Insulating cathedral\vaulted roof

[Steveb21]

Inquiry about completing a newly constructed cathedral/vaulted roof.

Rafters with a depth of 220mm

My current plan consists of the following:

- 50mm air gap (below the roof membrane)
- 120mm rigid board insulation
- 50mm gap/service void
- Plasterboard
- Skim coat

Questions:
*Is it acceptable to maintain the 50mm gap/service void above the plasterboard, or does the insulation need to be flush against the insulation for optimal efficiency? The reason for the 50mm gap/service void before the plasterboard is to facilitate easier installation of electrical wiring and spotlights. Any thoughts on this?

*Which rigid board would be recommend as the best option for this project?


Many thanks

 

[23vc]

Think you need insulation across rafters

 

[Steveb21]

Think you need insulation across rafters

Are you referring to insulated plasterboard to address cold bridging?

I spoke with my building officer regarding this matter. Since the rafters are quite substantial at 220mm, he has agreed that if I increase the insulation in between the rafters from 100mm to 120mm, I can then install standard plasterboard across the rafters.

 

[catlad]

I don't see any issues with what you propose. You could still have the insulated plasterboard across the rafters and then leave your 50mm service gap, making sure that you have a good vapour barrier.

 

[JohnD]

I used to have a building where the workshop was open to the roof, which had foam insulation boards laid over the roof timbers when it was built, on the same way sarking boards are laid. I thought that was a good method, and surely less workload and better insulation than cutting boards to fit between. It was very high for handling large objects.

It was an addition to a large Victorian building with an exposed timber roof, I have an idea it was called king-post design.

 

[stuart45]

I put 120mm insulation boards on top of the rafters on my extension. Made fixing the counter battens a bit more of a challenge.

 

[23vc]

Gap should be OK then. Don’t forget vapour barrier.
For insulation it’s foil backed PIR (several main brands)

 

[Steveb21]

Emailed celotex earlier and they confirmed leaving 50mm gap between insualtion and plasterboard is not an issue.

They did point out i need the VCL
.

So..
 
220 Rafters

50mm air gap (below the roof membrane)
- 120mm rigid board insulation
- 50mm gap/service void
- VCL over the rafters
- Plasterboard
- Skim coat

Only issue here is compromising the VCL when feeding through the electrics (from the 50mm service void) and installing spot lights.
Thoughts?

 

[23vc]

Not likely to be a big issue if done carefully

 

[JohnD]

I suppose you could seal the holes with fire foam or fire caulk.

The plasterboard is a fire barrier so I don't like holes in it.

 

[Steveb21]

Would it be possible to allow for more slack in the VCL around the spotlight areas? This would prevent it from being disturbed during the installation of the spotlights. If I secure the VCL tightly against the rafters, I'll need to cut into it when installing the lights.

 

[catlad]

If you fit your VCL to your rafters and then counter with 2 inch square timber which will be your service space , you shouldn't need to compromise the VCL

 

[Steveb21]

Even though its only 2"' I don't want to lose any more height off the ceiling.

 

[JohnD]

Around the lighting areas, you could batten the joists so the VCL can be fixed higher.

 

[catlad]

Even though its only 2"' I don't want to lose any more height off the ceiling.

Did you not start the thread asking about a service void! have you decided against it?