Service Void?

[Mulberry View]

My Architect specified a 50mm Service Void in the perimeter walls and a 25mm service void above the ceilings...

 

 

Would you do this?

[ProDave]

For walls, 25mm service void is plenty for cables, only use 50mm on walls that are going to have pipes in them.

 

Service void is pointless in a ceiling, you have the whole space between the joists for services, it is just unecessary work.

[Mulberry View]

1 minute ago, ProDave said:

For walls, 25mm service void is plenty for cables, only use 50mm on walls that are going to have pipes in them.

 

Service void is pointless in a ceiling, you have the whole space between the joists for services, it is just unecessary work.

 

Pretty much as I thought, particularly the ceiling!

 

Does the air gap in the perimeter walls contribute to the u-Value? Could/should I add a layer of rockwool in there?

[JohnMo]

5 minutes ago, ProDave said:

Service void is pointless in a ceiling, you have the whole space between the joists for services, it is just unecessary work.

Depends if you need a vapour barrier or not. I needed one, so did a 50mm service void in all ceiling to allow spots to be installed and running wires without compromise of the vapour/airtightness. But would only do this where the ceiling is with the roof.

Edited October 26, 2023 by JohnMo

[ProDave]

1 minute ago, JohnMo said:

Depends if you need a vapour barrier or not. I needed one, so did a 50mm service void in all ceiling to allow spots to be installed and running wires without compromise of the vapour/airtightness.

That would be an upstairs / roof ceiling.

 

The drawing show was an inter floor ceiling so no issues.  But good point depending on the roof make up and where the vapour / air tightness barrier is going, a service void may make sense on an upstairs ceiling,

[JohnMo]

@ProDaveI added that bit (possibly while you were looking at my reply), when I realised the drawing was showing a floor above.

[Mulberry View]

This is the detail at the roof junction...

 

4

 

But bear in mind that I'm aiming to ring all the bells in terms of air-tightness, so if a further layer under the roof Posis is a good idea, then I'm all for that.

[JohnMo]

With that roof buildup I wouldn't bother with the service space in the ceiling. As it's giving you nothing. But I would increase the depth of the insulation above the roof, to 150mm as a minimum.

 

 

[Conor]

We didn't leave a service void on the walls, just chased out using a hot wire cutter as needed.

[Mulberry View]

14 minutes ago, JohnMo said:

With that roof buildup I wouldn't bother with the service space in the ceiling. As it's giving you nothing. But I would increase the depth of the insulation above the roof, to 150mm as a minimum.

 

 

 

I've increased the floor insulation from 150mm (designed) to 200mm.

 

Do you think 150mm in the roof would be sufficient to provide an above-average performance? Is there any merit in adding rockwool to the Posi area?

 

Also, do I need any additional air-tightness barrier beyond what is drawn?

[JohnMo]

Not sure I would start adding insulation below and between the pozi rafters, as you start getting into hybrid roofs and very carefully details, otherwise you end up with condensation etc.

 

4 minutes ago, Mulberry View said:

Do you think 150mm in the roof would be sufficient

You could go up to 200mm then after that it's very diminished returns. 200mm will give a U value circa 0.1.

[FM2015]

Why wouldn't you chase everything into the EPS?  Then plasterboard straight to the EPS?

 

Some people say that battening off 50mm gives you space for a 40mm waste pipe but it doesn't....a pipe clip plus 40mm pipe is closer to 60mm.

 

It just seems like an expensive way of saving your electrician a bit of time.

 

 

[ChrisJ]

Defeating the purpose/point of ICF, just cut the services into the ICF, plasterboard directly onto the ICF

 

for the ceiling, I would use resilient channel, only because wood is typically a mess and RC allows the plasterboard to ‘float’ flat