Thermal bridging cock up

[cwr]

Kicking myself for not spotting this weeks ago...

 

At the gable wall of a vaulted ceiling there should have been insulation connecting the cavity insulation (blown EPS bead) to the roof insulation (125mm PIR between rafters, 60mm PIR below). However, the brickies have continued with the blockwork of the inner leaf right up to the underside of the membrane. I didn't make it particularly clear on the drawings, and somehow didn't spot it earlier so can only blame myself.

 

But what to do now? Slates are on one side, and battens on the other. Seriously considering the feasibility of carefully taking out the rafter on the inside of the inner leaf, removing top 200mm of block, making good the insulation/cavity closure and replacing the rafter. 

 

Crazy idea worth trying or should I live with this?

 

[PeterW]

You won’t get that out without damaging the roofing - on the side you can get to, you could cut the laths back 3 rafters and expose the top of the blockwork and remove it and then backfill with PIR. On the slated side it will be more difficult. 
 

Other option is 25mm of PIR between battens on the inside which will give you a service void too so you don’t need to chase blocks for electrics ..? Then board and skim as normal ..?

[Declan52]

With the way you have insulated between the truss in the pic in reality it's not going to be worth the amount of work needed to fix the issue. Cover it over and live with it.

[Iceverge]

On the side you can get to you could slot 25mm X 200 wide PIR above the membrane and between the slating battens. Then run a strip of membrane down over them. 

 

Otherwise parge coat taped to your airtightness layer inside and internally insulate with insulated plasterboard like @PeterW says. 

 

By the way full fill rafters with mineral wool would give the same u value. And be much easier and cheaper to do. 

 

Edited October 28, 2021 by Iceverge

[cwr]

On 28/10/2021 at 01:00, Iceverge said:

On the side you can get to you could slot 25mm X 200 wide PIR above the membrane and between the slating battens. Then run a strip of membrane down over them. 

 

 

This sounds like my best bet. Thanks for the suggestion. For the side that the slate are on I could put in some 25mm pir from the end, though wouldn't be able to put any membrane above it. Still worth doing?

 

[Iceverge]

yes

[cwr]

22 hours ago, Iceverge said:

yes

Riteo. Exposed side is done. 450mm wide strips between battens so the ends rest on rafters (one on inside of inner leaf, one in cavity) 600mm wide strip of membrane over it. Wonder what the roofers will make of it tomorrow...

I'll do the other side at the weekend, not as much rush and might be a bit safer in daylight.

[cwr]

On 28/10/2021 at 01:00, Iceverge said:

 

By the way full fill rafters with mineral wool would give the same u value. And be much easier and cheaper to do. 

 

Good point. Unfortunately too late tho as the PIR is now mostly in. Dunno why I didn't consider that.