Where to run services - Vaulted Roof

[NewToAllOfThis]

Our House design includes a vaulted roof, the rafters are 215mm deep. Plyboard will be fitted on top of the rafters and then insulation applied from below. It will then be finished with plasterboard.

 

I have been told that it has to be full insulation between the Plyboard and plasterboard so I am wondering how do I install electrical services for the lights.

 

 

[the_r_sole]

what type of insulation are you using? usually you'll get a better uvalue with insulation between and below rafters....either way you should include a service void (38mm straps) below the rafters to run the services...

[Conor]

50mm batons on the underside of the rafters then plasterboard on top. Acts as a service void for electrics etc. We're also leaving a larger void at the peak by bringing batons across under the main ridge steel to give a 150mm service void for the MVHR ducting.

[NewToAllOfThis]

I have been told that there cannot be a gap between insulation and plasterboard as condensation would form in the gap?

[Mr Punter]

No there should be no condensation in the gap, although it is common to have some thinner insulation under the rafters to prevent cold bridging.

[ProDave]

Why are you not insulating between the rafters? then you need less over the top.

 

Normally you would then have your air tight membrane, then battens to form a service void, then your plasterboard.

[Conor]

29 minutes ago, NewToAllOfThis said:

I have been told that there cannot be a gap between insulation and plasterboard as condensation would form in the gap?

Missed a detail - 50mm PIR insulation under the rafters, then airtightness membrane, all held in by 50mm batons screwed through to the rafters. That plus the gaps between 215mm rafters filled with rockwool will give you a u value of around 0.1. no ventilation required and no risk of condensation.

 

Who's advising you re insulation? Architect?

Edited September 9, 2020 by Conor

[Ferdinand]

4 hours ago, NewToAllOfThis said:

I have been told that there cannot be a gap between insulation and plasterboard as condensation would form in the gap?

 

This person needs to explain how condensation will form on the warm side of insulation. He's talking out of his Rs, unless there is something very very different going on.

 

Suggest you do a little background reading on the physical mechanisms of condensation which will help you get to grips with the issue - condensation forms where warm moist air contacts a relatively cool surface, and if your insulation is doing its job the inside surface will be warm by definition.

 

Example of difference : inside of a single glazed window, but less on the inside of a double glazed window as it is better insulated and the inside pane should be warmer.

 

The sort of condensation he is talking about is called "interstitial condensation" (ie between the layers in your structure). But in an interior-plasterboard-service void-insulation-outside sandwich, the service void temp will be far closer to the interior temp than the exterior temp, so warm and should stay dry.

Brief intro eg here:

https://www.designingbuildings.co.uk/wiki/Interstitial_condensation

 

Ferdinand

 

Edited September 9, 2020 by Ferdinand