Of course, it's only after one has his screed poured and set that he realises he forgot something...

I have a screed slab of approx 120 sqm and this is in a semi with the same on the other side. It sits on 175mm of pir and around the external perimeter I brought the wall PIR insulation down the wall to touch the floor PIR, isolating the heated screed from the external wall

In this crude text diagram that is a side section through, T is timber, N is insulation, B is blockwork (usually thermalite, occasionally dense concrete for load bearing points), S is screed and G is the ground

TN
TNSSSSSSSS
BNNNNNNN
GGGGGGG

Sometimes there are also small walls internally to the area of the slab that are load bearing and these are timber sitting on a block upstand. My concern is that i forgot to add a small 25mm strip of PIR between the screed and these block upstands, so all there is separating the screed from being in thermal contact with the block (that sits on the foundations) is a gap of probably 15-20mm created by the edging strip (a polystyrene-esque foam)

Another crude "drawing", the I being the edging strip:

T
TISSSSSSS
BNNNNNN
GGGGGG

I reckon there's approx 20m, maybe 25m of screed edge face (on a 50mm thick screed though at the edges this probably lessens due to the curve on the edging strip) where the screed is "more exposed" to block that is in thermal contact with the ground. It's still thermally broken, but not by the designed 25mm of PIR

My question is, is this such a calamity that I should look at getting a cutter out (bearing in mind this risks damage to the UFH) and cut 5-10mm off the edge of the screed all around this perimeter, and fit 25mm of PIR in place of the current edging strip?
If the financial impact of just letting more heat escape from this joint, over the time I'll keep the house, is less than the few days plus materials it'll take to change it then I'll just leave it.. (I have far more pressing things to attend to)

I'm assuming that, in terms of heat paths, the external walls that are well insulated are the most important, and that eventually these internals walls would establish their own heat gradient down to the ground below and would experience relatively lower heat loss.

I'm less worried about condensation for these because they're internal and eventually (when the screed is around 27 degrees or whatever the UFH will run at) will acquire some warmth that, being in the middle of the slab, shouldnt hit condensate temperatures often, if ever. I could replace the edging strip with pir.. I've even got some liquid PIR I could pour down and let turn into solid PIR filling the entire void. Job for a rainy day I suppose.

The edging strip is probably a polyethylene foam with a U value slightly worse than EPS - I haven't found any great references, but the few times it's cropped up its mentioned that the R value is similar to mineral wool