Thermal bridging from filling cavities under windows

[RachelGodfrey]

Hi all, 

 

I'm doing my first reno, an 80s brick and block house. I've more or less stripped the house as I needed to fill lots of holes in the breeze blocks caused by the original builders having used tongue and groove blocks instead of solid ones! Now I've filled them with mortar, I'm planning to batten out all walls, fill the gaps between with celotex (exterior walls) or acoustic rockwool (party walls) and then plasterboard over. 

 

The window reveal obviously has a cavity between the two skins which I want to insulate before battening. I'v tried cavity closers but they don't seem very effective as it takes ages to remove the polystyrene on the underside to the correct dimensions, seeing as the cavity is of variable width throughout. I tried using foam combined with spare bits of batten but now I'm wondering if I need to remove that as the battens might create thermal bridging? Would celotex offcuts and foam work instead?

 

Thanks 

Rachel

 

 

 

 

[Olf]

Yes, rigid insulation board stripes do the job.

Also for the reveal you can use XPS tile backer boards instead of plasterboard: that removes thermal bridging through the surface.

I would strongly recommend sandwiching an airtight layer under the surface board (regardless whether you choose xps or plasterboard), taped to the window on one end and sealed to inner skin - that way any microcracks caused by opening/closing of the window or structure movement won't rob you of warm air later. For such small area nothing fancy is needed, even stripe of DPC/DPM will do.

Saying that, what is your airtightness strategy for the walls? With them stripped, now this is the right time to make it right.