Insulation building regs

[Yiddo13]

my local building control are useless at being available to talk to.

How do I insulate for a pitched roof (staying pitched inside when complete) for a cold deck (?) roof.

Do I just need to insulate with celotex that fits to the bottom of the rafters with a 50mm gap at the top between the insulation and roofing membrane? Any advice would be appreciated. Thanks

 

[Ian H]

100mm between and full 50mm below.

 

[Yiddo13]

So it’s goes

Membrane
50mm gap
100mm insulation

That would leave a gap of 20mm between the insulation and plasterboard is that ok?

 

[Notch7]

So it’s goes

Membrane
50mm gap
100mm insulation

That would leave a gap of 20mm between the insulation and plasterboard is that ok?

100mm insulation flush to lower edge of rafter. Then 50mm insulation below the rafters, ie 150mm overall.

Effective vapour barrier to internal face.

 

[Yiddo13]

100mm insulation flush to lower edge of rafter. Then 50mm insulation below the rafters, ie 150mm overall.

Effective vapour barrier to internal face.

that would make the gap for ventilation 70mm instead of 50mm? is that ok?

Also i was under the impression i only needed to out insulation inbetween the beams and then insulated plasterboard underneath them would be ok?

buying 100mm and 50mm insulation is going to cost a bomb!

 

[Notch7]

that would make the gap for ventilation 70mm instead of 50mm? is that ok?

Also i was under the impression i only needed to out insulation inbetween the beams and then insulated plasterboard underneath them would be ok?

buying 100mm and 50mm insulation is going to cost a bomb!

Well it depends what has been specified for your project.

This calc gives 100mm and 42.5mm insulated plasterboard below to achieve 0.18u value

https://www.uvalue-calculator.co.uk...ilated/no sarking board/175mm/400mm/42.5/100/

Have a play to get to what matches your required u value and construction

 

[Yiddo13]

Well it depends what has been specified for your project.

This calc gives 100mm and 42.5mm insulated plasterboard below to achieve 0.18u value

https://www.uvalue-calculator.co.uk/calculator/pitched roof/insulation between and under rafters (partially filled)/ventilated/no sarking board/175mm/400mm/42.5/100/

Have a play to get to what matches your required u value and construction

thanks for that, says 90mm for me for 0.20u with 42.5 insulated plasterboard. in theory the building control would sign that off?

 

[Leofric]

It isn't the job of Building Control to design or specify the construction for your building work , but how are you applying for building regs approval ?
Have any drawings being prepared or has anything been specified for the construction ? Might have missed something but what is the roof finish ,it looks quite a low pitch

 

[noseall]

90mm for me for 0.20u with 42.5 insulated

Lol, you may as well go with 100mm and 50mm. Don't be fooled into thinking thinner (obscure sizes) is better. Availability is a massive boon. Also, applying the insulation in separates (as opposed to pre-insulated plasterboard) works out much better. You can fix the 50mm layer without worrying about landing the edges on a rafter and you have the benefit of being able to stagger the joints regards plasterboard v's the 50mm layer. Better job overall.
Don't forget to use foil tape and foam and foil back plasterboards.

Vaulted ceilings are not cheap. They are dusty expensive horrible jobs and I'm using the dreaded stuff for the next few days.