Painted mirror reflecting light

[StringJunky]

Imagine painting a mirror and then beaming a sun-like spectrum at it. Will that still reduce the heat-producing IR wavelengths passing right through to the other side? What I'm thinking is, if you papered the ceiling with mylar material and painted it, could that improve insulation?

[swansont]

It depends on the properties of the material. Some transmit thermal IR (polyethylene does) and it looks like mylar transmits a decent amount near 10-15 microns

https://www.researchgate.net/figure/IR-transmission-spectra-of-polymeric-substrates-Mylar-with-thickness-d-40-mm-1-and_fig2_327932890