We are having a cold roof, and we are going for as airtight as I can manage.  I’ve been boiling my brain about the roof space.  I’m familiar with a fully ventilated roof, I’m sitting under one as I type, and I’m reading on here about ventilation ‘airwashing’ insulation and reducing its efficiency.  As a result I started to wonder if a breathable ‘felt’ would remove the need for full loft ventilation.  So I was dead chuffed when I found this post….

 

 

in this thread….

 

 

Before I get too carried away, is this allowed in Blighty nowadays?

 

dont worry about airtightness in the loft, you actually want the reverse.

 

You need to seal up the ceiling and walls below.

 

If you have dabbed plasterboard on the external walls go up in the loft and foam the tops all the way round. If you have downlighters put airtight hoods on them all. rubber seals on loft hatch etc.

 

then leave the roof alone.

 

29 minutes ago, Dave Jones said:

You need to seal up the ceiling and walls below.

Yep, agreed.
 

So my earthwool type insulation will work just as well if it’s got a good airflow over it?

 

The other attraction of a sealed roof was the exclusion of insects, messy things.

13 hours ago, G and J said:

So my earthwool type insulation will work just as well if it’s got a good airflow over it?

Not quite as well, but current thinking is that it's unlikely to make a significant difference.

 

The last paper that I read on the topic is this one: https://www.passivhaustrust.org.uk/guidance_detail.php?gId=55