I put an emergency blanket over the window in my study because that's the only window where sunlight comes in directly, it's made an enormous difference and it was free lol
Fun little edit because the physicist in me will never die: I just did a rough estimate, and when the sun hits this window it's almost exactly the same as running a space heater at full blast (it's actually a little more).
With the insulation it's equivalent to a person standing in the room doing nothing.