Warmer in winter, cooler in summer.
If your home was built before the 1920s, its external walls are probably solid walls rather than cavity walls.
However solid walls can still be insulated. It’s a different process, and normally a more expensive one. However, it can lead to higher savings than with cavity wall insulation.
Internal wall insulation is done by fitting rigid insulation boards to the wall, or by building a stud wall filled in with insulation material such as mineral wool fibre.
Due to the thickness of the insulation, it will slightly reduce the floor area of the room(s) in which it is applied – around 100mm, and we may need to temporarily remove things like door frames.
It can also make it more complicated (though not impossible) to hang heavy items such as large picture frames.