVHR will be running so doors and windows normally shut, roof I can insulate to death as we’re not restricted on depth of insulation there.   I wish there was a system like u values for sound transmission.  

[scottishjohn]

On 27/05/2024 at 15:49, Professionally nosey said:

(150mm cavity)

why so large a cavity -6" cavity is huge  

[MAB]

On 27/05/2024 at 10:37, Professionally nosey said:

Hi.

 

We're planning a self build 2 storey side extension to our semi. The extension is about 3.6m wide and so we don't really want to go any smaller but that only leaves 800mm gap to the boundary.

 

We're looking at a fully rendered wall so are planning on aircrete for the inner leaf and standard 7n blocks for the outer with a 350mm full full cavity to hit the required u value.

 

The problem is at only 800mm wide it isn't going to be easy to get scaffolding into that gap. We could probably ask the neighbour if we could put some scaffolding on their side but their gap is also small and I'm worried that would make their side passage impassable.

 

Other option would be overhand laying of the blocks from the inside, not ideal as they're heavy but pointing doesn't need to be perfect as we're rendering afterwards anyway.

 

Any other suggestions/thoughts

 

Thanks

 

Our side passage is 1.0 metre max wide and scaffolding for our 2 storey side extension allowed access below no problem including builders wheel barrows, etc.

With your neighbours agreement could you remove any boundary fence temporarily between yours and neighbours side passages, giving you approx 2 x 800mm width in total for the scaffolding and access?

Afterwards if it is their fence you can promise them a nice new one as a replacement!

Edited May 31 by MAB

[Iceverge]

On 30/05/2024 at 14:21, G and J said:

I wish there was a system like u values for sound transmission.  

Me too. 

 

The white book from gypsum is about as close to anything I've found. 

 

In any case it depends on as built workmanship. 

 

1. Block air paths for the sound to travel through. 

2. Add mass

3. Counter resonance with a fluffy material somewhere in the buildup. 

4. Decouple the materials in a wall. 

 

A cavity wall can do these pretty well if wet plastered and using a full fill mineral wool insulation. 

 

PIR is crap. It doesn't deal with air paths unless you use some something dear like gafotape.

 

Blown cellulose is very good. It eliminates any gaps when done well.