Reflects 90% of solar infrared heat (infrared light) back through the window so it can't heat your room. Also very useful in attics, as a ceiling and/or floor lining. Also works the other way in winter, to reflect the room's heat to avoid heat loss. Both sides are identically reflective.
84" x 52", a very thin reflecting-coated plastic
The easiest way I've found to keep it against the window is with 10 or so small, neodymium ("rare earth metal") magnets against the various ferrous nails in the frame. It doesn't have any hole or slit to slide on a curtain rod, but you could cut several holes, and slide a curtain rod through it if desired.
Blackout level of darkness requires 4+ layers (99.99% light reflection)
I found that 4+ layers is the sweet spot for total blackout. With just 1 layer, even the sun appears no brighter than a streetlight. It blocks 90% of infrared heat which is just light, so 10% gets through in each layer. With 2 layers, 1% gets through (9% of the remaining 10% reflected). With 3 layers, only .1% gets through (.9% of the remaining 1% reflected). Etc.
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