How can I avoid creating a draft from the cold roof when using a twinwall flue and ventilated fire stops? Also, struggling to maintain part L airtightness.

Grateful for thoughts.

See page 16:

 

https://www.sauresta.com/static/files/3/54/354/Dumtraukiu_instrukcija_EN_Rasu_2018-10-26_No7.pdf

 

Surround with rockwool firerock + minimum spacing to combustibles.

 

 

Then use a silicone seal membrane (stretchy) to seal against the flue / join to your airtightness layer:

 

https://dumtakis.lt/sandarinimo-detales/356-dumtraukio-prasiskverbimo-manzetas.html

 

 

I'm going to back that up with some cement board and high temperature sealant to avoid it being a weak point for mice to come visit in our place. It's stick built. Swap the OSB for the fireproof cement boards local to the flue. Use high temperature silicone type sealant between flue and the cement board. Then go over the top of both with the silicone seal membrane taped onto the airtightness layer.

 

 

Low / high temperature tape is required for joining that membrane to others if it is at roof level. e.g. 

 

https://dumtakis.lt/sandarinimo-detales/841-klijavimo-juosta-delta-multiband-m60.html

(i.e. buy a flue that doesn't require ventilated fire stops)

What is the requirement for ventilated fire stops then? I don't understand this requirement. Is it something to do specifically with the manufacturer of the twinwall?

Different vendors have different requirements based on how hot their twinwall gets in the event of a flue fire.