Well, we are still midst renovations - ill health has lost us some time - but I am sure that the house would have been very much colder; picture snow, then ice sat in the finlock gutter, and that cold bridging in to the top of all bedrooms through the width of solid concrete... when the sun shone, eventually, the thin black aluminium gutters melted the snow soon enough.
The rooms with internal insulation finished saw no signs of melting snow on the roof above.
In addition we are not busy worrying about water ingress from possible unseen leaks in the finlock gutters ... never need to worry about that again.
This is a long term investment and improvement to the house moving on from the 1950s finlock gutter solution to a shortage of steel. WE also had the roof stripped, part insulated between the rafters (would have liked to do the entire roof), lined with modern breathable membrane, and tiles refixed, verges re-pointed, etc.
Our house is not letting in water and is letting in less wind in the roof whilst still having ventilation strip along the entire length of the eaves. We are in a good place. The insulation once complete will leave the house comfortable in all but the coldest weather with a minimum of heating ... our heating is designed for this completion of insulation, so we are not quite in 'paradise' yet!
When buying you need to look at the current state of the finlocks - are they sagging badly? Are they basically okay. If they have suffered major water/freeze damage they may be structurally unstable in places, and this is not good, will need remedial works.
Good luck.
