[masklinn]

Thankfully insulation tends to work both ways, it keeps warmth in and out.

One of my neighbours redid all the insulation of their house and lowered the high point of the south-facing living room by 10-15C (and made the house much easier to heat in the winter, they can now get by on just the fireplace).

An other good option if you have the space (and money), especially with a south-facing living room, is to add a "sacrificial" sunroom (/ enclosed patio): at the cost of a bit of light, you get extra living space in spring and autumn, and the sunroom will insulate the living room in winter and summer (for the latter especially if it has an opaque roof e.g. tiling).