Real benefit of thermal break course

[SuperPav]

We're going up a storey on our bungalow, which is 340-350mm solid walls (Costwold stone outer, brick inner tied with rubble infill). Obviously the downstairs will never be particularly high performance thermally. The upstairs is 100mm celcon thin joint inner leaf, 100-110mm stone outer, with EPS beads full fill in the residual cavity. Realistically, probably around ~0.22 U Value.

 

My question is where we're building up off the existing wall, the current plan has been to lay a single 65mm course of Marmox thermoblock on top of the existing inner brick leaf after removing the wallplate, then a coursing layer of 65mm celcon bricks, then sit Posijoists on top of that.

 

This is OK but is causing a few issues around openings with lintels, and can't very easily raise the whole thing higher as we've got some strict planning restrictions. It would be much easier if we could lose the Marmox course, as that would allow us to have two courses of 65mm celcon bricks, and over full height openings we'd just use 140mm concrete lintels on end to match the two courses. Joists can just sit on that then, nice and simple. Would also be cheaper as we can lose the Marmox...

 

Obviously this creates a thermal bridge to the upper inner leaf and joist ends, as they're sat right on top of the inner leaf of the solid wall.

 

My question is how much difference/impact would a thermal break here really make?

 

 

The alternative would be to lower all the "full height opening" lintels to 140mm below current wall plate level, and then build up from them as per the rest of the wall...