Cranked roof insulation

[JohnBishop]

Hi All,

   I have a cranked roof and I was initially thinking about fitting 25mm Celotex into the gap to address the thermal bridging however there are joists protruding and I think in most cases I won't be able to squeeze in Celotex.
Can I use SuperFoil instead?
Best

[nod]

Superfoil is ideal for this From memory = to 60 mil of pir 

[Redbeard]

Check the spec for whatever multifoil you have in mind, as there may be battens required for air-gaps either side, so thin might = 'fat' after all.

[JohnBishop]

7 hours ago, Redbeard said:

Check the spec for whatever multifoil you have in mind, as there may be battens required for air-gaps either side, so thin might = 'fat' after all.

Last time I have ordered Superfoil but it was floor insulation so this one will be different.
I look at these https://www.builderdepot.co.uk/?q=superfoil

There is breathable Superfoil SF19BB: https://www.builderdepot.co.uk/1200mm-x-10m-superfoil-sf19bb-breathable-multi-foil-insulation?queryID=2263a3e602ffb7156e458879776cdf73&objectID=40602&indexName=production_default_products#

and a bit more expensive SF40 https://www.builderdepot.co.uk/1500mm-x-10m-superfoil-sf40-multi-foil-insulation?queryID=2263a3e602ffb7156e458879776cdf73&objectID=22352&indexName=production_default_products

In my case the rafters are about 33-37cm apart and the cranked bit is about 70cm deep/long so the only thing I can do is to cut ~35cm W and 80cm L Superfoil strips and slide them into the void then staple on top unless there is a better way.

[Redbeard]

What does the manuf'r say about air gaps on either side (i.e. top and bottom)  of the insulation? Can you comply in the space you have? Can you get it air-tight? If it is only stapled, and you cannot get in to get air-tightness tape on then there is a good chance that it will just be something shiny sitting in a breeze.

[JohnBishop]

42 minutes ago, Redbeard said:

What does the manuf'r say about air gaps on either side (i.e. top and bottom)  of the insulation? Can you comply in the space you have? Can you get it air-tight? If it is only stapled, and you cannot get in to get air-tightness tape on then there is a good chance that it will just be something shiny sitting in a breeze.

I cannot find this information. Their installation manual is loaded with colours, my eyes hurt.
In that case it does not make sense to put it in because there is no access to attach it at the bottom. It says in the manual that for over rafter installation full roof strip is required.
On the other hand under rafter installation does not make sense because I want to insulate just the cranked bit not the whole roof e.g. for loft extension

[JohnBishop]

21 hours ago, Redbeard said:

What does the manuf'r say about air gaps on either side (i.e. top and bottom)  of the insulation? Can you comply in the space you have? Can you get it air-tight? If it is only stapled, and you cannot get in to get air-tightness tape on then there is a good chance that it will just be something shiny sitting in a breeze.

I understand this cannot be airtight but I don't feel a breeze in that space moreover foil reflects the heat otherwise people would not use foil blankets outdoors. Ideally you want to be perfectly wrapped in it but it still works even if it isn't.