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

 

 

Would you do this?

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.

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?

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, 20233 yr by JohnMo

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,

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

This is the detail at the roof junction...

 

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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.

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.

 

 

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

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?

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.

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.

 

 

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