insulation and R values

[u25kr]

Hi
I am converting an old stone outbuilding into a granny flat, and my architect has specified a u value of 0.19w/m2k for the walls. I think this corresponds to a R value of around 5.4. I am looking at various products that will enable me to achieve this level of insulation and wondered if anyone has any thoughts on what to use.

I am currently considering Framethern 32 slabs,(140mm), with Knauf thermal laminate plus, (45mm) and plasterboard finsh, (9.5mm) which will achieve the desired r value. However this will be very costly, so does anyone know of similar products that would come in cheaper?

Thanks for your help
Kevin