Considering a thermal bridge on purpose.

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Latest place has a 25mm PIR upstand all around perimeter of slab finishing flush at FFL. I think the slab is specced at around .15 U value so you know what i'm working with. 

At the minute my french doors are set to sit ontop of DPC brickwork with around 50mm sticking up. 30mm cill blues 20mm ally threshold. Door frame set back 30mm into cavity so effectively bridging this PIR at floor level.


I am considering ripping out 1 course of brickwork below french doors and having them made 75mm higher. I can then use a 30mm cill and bigger section (70mm ish) PVC bottom rail. I think using the PVC bottom rail gives better performance as not relying on the draught seal under the doors, it's against the face as the rebate is all around. Only snag is I will be ripping out the 25mm PIR behind the door to achieve the setback. The edge of the slab will be upto the PVC cill.


In practice would this scenario be any worse than the pvc cill bridging over the PIR as per original way? I suspect not but looking for advice. Don't need any calls done just your opinions please. Obviously I prefer option 2 because it gives be that better draught seal performance and a lower profile cill as it is effectively set 75mm down.





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