Can I use Squid to keep heat from the sun out?

Yes. Squid reflects a large proportion of heat from the sun before it enters the room. Depending on the colour, 25% to 36% of the sun's energy is reflected.
> Chalk: 36%
> Bone: 33%
> Oak: 28%
> Ash: 25%
> Rock: 25%

Because Squid sticks straight to the glass, the effect of thermal heating is also reduced. This is what happens when the energy from the sun heats up the air between the window and the curtain (or blind) and so also the room. Sticking Squid on your window therefore helps keep heat from the sun out.