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.