Roof insulation, pitched vaulted ceiling???

[ianwainwrightjones]

Hi back again! Finally got the roof done and tiled.
Looking for some info regarding thickness of roof insulation required to achieve the U value.
I have a low pitched lean to roof on my new extension. I have 200mm rafters so have plenty of room for insulation and air gap. 400mm centres, breathable membrane and a vented soffit approx 20mtr square floor space.
Can anyone give me some help with regards to what's required before I go and order the totally wrong stuff!😊
Cheers Ian

Ps I was going to use celotex

 

[freddiemercurystwin]

This a domestic extension? Out of interest why do have you a soffit vents if you have a breathable membrane?

 

[ianwainwrightjones]

Yes it is a domestic. With regards the vents, I was told it would be better vented, but I know I didn't need to be when using a breathable membrane. Just mis advised really. This roof has been the headache of a lifetime. I will be glad when it's completed so I can concentrate on the bits I'm qualified for! Any advice would be gratefully appreciated!

 

[freddiemercurystwin]

Its a vaulted roof? As opposed to you fitting a flat ceiling?

 

[ianwainwrightjones]

Yes vaulted ceiling but plaster boards on undercloak. I'm I right in saying I need to use foil backed boards too?regards Ian

 

[freddiemercurystwin]

OK, you need to achieve a U Value of 0.16 so if you used the U Value calculator you will find you need 90mm between and a 50mm continuous layer beneath.

According to the calculator you cannot achieve this U Value with just insulation between.

You can use pretty much any 'PIR' type insulation so no need to limit yourself to Celotex.