Close cavity or leave as is?

[Micky Dripping]

Hi all,

I am about to replace all the fascia on a single storey extension that has a warm flat roof installed and it will expose an unclosed cavity.
The extension walls are entirely built with Celcon blocks, 4" inside and 5" outside leaf, presumably this was for thermal reasons, as they are expensive and not particularly sturdy in all but compression.
Anyway, the cavity is not filled and is open at the top, so negating any thermal benefit of the outer Celcon skin. This doesn't seem right to me and I was wondering if the cavity shouldn't be closed somehow, anything to stop the flow of air and therefore the transfer of heat, but preferably a thermal sock of some type.
The extension is getting on for 20 years old now, so regs will have changed, but I fail to see the point of using expensive thermal blocks on the outside skin if the cavity is open to atmosphere.
The roof has recently been replaced, but the cavity has never been closed since build.

Am I missing something in my assumptions or am I missing a cavity closer?

Many thanks

 

[vinn]

If I understand what your question is:
Then you could bed down off-cuts of slate or similar across the cavity in each joist bay.
But more important is that the the warm roof insulation crosses the cavity to butt up to the back of the fascia.

 

[Micky Dripping]

Thanks vinn,

The insulation in the warm roof goes over the cavity, but it's not in contact as its on the 9x3 joists.

I've filled the gaps between the joists, roof and inner leaf with the same PU insulation as in the roof. If closing the cavity isn't a problem, I was going to use some sort of sock.

My thinking is that a lot of heat goes through the inner leaf and straight up and out of the cavity, bringing the insulation to the outer leaf would also solve that, as you suggest.