Insulation under doors

[JohnnyB]

I am looking for something to use under the doors frames in anew build, so there's not a thermal bridge between the brickwork and the screed.  25mm of celotex between the screed and the bricks seems rather thin. 

The architect is suggesting I use Marmox blocks but that seems overkill to support a door frame.  

Is there anything else that is suitable for this that will work out cheaper?

 

[Conor]

Compact foam 

[MikeGrahamT21]

Or another equivalent of compacfoam, one I used recently called Brigifoam. 
 

https://www.affixit.co.uk/brigi-foam.html

 

made by a company called iso-chemie who only sell to the trade, and these are a middle man who then sell it rebranded to the public, very good stuff and ideal for what you are wanting, and very cost effective compared to compacfoam.

[JohnnyB]

Following on from this I'm wondering how I can insulate between the back of the door cill where it meets the concrete.  We are having timber doors and windows and with a level threshold some of the timber cill is level with the screed.  In another area I have 25mm of celotex between the screed and the door frame but I'm wondering if there is something better I could be doing on my door frames

 

I've ended up ordering foamglas blocks for below the door frame.  The difference in price between those and compacfoam, brigi foam or marmox blocks is minimal and the foamglas blocks can be on the outside skin of the wall without covering so they work best for me.  The brigi foam company said their foam can be under the ground and exposed to the type 1 under paving slabs but the price is about the same so I decided not to use it here.