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I'm no expert in radiator sizing but I don't think it was miss sized
It was underpowered as it couldn’t heat the room. Was it single panel? A quick calc puts the required power at around 1000 watts but a single panel type ii at 100*60 will put out 800 and typically you over spec the rads and let TRVs modulate the output.
Not to say insulating is the wrong thing to do - with a bigger red you just piss more energy down the drain.
That is bonkers
Was the CH radiator dinner plate sized?!