IR and visible light are both electromagnetic radiation; I certainly am not challenging that. However none of these materials has a "flat" reflectance spectrum in either of two dimensions: wavelength and angle of reflection. I am unable to find the reference, but I recall seeing reflectix listed as only 86% reflective (I did say 80s, not 80) at the blue end of the visible EM spectrum. "Space blankets" (a slightly different execution of the aluminized polymer sheet) are ~95% reflective in the thermal (0.8-3nm) range but only 70% in the blue. I grant that this is argument by analogy and not conclusive for Reflectix, but I'd be surprised if they sweated blue reflectance in a product designed to bounce thermal radiant energy.
Your using a meter is a good thing. Does your meter have a spectral feature, or are you taking a "bin" value across its detector sensitivity? How is that sensitivity at the red and blue ends? What trend do you observe for decreasing (more grazing) angles of reflection? I am not trolling you, germania. I am asking questions because reflectivity is not a simple scalar quantity, even though it is presented as such for simplicity's sake.
sayyes, none of this really matters practically. Reflectix will do the job quite well.