Insulating Vaulted Ceiling

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I'm using scissor trusses to create a vaulted ceiling in a new bedroom im building above garage.

The drawings detail 150mm celotex between 1st floor joists and 50mm under the joists. This seperates the garage from bedroom.

The original plans were for a standard ceiling, but I've managed to convince the BCO to let me use scissor trusses..... ie the original drawings don't deal with the insulation detail for a vaulted ceiling.

Do you think 100mm celotex between rafters and 50 mm under them followed by 12.5mm foil backed plasterboard and skim would suffice? Do I need to leave 50mm airgap between the celotex and the roofing membrane?
 
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Would say yes and yes.

What sectional size timber will the trusses be? As you might not be able to leave 50mm air gap at the truss legs.

You’ll also have a lot more than a 50mm gap for the rest, as you will be insulating the ceiling chord of the truss won’t you?
 
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Oh yeah, you're correct. So..... the 100mm celotex would go at the ceiling, then 50mm underneath.

Like this......

20220220_101751.jpg
 
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Wonder if I'll need to ventilate the roof above the insulation from the eaves/soffits up to the ridge.... or is the use of breathable membrane sufficient.

Also, how do you install plasterboard over VCL without piercing it? I presume you just screw through it.
 
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Yes just like your drawing.

we used insulation washers and long screws. In both the insulation and plasterboard. These get covered when the skin goes on.

There was eaves an Dry ridge ventilation.
VCL layer does get punctured, when screwing through, but there’s no other way. But shouldn’t be a concern.

(y)

Ps, Any velux going in?
 
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I had 100mm between and 40mm overboarded (foil backed PIR).
I taped all joints on both layers. I just used normal 12.5mm plasterboard with long screws.

Dont forget to mark where the joists are on the walls otherwise screwing in the plasterboard is a guessing game...
 

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