I'm in the process of adding some thermal insulation to the single leaf wall of a utility room. The existing power sockets are wired as a radial circuit with 2.5mm^2 T&E fed from a 20A MCB. I'll be sticking 65mm foam insulated plasterboard over the cable and am uncertain what derating factor I should apply to the cable.
None of the installation methods shown in the cable section of the DIY Wiki show an insulated masonry wall . I'm assuming that the heat loss through the blockwork will be less than through a thin layer of plasterboard and that this arrangement would be equivalent to something between methods 102 (in a stud wall with thermal insulation with the cable touching the inner wall surface) and 103 (in a stud wall with thermal insulation with the cable NOT touching the inner wall surface).
If method 102 applies then I'd be Ok with the 2.5mm^2 and 20A MCB but if it's as bad as 103 then I'd need to go for 4.0nn^2 and a 16A MCB.
What derating factor should I be assuming for this arrangement?